QEMU-Devel Archive on lore.kernel.org
 help / color / Atom feed
* [RFC 0/3] acpi: cphp: add CPHP_GET_CPU_ID_CMD command to cpu hotplug MMIO interface
@ 2019-10-09 13:22 Igor Mammedov
  2019-10-09 13:22 ` [RFC 1/3] acpi: cpuhp: fix 'Command data' description is spec Igor Mammedov
                   ` (3 more replies)
  0 siblings, 4 replies; 31+ messages in thread
From: Igor Mammedov @ 2019-10-09 13:22 UTC (permalink / raw)
  To: qemu-devel
  Cc: Eduardo Habkost, Michael S. Tsirkin, Laszlo Ersek, Gerd Hoffmann,
	Paolo Bonzini, Philippe Mathieu-Daudé,
	Richard Henderson

As an alternative to passing to firmware topology info via new fwcfg files
so it could recreate APIC IDs based on it and order CPUs are enumerated,

extend CPU hotplug interface to return APIC ID as response to the new command
CPHP_GET_CPU_ID_CMD.


CC: Laszlo Ersek <lersek@redhat.com>
CC: Eduardo Habkost <ehabkost@redhat.com>
CC: "Michael S. Tsirkin" <mst@redhat.com>
CC: Gerd Hoffmann <kraxel@redhat.com>
CC: Paolo Bonzini <pbonzini@redhat.com>
CC: Philippe Mathieu-Daudé <philmd@redhat.com>
CC: Richard Henderson <rth@twiddle.net>
 
Igor Mammedov (3):
  acpi: cpuhp: fix 'Command data' description is spec
  acpi: cpuhp: add typical usecases into spec
  acpi: cpuhp: add CPHP_GET_CPU_ID_CMD command

 docs/specs/acpi_cpu_hotplug.txt | 37 ++++++++++++++++++++++++++++++---
 hw/acpi/cpu.c                   | 15 +++++++++++++
 hw/acpi/trace-events            |  1 +
 3 files changed, 50 insertions(+), 3 deletions(-)

-- 
2.18.1



^ permalink raw reply	[flat|nested] 31+ messages in thread

* [RFC 1/3] acpi: cpuhp: fix 'Command data' description is spec
  2019-10-09 13:22 [RFC 0/3] acpi: cphp: add CPHP_GET_CPU_ID_CMD command to cpu hotplug MMIO interface Igor Mammedov
@ 2019-10-09 13:22 ` Igor Mammedov
  2019-10-10 12:33   ` Laszlo Ersek
  2019-10-10 13:31   ` Laszlo Ersek
  2019-10-09 13:22 ` [RFC 2/3] acpi: cpuhp: add typical usecases into spec Igor Mammedov
                   ` (2 subsequent siblings)
  3 siblings, 2 replies; 31+ messages in thread
From: Igor Mammedov @ 2019-10-09 13:22 UTC (permalink / raw)
  To: qemu-devel
  Cc: Eduardo Habkost, Michael S. Tsirkin, Laszlo Ersek, Gerd Hoffmann,
	Paolo Bonzini, Philippe Mathieu-Daudé,
	Richard Henderson

QEMU returns 0, in case of erro or invalid value in 'Command field',
make spec match reality, i.e.

Also fix returned value description  in case 'Command field' == 0x0,
it's in not PXM but CPU selector value with pending event

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
---
 docs/specs/acpi_cpu_hotplug.txt | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/docs/specs/acpi_cpu_hotplug.txt b/docs/specs/acpi_cpu_hotplug.txt
index ee219c8358..ac5903b2b1 100644
--- a/docs/specs/acpi_cpu_hotplug.txt
+++ b/docs/specs/acpi_cpu_hotplug.txt
@@ -44,9 +44,10 @@ read access:
            3-7: reserved and should be ignored by OSPM
     [0x5-0x7] reserved
     [0x8] Command data: (DWORD access)
-          in case of error or unsupported command reads is 0xFFFFFFFF
+          in case of error or unsupported command reads is 0x0
           current 'Command field' value:
-              0: returns PXM value corresponding to device
+              0: returns CPU selector value corresponding to a CPU with
+                 pending event.
 
 write access:
     offset:
-- 
2.18.1



^ permalink raw reply	[flat|nested] 31+ messages in thread

* [RFC 2/3] acpi: cpuhp: add typical usecases into spec
  2019-10-09 13:22 [RFC 0/3] acpi: cphp: add CPHP_GET_CPU_ID_CMD command to cpu hotplug MMIO interface Igor Mammedov
  2019-10-09 13:22 ` [RFC 1/3] acpi: cpuhp: fix 'Command data' description is spec Igor Mammedov
@ 2019-10-09 13:22 ` Igor Mammedov
  2019-10-10 13:04   ` Laszlo Ersek
  2019-10-10 14:13   ` Laszlo Ersek
  2019-10-09 13:22 ` [RFC 3/3] acpi: cpuhp: add CPHP_GET_CPU_ID_CMD command Igor Mammedov
  2019-10-10  9:56 ` [RFC 0/3] acpi: cphp: add CPHP_GET_CPU_ID_CMD command to cpu hotplug MMIO interface Michael S. Tsirkin
  3 siblings, 2 replies; 31+ messages in thread
From: Igor Mammedov @ 2019-10-09 13:22 UTC (permalink / raw)
  To: qemu-devel
  Cc: Eduardo Habkost, Michael S. Tsirkin, Laszlo Ersek, Gerd Hoffmann,
	Paolo Bonzini, Philippe Mathieu-Daudé,
	Richard Henderson

Clarify values of "CPU selector' register and add workflows for
  * finding CPU with pending 'insert/remove' event
  * enumerating present/non present CPUs

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
---
 docs/specs/acpi_cpu_hotplug.txt | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/docs/specs/acpi_cpu_hotplug.txt b/docs/specs/acpi_cpu_hotplug.txt
index ac5903b2b1..43c5a193f0 100644
--- a/docs/specs/acpi_cpu_hotplug.txt
+++ b/docs/specs/acpi_cpu_hotplug.txt
@@ -54,6 +54,7 @@ write access:
     [0x0-0x3] CPU selector: (DWORD access)
               selects active CPU device. All following accesses to other
               registers will read/store data from/to selected CPU.
+              Valid values: [0 .. max_cpus)
     [0x4] CPU device control fields: (1 byte access)
         bits:
             0: reserved, OSPM must clear it before writing to register.
@@ -93,3 +94,24 @@ Selecting CPU device beyond possible range has no effect on platform:
      ignored
    - read accesses to CPU hot-plug registers not documented above return
      all bits set to 0.
+
+Typical usecases:
+   - Get a cpu with pending event
+     1. write 0x0 into 'Command field' register
+     2. read from 'Command data' register, CPU selector value (CPU's UID in ACPI
+        tables) and event for selected CPU from 'CPU device status fields'
+        register. If there aren't pending events, CPU selector value doesn't
+        change and 'insert' and 'remove' bits are not set.
+   - Enumerate CPUs present/non present CPUs.
+     1. set iterator to 0x0
+     2. write 0x0 into 'Command field' register and then iterator
+        into 'CPU selector' register.
+     3. read 'enabled' flag for selected CPU from 'CPU device status fields'
+        register
+     4. to continue to the next CPU, increment iterator and repeat step 2
+     5. read 'Command data' register
+     5.1 if 'Command data' register matches iterator continue to step 3.
+         (read presence bit for the next CPU)
+     5.2 if 'Command data' register has not changed, there is not CPU
+         corresponding to iterator value and the last valid iterator value
+         equals to 'max_cpus' + 1
-- 
2.18.1



^ permalink raw reply	[flat|nested] 31+ messages in thread

* [RFC 3/3] acpi: cpuhp: add CPHP_GET_CPU_ID_CMD command
  2019-10-09 13:22 [RFC 0/3] acpi: cphp: add CPHP_GET_CPU_ID_CMD command to cpu hotplug MMIO interface Igor Mammedov
  2019-10-09 13:22 ` [RFC 1/3] acpi: cpuhp: fix 'Command data' description is spec Igor Mammedov
  2019-10-09 13:22 ` [RFC 2/3] acpi: cpuhp: add typical usecases into spec Igor Mammedov
@ 2019-10-09 13:22 ` Igor Mammedov
  2019-10-10 14:56   ` Laszlo Ersek
  2019-10-10  9:56 ` [RFC 0/3] acpi: cphp: add CPHP_GET_CPU_ID_CMD command to cpu hotplug MMIO interface Michael S. Tsirkin
  3 siblings, 1 reply; 31+ messages in thread
From: Igor Mammedov @ 2019-10-09 13:22 UTC (permalink / raw)
  To: qemu-devel
  Cc: Eduardo Habkost, Michael S. Tsirkin, Laszlo Ersek, Gerd Hoffmann,
	Paolo Bonzini, Philippe Mathieu-Daudé,
	Richard Henderson

Extend CPU hotplug interface to return architecture specific
identifier for current CPU (in case of x86, it's APIC ID).

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
---
TODO:
  * cripple it to behave old way on old machine types so that
    new firmware started on new QEMU won't see a difference
    when migrated to an old QEMU (i.e. QEMU that doesn't support
    this command)
---
 docs/specs/acpi_cpu_hotplug.txt | 10 +++++++++-
 hw/acpi/cpu.c                   | 15 +++++++++++++++
 hw/acpi/trace-events            |  1 +
 3 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/docs/specs/acpi_cpu_hotplug.txt b/docs/specs/acpi_cpu_hotplug.txt
index 43c5a193f0..0438678249 100644
--- a/docs/specs/acpi_cpu_hotplug.txt
+++ b/docs/specs/acpi_cpu_hotplug.txt
@@ -32,7 +32,9 @@ Register block size:
 
 read access:
     offset:
-    [0x0-0x3] reserved
+    [0x0-0x3] Command data 2: (DWORD access)
+              upper 32 bit of 'Command data' if returned data value is 64 bit.
+              in case of error or unsupported command reads is 0x0
     [0x4] CPU device status fields: (1 byte access)
         bits:
            0: Device is enabled and may be used by guest
@@ -87,6 +89,8 @@ write access:
               2: stores value into OST status register, triggers
                  ACPI_DEVICE_OST QMP event from QEMU to external applications
                  with current values of OST event and status registers.
+              3: OSPM reads architecture specific value identifying CPU
+                 (x86: APIC ID)
             other values: reserved
 
 Selecting CPU device beyond possible range has no effect on platform:
@@ -115,3 +119,7 @@ Typical usecases:
      5.2 if 'Command data' register has not changed, there is not CPU
          corresponding to iterator value and the last valid iterator value
          equals to 'max_cpus' + 1
+   - Get architecture specific id for a CPU
+     1. pick a CPU to read from using 'CPU selector' register
+     2. write 0x3 int0 'Command field' register
+     3. read architecture specific id from 'Command data' register
diff --git a/hw/acpi/cpu.c b/hw/acpi/cpu.c
index 87f30a31d7..701542d860 100644
--- a/hw/acpi/cpu.c
+++ b/hw/acpi/cpu.c
@@ -12,11 +12,13 @@
 #define ACPI_CPU_FLAGS_OFFSET_RW 4
 #define ACPI_CPU_CMD_OFFSET_WR 5
 #define ACPI_CPU_CMD_DATA_OFFSET_RW 8
+#define ACPI_CPU_CMD_DATA2_OFFSET_RW 0
 
 enum {
     CPHP_GET_NEXT_CPU_WITH_EVENT_CMD = 0,
     CPHP_OST_EVENT_CMD = 1,
     CPHP_OST_STATUS_CMD = 2,
+    CPHP_GET_CPU_ID_CMD = 3,
     CPHP_CMD_MAX
 };
 
@@ -74,11 +76,24 @@ static uint64_t cpu_hotplug_rd(void *opaque, hwaddr addr, unsigned size)
         case CPHP_GET_NEXT_CPU_WITH_EVENT_CMD:
            val = cpu_st->selector;
            break;
+        case CPHP_GET_CPU_ID_CMD:
+           val = cpu_to_le64(cdev->arch_id) & 0xFFFFFFFF;
+           break;
         default:
            break;
         }
         trace_cpuhp_acpi_read_cmd_data(cpu_st->selector, val);
         break;
+    case ACPI_CPU_CMD_DATA2_OFFSET_RW:
+        switch (cpu_st->command) {
+        case CPHP_GET_CPU_ID_CMD:
+           val = cpu_to_le64(cdev->arch_id) >> 32;
+           break;
+        default:
+           break;
+        }
+        trace_cpuhp_acpi_read_cmd_data2(cpu_st->selector, val);
+        break;
     default:
         break;
     }
diff --git a/hw/acpi/trace-events b/hw/acpi/trace-events
index 96b8273297..afbc77de1c 100644
--- a/hw/acpi/trace-events
+++ b/hw/acpi/trace-events
@@ -23,6 +23,7 @@ cpuhp_acpi_read_flags(uint32_t idx, uint8_t flags) "idx[0x%"PRIx32"] flags: 0x%"
 cpuhp_acpi_write_idx(uint32_t idx) "set active cpu idx: 0x%"PRIx32
 cpuhp_acpi_write_cmd(uint32_t idx, uint8_t cmd) "idx[0x%"PRIx32"] cmd: 0x%"PRIx8
 cpuhp_acpi_read_cmd_data(uint32_t idx, uint32_t data) "idx[0x%"PRIx32"] data: 0x%"PRIx32
+cpuhp_acpi_read_cmd_data2(uint32_t idx, uint32_t data) "idx[0x%"PRIx32"] data: 0x%"PRIx32
 cpuhp_acpi_cpu_has_events(uint32_t idx, bool ins, bool rm) "idx[0x%"PRIx32"] inserting: %d, removing: %d"
 cpuhp_acpi_clear_inserting_evt(uint32_t idx) "idx[0x%"PRIx32"]"
 cpuhp_acpi_clear_remove_evt(uint32_t idx) "idx[0x%"PRIx32"]"
-- 
2.18.1



^ permalink raw reply	[flat|nested] 31+ messages in thread

* Re: [RFC 0/3] acpi: cphp: add CPHP_GET_CPU_ID_CMD command to cpu hotplug MMIO interface
  2019-10-09 13:22 [RFC 0/3] acpi: cphp: add CPHP_GET_CPU_ID_CMD command to cpu hotplug MMIO interface Igor Mammedov
                   ` (2 preceding siblings ...)
  2019-10-09 13:22 ` [RFC 3/3] acpi: cpuhp: add CPHP_GET_CPU_ID_CMD command Igor Mammedov
@ 2019-10-10  9:56 ` Michael S. Tsirkin
  2019-10-10 13:39   ` Igor Mammedov
  3 siblings, 1 reply; 31+ messages in thread
From: Michael S. Tsirkin @ 2019-10-10  9:56 UTC (permalink / raw)
  To: Igor Mammedov
  Cc: Eduardo Habkost, Laszlo Ersek, qemu-devel, Gerd Hoffmann,
	Paolo Bonzini, Philippe Mathieu-Daudé,
	Richard Henderson

On Wed, Oct 09, 2019 at 09:22:49AM -0400, Igor Mammedov wrote:
> As an alternative to passing to firmware topology info via new fwcfg files
> so it could recreate APIC IDs based on it and order CPUs are enumerated,
> 
> extend CPU hotplug interface to return APIC ID as response to the new command
> CPHP_GET_CPU_ID_CMD.

One big piece missing here is motivation:
Who's going to use this interface?

So far CPU hotplug was used by the ACPI, so we didn't
really commit to a fixed interface too strongly.

Is this a replacement to Laszlo's fw cfg interface?
If yes is the idea that OVMF going to depend on CPU hotplug directly then?
It does not depend on it now, does it?

If answers to all of the above is yes, then I don't really like it: it
is better to keep all paravirt stuff in one place, namely in fw cfg.



> 
> CC: Laszlo Ersek <lersek@redhat.com>
> CC: Eduardo Habkost <ehabkost@redhat.com>
> CC: "Michael S. Tsirkin" <mst@redhat.com>
> CC: Gerd Hoffmann <kraxel@redhat.com>
> CC: Paolo Bonzini <pbonzini@redhat.com>
> CC: Philippe Mathieu-Daudé <philmd@redhat.com>
> CC: Richard Henderson <rth@twiddle.net>
>  
> Igor Mammedov (3):
>   acpi: cpuhp: fix 'Command data' description is spec
>   acpi: cpuhp: add typical usecases into spec
>   acpi: cpuhp: add CPHP_GET_CPU_ID_CMD command
> 
>  docs/specs/acpi_cpu_hotplug.txt | 37 ++++++++++++++++++++++++++++++---
>  hw/acpi/cpu.c                   | 15 +++++++++++++
>  hw/acpi/trace-events            |  1 +
>  3 files changed, 50 insertions(+), 3 deletions(-)
> 
> -- 
> 2.18.1


^ permalink raw reply	[flat|nested] 31+ messages in thread

* Re: [RFC 1/3] acpi: cpuhp: fix 'Command data' description is spec
  2019-10-09 13:22 ` [RFC 1/3] acpi: cpuhp: fix 'Command data' description is spec Igor Mammedov
@ 2019-10-10 12:33   ` Laszlo Ersek
  2019-10-10 13:31   ` Laszlo Ersek
  1 sibling, 0 replies; 31+ messages in thread
From: Laszlo Ersek @ 2019-10-10 12:33 UTC (permalink / raw)
  To: Igor Mammedov, qemu-devel
  Cc: Eduardo Habkost, Michael S. Tsirkin, Gerd Hoffmann,
	Paolo Bonzini, Philippe Mathieu-Daudé,
	Richard Henderson

On 10/09/19 15:22, Igor Mammedov wrote:
> QEMU returns 0, in case of erro or invalid value in 'Command field',
> make spec match reality, i.e.

Unfinished thought?

> 
> Also fix returned value description  in case 'Command field' == 0x0,

double space

> it's in not PXM but CPU selector value with pending event

suggest dropping "in"

> 
> Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> ---
>  docs/specs/acpi_cpu_hotplug.txt | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/docs/specs/acpi_cpu_hotplug.txt b/docs/specs/acpi_cpu_hotplug.txt
> index ee219c8358..ac5903b2b1 100644
> --- a/docs/specs/acpi_cpu_hotplug.txt
> +++ b/docs/specs/acpi_cpu_hotplug.txt
> @@ -44,9 +44,10 @@ read access:
>             3-7: reserved and should be ignored by OSPM
>      [0x5-0x7] reserved
>      [0x8] Command data: (DWORD access)

since we're fixing this dword field description, can you spell out the
endianness too?

(I think endianness is relevant here, because patch#2 suggests selectors
can be looped over. So selectors are actual integers, not just 32-bit
opaque blobs.)

> -          in case of error or unsupported command reads is 0xFFFFFFFF
> +          in case of error or unsupported command reads is 0x0
>            current 'Command field' value:
> -              0: returns PXM value corresponding to device
> +              0: returns CPU selector value corresponding to a CPU with
> +                 pending event.
>  
>  write access:
>      offset:
> 

Thanks!
Laszlo


^ permalink raw reply	[flat|nested] 31+ messages in thread

* Re: [RFC 2/3] acpi: cpuhp: add typical usecases into spec
  2019-10-09 13:22 ` [RFC 2/3] acpi: cpuhp: add typical usecases into spec Igor Mammedov
@ 2019-10-10 13:04   ` Laszlo Ersek
  2019-10-10 13:15     ` Laszlo Ersek
  2019-10-10 14:13   ` Laszlo Ersek
  1 sibling, 1 reply; 31+ messages in thread
From: Laszlo Ersek @ 2019-10-10 13:04 UTC (permalink / raw)
  To: Igor Mammedov, qemu-devel
  Cc: Eduardo Habkost, Michael S. Tsirkin, Gerd Hoffmann,
	Paolo Bonzini, Philippe Mathieu-Daudé,
	Richard Henderson

On 10/09/19 15:22, Igor Mammedov wrote:
> Clarify values of "CPU selector' register and add workflows for

mismatched quotes (double vs. single)

>   * finding CPU with pending 'insert/remove' event
>   * enumerating present/non present CPUs
> 
> Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> ---
>  docs/specs/acpi_cpu_hotplug.txt | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/docs/specs/acpi_cpu_hotplug.txt b/docs/specs/acpi_cpu_hotplug.txt
> index ac5903b2b1..43c5a193f0 100644
> --- a/docs/specs/acpi_cpu_hotplug.txt
> +++ b/docs/specs/acpi_cpu_hotplug.txt
> @@ -54,6 +54,7 @@ write access:
>      [0x0-0x3] CPU selector: (DWORD access)

Please clarify the endianness.

>                selects active CPU device. All following accesses to other
>                registers will read/store data from/to selected CPU.
> +              Valid values: [0 .. max_cpus)

Nice; appreciate the bracket on the left side vs. the paren on the right
side!

>      [0x4] CPU device control fields: (1 byte access)
>          bits:
>              0: reserved, OSPM must clear it before writing to register.
> @@ -93,3 +94,24 @@ Selecting CPU device beyond possible range has no effect on platform:
>       ignored
>     - read accesses to CPU hot-plug registers not documented above return
>       all bits set to 0.
> +
> +Typical usecases:
> +   - Get a cpu with pending event
> +     1. write 0x0 into 'Command field' register
> +     2. read from 'Command data' register, CPU selector value (CPU's UID in ACPI
> +        tables)

OK.

I suggest putting this as: "read the CPU selector value (the CPU's UID
in the ACPI tables) from the 'Command data' register"

> and event for selected CPU from 'CPU device status fields'

OK.

> +        register. If there aren't pending events, CPU selector value doesn't

OK.

I suggest s/aren't/are no/

> +        change

So this feels important: *change* is relative to a previous value. In
order to determine change, I have to

- either read the "command data" register before writing 0x0 to
"command", and then compare the old value against the new value

- or even set "command data" to a bogus value myself (against which I
can compare the new value, after writing the command register).

So, what is the previous selector value that the change is relative to?

> and 'insert' and 'remove' bits are not set.

Ah, so is the order of steps actually this:

1. write 0x0 to command

2. read device status field

3. if bit#1 or bit#2 is set (insert or remove event), read CPU selector
affected by those event(s) from the command data field

4. otherwise, no pending event

?

> +   - Enumerate CPUs present/non present CPUs.
> +     1. set iterator to 0x0

OK

> +     2. write 0x0 into 'Command field' register

... this may update the device status field, and the command data field
(to a selector with pending events)

> and then iterator
> +        into 'CPU selector' register.

... so in case command 0x0 selected a CPU with pending events, we ignore
that, and select our iterator anyway. OK.

> +     3. read 'enabled' flag for selected CPU from 'CPU device status fields'
> +        register

OK

> +     4. to continue to the next CPU, increment iterator

OK

> and repeat step 2

not sure why writing 0x0 to "command" again is useful, but I'll see it
below; OK

> +     5. read 'Command data' register

oookay... so if writing 0x0 to command selected a CPU with pending
events, we get the selector of *that* CPU (regardless of what iterator
we have presently)

Otherwise we get an indeterminate value.

> +     5.1 if 'Command data' register matches iterator continue to step 3.

uhhh... what? :) At this point, the command data register can be in two
states:

- if the last 0x0 command selected a CPU with events pending, then that
selector is available in the command data register.

I don't understand why comparing that against the iterator is helpful.

- If there was no CPU with pending events, we're comparing an
indeterminate value against the iterator. Why?

I think the "command data" field must change under some circumstances
that are currently not documented. (I.e. it seems like "command data"
does not *only* change when command 0x0 can find a CPU with pending events.)

Thanks
Laszlo

> +         (read presence bit for the next CPU)
> +     5.2 if 'Command data' register has not changed, there is not CPU
> +         corresponding to iterator value and the last valid iterator value
> +         equals to 'max_cpus' + 1
> 



^ permalink raw reply	[flat|nested] 31+ messages in thread

* Re: [RFC 2/3] acpi: cpuhp: add typical usecases into spec
  2019-10-10 13:04   ` Laszlo Ersek
@ 2019-10-10 13:15     ` Laszlo Ersek
  0 siblings, 0 replies; 31+ messages in thread
From: Laszlo Ersek @ 2019-10-10 13:15 UTC (permalink / raw)
  To: Igor Mammedov, qemu-devel
  Cc: Eduardo Habkost, Michael S. Tsirkin, Gerd Hoffmann,
	Paolo Bonzini, Philippe Mathieu-Daudé,
	Richard Henderson

On 10/10/19 15:04, Laszlo Ersek wrote:
> On 10/09/19 15:22, Igor Mammedov wrote:
>> Clarify values of "CPU selector' register and add workflows for
> 
> mismatched quotes (double vs. single)
> 
>>   * finding CPU with pending 'insert/remove' event
>>   * enumerating present/non present CPUs
>>
>> Signed-off-by: Igor Mammedov <imammedo@redhat.com>
>> ---
>>  docs/specs/acpi_cpu_hotplug.txt | 22 ++++++++++++++++++++++
>>  1 file changed, 22 insertions(+)
>>
>> diff --git a/docs/specs/acpi_cpu_hotplug.txt b/docs/specs/acpi_cpu_hotplug.txt
>> index ac5903b2b1..43c5a193f0 100644
>> --- a/docs/specs/acpi_cpu_hotplug.txt
>> +++ b/docs/specs/acpi_cpu_hotplug.txt
>> @@ -54,6 +54,7 @@ write access:
>>      [0x0-0x3] CPU selector: (DWORD access)
> 
> Please clarify the endianness.
> 
>>                selects active CPU device. All following accesses to other
>>                registers will read/store data from/to selected CPU.
>> +              Valid values: [0 .. max_cpus)
> 
> Nice; appreciate the bracket on the left side vs. the paren on the right
> side!
> 
>>      [0x4] CPU device control fields: (1 byte access)
>>          bits:
>>              0: reserved, OSPM must clear it before writing to register.
>> @@ -93,3 +94,24 @@ Selecting CPU device beyond possible range has no effect on platform:
>>       ignored
>>     - read accesses to CPU hot-plug registers not documented above return
>>       all bits set to 0.
>> +
>> +Typical usecases:
>> +   - Get a cpu with pending event
>> +     1. write 0x0 into 'Command field' register
>> +     2. read from 'Command data' register, CPU selector value (CPU's UID in ACPI
>> +        tables)
> 
> OK.
> 
> I suggest putting this as: "read the CPU selector value (the CPU's UID
> in the ACPI tables) from the 'Command data' register"
> 
>> and event for selected CPU from 'CPU device status fields'
> 
> OK.
> 
>> +        register. If there aren't pending events, CPU selector value doesn't
> 
> OK.
> 
> I suggest s/aren't/are no/
> 
>> +        change
> 
> So this feels important: *change* is relative to a previous value. In
> order to determine change, I have to
> 
> - either read the "command data" register before writing 0x0 to
> "command", and then compare the old value against the new value
> 
> - or even set "command data" to a bogus value myself (against which I
> can compare the new value, after writing the command register).
> 
> So, what is the previous selector value that the change is relative to?
> 
>> and 'insert' and 'remove' bits are not set.
> 
> Ah, so is the order of steps actually this:
> 
> 1. write 0x0 to command
> 
> 2. read device status field
> 
> 3. if bit#1 or bit#2 is set (insert or remove event), read CPU selector
> affected by those event(s) from the command data field
> 
> 4. otherwise, no pending event
> 
> ?
> 
>> +   - Enumerate CPUs present/non present CPUs.
>> +     1. set iterator to 0x0
> 
> OK
> 
>> +     2. write 0x0 into 'Command field' register
> 
> ... this may update the device status field, and the command data field
> (to a selector with pending events)
> 
>> and then iterator
>> +        into 'CPU selector' register.
> 
> ... so in case command 0x0 selected a CPU with pending events, we ignore
> that, and select our iterator anyway. OK.
> 
>> +     3. read 'enabled' flag for selected CPU from 'CPU device status fields'
>> +        register
> 
> OK
> 
>> +     4. to continue to the next CPU, increment iterator
> 
> OK
> 
>> and repeat step 2
> 
> not sure why writing 0x0 to "command" again is useful, but I'll see it
> below; OK
> 
>> +     5. read 'Command data' register
> 
> oookay... so if writing 0x0 to command selected a CPU with pending
> events, we get the selector of *that* CPU (regardless of what iterator
> we have presently)
> 
> Otherwise we get an indeterminate value.
> 
>> +     5.1 if 'Command data' register matches iterator continue to step 3.
> 
> uhhh... what? :) At this point, the command data register can be in two
> states:
> 
> - if the last 0x0 command selected a CPU with events pending, then that
> selector is available in the command data register.
> 
> I don't understand why comparing that against the iterator is helpful.
> 
> - If there was no CPU with pending events, we're comparing an
> indeterminate value against the iterator. Why?
> 
> I think the "command data" field must change under some circumstances
> that are currently not documented. (I.e. it seems like "command data"
> does not *only* change when command 0x0 can find a CPU with pending events.)

After looking at cpu_hotplug_rd(), I think I know what's going on.

Every time command 0 is written, and there is no CPU with pending
events, the command data register will read as 0!

This seems like a core piece of information, and it's not documented in
the text file anywhere. It only says (with patch#1 applied),

  in case of error or unsupported command reads is 0x0

Command 0 is *not* unsupported. Therefore, this documentation is only
self-consistent if:

- selecting a non-existent (>=max_cpus) CPU via the selector register is
an *error*

- asking for a CPU with pending events (with command 0x0), and finding
none, is also an *error*.

Let me re-read the patch set with this information in mind.

Thanks
Laszlo


>> +         (read presence bit for the next CPU)
>> +     5.2 if 'Command data' register has not changed, there is not CPU
>> +         corresponding to iterator value and the last valid iterator value
>> +         equals to 'max_cpus' + 1
>>
> 



^ permalink raw reply	[flat|nested] 31+ messages in thread

* Re: [RFC 1/3] acpi: cpuhp: fix 'Command data' description is spec
  2019-10-09 13:22 ` [RFC 1/3] acpi: cpuhp: fix 'Command data' description is spec Igor Mammedov
  2019-10-10 12:33   ` Laszlo Ersek
@ 2019-10-10 13:31   ` Laszlo Ersek
  2019-10-10 13:36     ` Laszlo Ersek
  1 sibling, 1 reply; 31+ messages in thread
From: Laszlo Ersek @ 2019-10-10 13:31 UTC (permalink / raw)
  To: Igor Mammedov, qemu-devel
  Cc: Eduardo Habkost, Michael S. Tsirkin, Gerd Hoffmann,
	Paolo Bonzini, Philippe Mathieu-Daudé,
	Richard Henderson

2nd round:

On 10/09/19 15:22, Igor Mammedov wrote:
> QEMU returns 0, in case of erro or invalid value in 'Command field',
> make spec match reality, i.e.

AHA! so this is exactly where you meant to list the particular cases
when "command data" reads as 0:
- CPU >= max_cpus selected,
- CPU with pending events asked for, but none found

> Also fix returned value description  in case 'Command field' == 0x0,
> it's in not PXM but CPU selector value with pending event
> 
> Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> ---
>  docs/specs/acpi_cpu_hotplug.txt | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/docs/specs/acpi_cpu_hotplug.txt b/docs/specs/acpi_cpu_hotplug.txt
> index ee219c8358..ac5903b2b1 100644
> --- a/docs/specs/acpi_cpu_hotplug.txt
> +++ b/docs/specs/acpi_cpu_hotplug.txt
> @@ -44,9 +44,10 @@ read access:
>             3-7: reserved and should be ignored by OSPM
>      [0x5-0x7] reserved
>      [0x8] Command data: (DWORD access)
> -          in case of error or unsupported command reads is 0xFFFFFFFF
> +          in case of error or unsupported command reads is 0x0
>            current 'Command field' value:
> -              0: returns PXM value corresponding to device
> +              0: returns CPU selector value corresponding to a CPU with
> +                 pending event.
>  
>  write access:
>      offset:
> 

How about:

    [0x8] Command data: (DWORD access, little endian)
          If the "CPU selector" value last stored by the guest refers to
          an impossible CPU, then 0.
          Otherwise, if the "Command field" value last stored by the
          guest differs from 0, then 0.
          Otherwise, if there is at least one CPU with a pending event,
          then that CPU has been selected; the command data register
          returns that selector.
          Otherwise, 0.

Thanks
Laszlo


^ permalink raw reply	[flat|nested] 31+ messages in thread

* Re: [RFC 1/3] acpi: cpuhp: fix 'Command data' description is spec
  2019-10-10 13:31   ` Laszlo Ersek
@ 2019-10-10 13:36     ` Laszlo Ersek
  0 siblings, 0 replies; 31+ messages in thread
From: Laszlo Ersek @ 2019-10-10 13:36 UTC (permalink / raw)
  To: Igor Mammedov, qemu-devel
  Cc: Eduardo Habkost, Michael S. Tsirkin, Gerd Hoffmann,
	Paolo Bonzini, Philippe Mathieu-Daudé,
	Richard Henderson

On 10/10/19 15:31, Laszlo Ersek wrote:
> 2nd round:
> 
> On 10/09/19 15:22, Igor Mammedov wrote:
>> QEMU returns 0, in case of erro or invalid value in 'Command field',
>> make spec match reality, i.e.
> 
> AHA! so this is exactly where you meant to list the particular cases
> when "command data" reads as 0:
> - CPU >= max_cpus selected,
> - CPU with pending events asked for, but none found
> 
>> Also fix returned value description  in case 'Command field' == 0x0,
>> it's in not PXM but CPU selector value with pending event
>>
>> Signed-off-by: Igor Mammedov <imammedo@redhat.com>
>> ---
>>  docs/specs/acpi_cpu_hotplug.txt | 5 +++--
>>  1 file changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/docs/specs/acpi_cpu_hotplug.txt b/docs/specs/acpi_cpu_hotplug.txt
>> index ee219c8358..ac5903b2b1 100644
>> --- a/docs/specs/acpi_cpu_hotplug.txt
>> +++ b/docs/specs/acpi_cpu_hotplug.txt
>> @@ -44,9 +44,10 @@ read access:
>>             3-7: reserved and should be ignored by OSPM
>>      [0x5-0x7] reserved
>>      [0x8] Command data: (DWORD access)
>> -          in case of error or unsupported command reads is 0xFFFFFFFF
>> +          in case of error or unsupported command reads is 0x0
>>            current 'Command field' value:
>> -              0: returns PXM value corresponding to device
>> +              0: returns CPU selector value corresponding to a CPU with
>> +                 pending event.
>>  
>>  write access:
>>      offset:
>>
> 
> How about:
> 
>     [0x8] Command data: (DWORD access, little endian)
>           If the "CPU selector" value last stored by the guest refers to
>           an impossible CPU, then 0.
>           Otherwise, if the "Command field" value last stored by the
>           guest differs from 0, then 0.
>           Otherwise, if there is at least one CPU with a pending event,
>           then that CPU has been selected; the command data register
>           returns that selector.
>           Otherwise, 0.

Hmmm not exactly. Let me try again.

    [0x8] Command data: (DWORD access, little endian)
          If the "CPU selector" value last stored by the guest refers to
          an impossible CPU, then 0.
          Otherwise, if the "Command field" value last stored by the
          guest differs from 0, then 0.
          Otherwise, the currently selected CPU.

Thanks,
Laszlo


^ permalink raw reply	[flat|nested] 31+ messages in thread

* Re: [RFC 0/3] acpi: cphp: add CPHP_GET_CPU_ID_CMD command to cpu hotplug MMIO interface
  2019-10-10  9:56 ` [RFC 0/3] acpi: cphp: add CPHP_GET_CPU_ID_CMD command to cpu hotplug MMIO interface Michael S. Tsirkin
@ 2019-10-10 13:39   ` Igor Mammedov
  2019-10-10 13:59     ` Michael S. Tsirkin
  2019-10-10 14:16     ` Eduardo Habkost
  0 siblings, 2 replies; 31+ messages in thread
From: Igor Mammedov @ 2019-10-10 13:39 UTC (permalink / raw)
  To: Michael S. Tsirkin
  Cc: Eduardo Habkost, Laszlo Ersek, qemu-devel, Gerd Hoffmann,
	Paolo Bonzini, Philippe Mathieu-Daudé,
	Richard Henderson

On Thu, 10 Oct 2019 05:56:55 -0400
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Wed, Oct 09, 2019 at 09:22:49AM -0400, Igor Mammedov wrote:
> > As an alternative to passing to firmware topology info via new fwcfg files
> > so it could recreate APIC IDs based on it and order CPUs are enumerated,
> > 
> > extend CPU hotplug interface to return APIC ID as response to the new command
> > CPHP_GET_CPU_ID_CMD.  
> 
> One big piece missing here is motivation:
I thought the only willing reader was Laszlo (who is aware of context)
so I skipped on details and confused others :/

> Who's going to use this interface?
In current state it's for firmware, since ACPI tables can cheat
by having APIC IDs statically built in.

If we were creating CPU objects in ACPI dynamically
we would be using this command as well. It would save
us quite a bit space in ACPI blob but it would be a pain
to debug and diagnose problems in ACPI tables, so I'd rather
stay with static CPU descriptions in ACPI tables for the sake
of maintenance.

> So far CPU hotplug was used by the ACPI, so we didn't
> really commit to a fixed interface too strongly.
> 
> Is this a replacement to Laszlo's fw cfg interface?
> If yes is the idea that OVMF going to depend on CPU hotplug directly then?
> It does not depend on it now, does it?
It doesn't, but then it doesn't support cpu hotplug,
OVMF(SMM) needs to cooperate with QEMU "and" ACPI tables to perform
the task and using the same interface/code path between all involved
parties makes the task easier with the least amount of duplicated
interfaces and more robust.

Re-implementing alternative interface for firmware (fwcfg or what not)
would work as well, but it's only question of time when ACPI and
this new interface disagree on how world works and process falls
apart.

> If answers to all of the above is yes, then I don't really like it: it
> is better to keep all paravirt stuff in one place, namely in fw cfg.
Lets discuss, what cpu hotplug fwcfg interface could look like in 
 [PATCH 3/4] hw/i386: add facility to expose CPU topology over  fw-cfg
mail thread and clarify (dis)likes with concrete reasons.

So far I managed to convince myself that we ought to reuse
and extend current CPU hotplug interface with firmware features,
to endup with consolidated cpu hotplug process without
introducing duplicate ABIs, but I could be wrong so
lets see if fwcfg will be the better approach.

 
> > CC: Laszlo Ersek <lersek@redhat.com>
> > CC: Eduardo Habkost <ehabkost@redhat.com>
> > CC: "Michael S. Tsirkin" <mst@redhat.com>
> > CC: Gerd Hoffmann <kraxel@redhat.com>
> > CC: Paolo Bonzini <pbonzini@redhat.com>
> > CC: Philippe Mathieu-Daudé <philmd@redhat.com>
> > CC: Richard Henderson <rth@twiddle.net>
> >  
> > Igor Mammedov (3):
> >   acpi: cpuhp: fix 'Command data' description is spec
> >   acpi: cpuhp: add typical usecases into spec
> >   acpi: cpuhp: add CPHP_GET_CPU_ID_CMD command
> > 
> >  docs/specs/acpi_cpu_hotplug.txt | 37 ++++++++++++++++++++++++++++++---
> >  hw/acpi/cpu.c                   | 15 +++++++++++++
> >  hw/acpi/trace-events            |  1 +
> >  3 files changed, 50 insertions(+), 3 deletions(-)
> > 
> > -- 
> > 2.18.1  
> 



^ permalink raw reply	[flat|nested] 31+ messages in thread

* Re: [RFC 0/3] acpi: cphp: add CPHP_GET_CPU_ID_CMD command to cpu hotplug MMIO interface
  2019-10-10 13:39   ` Igor Mammedov
@ 2019-10-10 13:59     ` Michael S. Tsirkin
  2019-10-10 15:57       ` Igor Mammedov
  2019-10-10 14:16     ` Eduardo Habkost
  1 sibling, 1 reply; 31+ messages in thread
From: Michael S. Tsirkin @ 2019-10-10 13:59 UTC (permalink / raw)
  To: Igor Mammedov
  Cc: Eduardo Habkost, Laszlo Ersek, qemu-devel, Gerd Hoffmann,
	Paolo Bonzini, Philippe Mathieu-Daudé,
	Richard Henderson

On Thu, Oct 10, 2019 at 03:39:12PM +0200, Igor Mammedov wrote:
> On Thu, 10 Oct 2019 05:56:55 -0400
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > On Wed, Oct 09, 2019 at 09:22:49AM -0400, Igor Mammedov wrote:
> > > As an alternative to passing to firmware topology info via new fwcfg files
> > > so it could recreate APIC IDs based on it and order CPUs are enumerated,
> > > 
> > > extend CPU hotplug interface to return APIC ID as response to the new command
> > > CPHP_GET_CPU_ID_CMD.  
> > 
> > One big piece missing here is motivation:
> I thought the only willing reader was Laszlo (who is aware of context)
> so I skipped on details and confused others :/
> 
> > Who's going to use this interface?
> In current state it's for firmware, since ACPI tables can cheat
> by having APIC IDs statically built in.
> 
> If we were creating CPU objects in ACPI dynamically
> we would be using this command as well.

I'm not sure how it's even possible to create devices dynamically. Well
I guess it's possible with LoadTable. Is this what you had in
mind?


> It would save
> us quite a bit space in ACPI blob but it would be a pain
> to debug and diagnose problems in ACPI tables, so I'd rather
> stay with static CPU descriptions in ACPI tables for the sake
> of maintenance.
> > So far CPU hotplug was used by the ACPI, so we didn't
> > really commit to a fixed interface too strongly.
> > 
> > Is this a replacement to Laszlo's fw cfg interface?
> > If yes is the idea that OVMF going to depend on CPU hotplug directly then?
> > It does not depend on it now, does it?
> It doesn't, but then it doesn't support cpu hotplug,
> OVMF(SMM) needs to cooperate with QEMU "and" ACPI tables to perform
> the task and using the same interface/code path between all involved
> parties makes the task easier with the least amount of duplicated
> interfaces and more robust.
> 
> Re-implementing alternative interface for firmware (fwcfg or what not)
> would work as well, but it's only question of time when ACPI and
> this new interface disagree on how world works and process falls
> apart.

Then we should consider switching acpi to use fw cfg.
Or build another interface that can scale.

> > If answers to all of the above is yes, then I don't really like it: it
> > is better to keep all paravirt stuff in one place, namely in fw cfg.
> Lets discuss, what cpu hotplug fwcfg interface could look like in 
>  [PATCH 3/4] hw/i386: add facility to expose CPU topology over  fw-cfg
> mail thread and clarify (dis)likes with concrete reasons.
> 
> So far I managed to convince myself that we ought to reuse
> and extend current CPU hotplug interface with firmware features,
> to endup with consolidated cpu hotplug process without
> introducing duplicate ABIs, but I could be wrong so
> lets see if fwcfg will be the better approach.
> 
>  
> > > CC: Laszlo Ersek <lersek@redhat.com>
> > > CC: Eduardo Habkost <ehabkost@redhat.com>
> > > CC: "Michael S. Tsirkin" <mst@redhat.com>
> > > CC: Gerd Hoffmann <kraxel@redhat.com>
> > > CC: Paolo Bonzini <pbonzini@redhat.com>
> > > CC: Philippe Mathieu-Daudé <philmd@redhat.com>
> > > CC: Richard Henderson <rth@twiddle.net>
> > >  
> > > Igor Mammedov (3):
> > >   acpi: cpuhp: fix 'Command data' description is spec
> > >   acpi: cpuhp: add typical usecases into spec
> > >   acpi: cpuhp: add CPHP_GET_CPU_ID_CMD command
> > > 
> > >  docs/specs/acpi_cpu_hotplug.txt | 37 ++++++++++++++++++++++++++++++---
> > >  hw/acpi/cpu.c                   | 15 +++++++++++++
> > >  hw/acpi/trace-events            |  1 +
> > >  3 files changed, 50 insertions(+), 3 deletions(-)
> > > 
> > > -- 
> > > 2.18.1  
> > 


^ permalink raw reply	[flat|nested] 31+ messages in thread

* Re: [RFC 2/3] acpi: cpuhp: add typical usecases into spec
  2019-10-09 13:22 ` [RFC 2/3] acpi: cpuhp: add typical usecases into spec Igor Mammedov
  2019-10-10 13:04   ` Laszlo Ersek
@ 2019-10-10 14:13   ` Laszlo Ersek
  1 sibling, 0 replies; 31+ messages in thread
From: Laszlo Ersek @ 2019-10-10 14:13 UTC (permalink / raw)
  To: Igor Mammedov, qemu-devel
  Cc: Eduardo Habkost, Michael S. Tsirkin, Gerd Hoffmann,
	Paolo Bonzini, Philippe Mathieu-Daudé,
	Richard Henderson

On 10/09/19 15:22, Igor Mammedov wrote:
> Clarify values of "CPU selector' register and add workflows for
>   * finding CPU with pending 'insert/remove' event
>   * enumerating present/non present CPUs
> 
> Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> ---
>  docs/specs/acpi_cpu_hotplug.txt | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/docs/specs/acpi_cpu_hotplug.txt b/docs/specs/acpi_cpu_hotplug.txt
> index ac5903b2b1..43c5a193f0 100644
> --- a/docs/specs/acpi_cpu_hotplug.txt
> +++ b/docs/specs/acpi_cpu_hotplug.txt
> @@ -54,6 +54,7 @@ write access:
>      [0x0-0x3] CPU selector: (DWORD access)
>                selects active CPU device. All following accesses to other
>                registers will read/store data from/to selected CPU.
> +              Valid values: [0 .. max_cpus)
>      [0x4] CPU device control fields: (1 byte access)
>          bits:
>              0: reserved, OSPM must clear it before writing to register.
> @@ -93,3 +94,24 @@ Selecting CPU device beyond possible range has no effect on platform:
>       ignored
>     - read accesses to CPU hot-plug registers not documented above return
>       all bits set to 0.
> +
> +Typical usecases:
> +   - Get a cpu with pending event
> +     1. write 0x0 into 'Command field' register
> +     2. read from 'Command data' register, CPU selector value (CPU's UID in ACPI
> +        tables) and event for selected CPU from 'CPU device status fields'
> +        register. If there aren't pending events, CPU selector value doesn't
> +        change and 'insert' and 'remove' bits are not set.

Okay, so based on the "Command data" documentation I'm suggesting in
<http://mid.mail-archive.com/cd0713b5-fd64-d3e1-7f83-3a0725b819a3@redhat.com>,
I propose:

1. Store 0x0 to the 'CPU selector' register.
2. Store 0x0 to the 'Command field' register.
3. Read the 'CPU device status fields' register.
4. If both bit#1 and bit#2 are clear in the value read, there is no CPU
   with a pending event.
5. Otherwise, read the 'Command data' register. The value read is the
   selector of the CPU with the pending event (which is already
   selected).

> +   - Enumerate CPUs present/non present CPUs.
> +     1. set iterator to 0x0
> +     2. write 0x0 into 'Command field' register and then iterator
> +        into 'CPU selector' register.
> +     3. read 'enabled' flag for selected CPU from 'CPU device status fields'
> +        register
> +     4. to continue to the next CPU, increment iterator and repeat step 2
> +     5. read 'Command data' register
> +     5.1 if 'Command data' register matches iterator continue to step 3.
> +         (read presence bit for the next CPU)
> +     5.2 if 'Command data' register has not changed, there is not CPU
> +         corresponding to iterator value and the last valid iterator value
> +         equals to 'max_cpus' + 1
> 

How about:

01. Set the present CPU count to 0.
02. Set the iterator to 0.
03. Store 0x0 to the 'Command field' register.
04. Store 0x0 to the 'CPU selector' register.
05. Read the 'CPU device status fields' register.
06. If bit#0 is set, increment the present CPU count.
07. Increment the iterator.
08. Store the iterator to the 'CPU selector' register.
09. Read the 'Command data' register.
10. If the value read is zero, then the iterator equals "max_cpus";
    exit now.
11. Goto 05.

Thanks
Laszlo


^ permalink raw reply	[flat|nested] 31+ messages in thread

* Re: [RFC 0/3] acpi: cphp: add CPHP_GET_CPU_ID_CMD command to cpu hotplug MMIO interface
  2019-10-10 13:39   ` Igor Mammedov
  2019-10-10 13:59     ` Michael S. Tsirkin
@ 2019-10-10 14:16     ` Eduardo Habkost
  2019-10-10 14:49       ` Michael S. Tsirkin
  2019-10-10 17:09       ` Igor Mammedov
  1 sibling, 2 replies; 31+ messages in thread
From: Eduardo Habkost @ 2019-10-10 14:16 UTC (permalink / raw)
  To: Igor Mammedov
  Cc: Michael S. Tsirkin, Laszlo Ersek, qemu-devel, Gerd Hoffmann,
	Paolo Bonzini, Philippe Mathieu-Daudé,
	Richard Henderson

On Thu, Oct 10, 2019 at 03:39:12PM +0200, Igor Mammedov wrote:
> On Thu, 10 Oct 2019 05:56:55 -0400
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > On Wed, Oct 09, 2019 at 09:22:49AM -0400, Igor Mammedov wrote:
> > > As an alternative to passing to firmware topology info via new fwcfg files
> > > so it could recreate APIC IDs based on it and order CPUs are enumerated,
> > > 
> > > extend CPU hotplug interface to return APIC ID as response to the new command
> > > CPHP_GET_CPU_ID_CMD.  
> > 
> > One big piece missing here is motivation:
> I thought the only willing reader was Laszlo (who is aware of context)
> so I skipped on details and confused others :/
> 
> > Who's going to use this interface?
> In current state it's for firmware, since ACPI tables can cheat
> by having APIC IDs statically built in.
> 
> If we were creating CPU objects in ACPI dynamically
> we would be using this command as well. It would save
> us quite a bit space in ACPI blob but it would be a pain
> to debug and diagnose problems in ACPI tables, so I'd rather
> stay with static CPU descriptions in ACPI tables for the sake
> of maintenance.
> 
> > So far CPU hotplug was used by the ACPI, so we didn't
> > really commit to a fixed interface too strongly.
> > 
> > Is this a replacement to Laszlo's fw cfg interface?
> > If yes is the idea that OVMF going to depend on CPU hotplug directly then?
> > It does not depend on it now, does it?
> It doesn't, but then it doesn't support cpu hotplug,
> OVMF(SMM) needs to cooperate with QEMU "and" ACPI tables to perform
> the task and using the same interface/code path between all involved
> parties makes the task easier with the least amount of duplicated
> interfaces and more robust.
> 
> Re-implementing alternative interface for firmware (fwcfg or what not)
> would work as well, but it's only question of time when ACPI and
> this new interface disagree on how world works and process falls
> apart.
> 
> > If answers to all of the above is yes, then I don't really like it: it
> > is better to keep all paravirt stuff in one place, namely in fw cfg.
> Lets discuss, what cpu hotplug fwcfg interface could look like in 
>  [PATCH 3/4] hw/i386: add facility to expose CPU topology over  fw-cfg
> mail thread and clarify (dis)likes with concrete reasons.
> 
> So far I managed to convince myself that we ought to reuse
> and extend current CPU hotplug interface with firmware features,
> to endup with consolidated cpu hotplug process without
> introducing duplicate ABIs, but I could be wrong so
> lets see if fwcfg will be the better approach.
> 

I was more inclined towards the approach in this patch, because I
see it as just a bug fix in the CPU hotplug interface (which
should have been using the hardware CPU identifier as the CPU
selector since the beginning).

Providing the missing information in fw_cfg isn't necessarily
bad, but please document it explicitly as a
  hotplug_cpu_selector => cpu_hardware_id
mapping, so people won't use "CPU index" as a generic identifier
elsewhere.

-- 
Eduardo


^ permalink raw reply	[flat|nested] 31+ messages in thread

* Re: [RFC 0/3] acpi: cphp: add CPHP_GET_CPU_ID_CMD command to cpu hotplug MMIO interface
  2019-10-10 14:16     ` Eduardo Habkost
@ 2019-10-10 14:49       ` Michael S. Tsirkin
  2019-10-10 17:09       ` Igor Mammedov
  1 sibling, 0 replies; 31+ messages in thread
From: Michael S. Tsirkin @ 2019-10-10 14:49 UTC (permalink / raw)
  To: Eduardo Habkost
  Cc: Laszlo Ersek, qemu-devel, Gerd Hoffmann, Paolo Bonzini,
	Igor Mammedov, Philippe Mathieu-Daudé,
	Richard Henderson

On Thu, Oct 10, 2019 at 11:16:52AM -0300, Eduardo Habkost wrote:
> On Thu, Oct 10, 2019 at 03:39:12PM +0200, Igor Mammedov wrote:
> > On Thu, 10 Oct 2019 05:56:55 -0400
> > "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > 
> > > On Wed, Oct 09, 2019 at 09:22:49AM -0400, Igor Mammedov wrote:
> > > > As an alternative to passing to firmware topology info via new fwcfg files
> > > > so it could recreate APIC IDs based on it and order CPUs are enumerated,
> > > > 
> > > > extend CPU hotplug interface to return APIC ID as response to the new command
> > > > CPHP_GET_CPU_ID_CMD.  
> > > 
> > > One big piece missing here is motivation:
> > I thought the only willing reader was Laszlo (who is aware of context)
> > so I skipped on details and confused others :/
> > 
> > > Who's going to use this interface?
> > In current state it's for firmware, since ACPI tables can cheat
> > by having APIC IDs statically built in.
> > 
> > If we were creating CPU objects in ACPI dynamically
> > we would be using this command as well. It would save
> > us quite a bit space in ACPI blob but it would be a pain
> > to debug and diagnose problems in ACPI tables, so I'd rather
> > stay with static CPU descriptions in ACPI tables for the sake
> > of maintenance.
> > 
> > > So far CPU hotplug was used by the ACPI, so we didn't
> > > really commit to a fixed interface too strongly.
> > > 
> > > Is this a replacement to Laszlo's fw cfg interface?
> > > If yes is the idea that OVMF going to depend on CPU hotplug directly then?
> > > It does not depend on it now, does it?
> > It doesn't, but then it doesn't support cpu hotplug,
> > OVMF(SMM) needs to cooperate with QEMU "and" ACPI tables to perform
> > the task and using the same interface/code path between all involved
> > parties makes the task easier with the least amount of duplicated
> > interfaces and more robust.
> > 
> > Re-implementing alternative interface for firmware (fwcfg or what not)
> > would work as well, but it's only question of time when ACPI and
> > this new interface disagree on how world works and process falls
> > apart.
> > 
> > > If answers to all of the above is yes, then I don't really like it: it
> > > is better to keep all paravirt stuff in one place, namely in fw cfg.
> > Lets discuss, what cpu hotplug fwcfg interface could look like in 
> >  [PATCH 3/4] hw/i386: add facility to expose CPU topology over  fw-cfg
> > mail thread and clarify (dis)likes with concrete reasons.
> > 
> > So far I managed to convince myself that we ought to reuse
> > and extend current CPU hotplug interface with firmware features,
> > to endup with consolidated cpu hotplug process without
> > introducing duplicate ABIs, but I could be wrong so
> > lets see if fwcfg will be the better approach.
> > 
> 
> I was more inclined towards the approach in this patch, because I
> see it as just a bug fix in the CPU hotplug interface (which
> should have been using the hardware CPU identifier as the CPU
> selector since the beginning).

Well if ACPI is going to be using this, then that's different.
Igor do you see any need for that?

> Providing the missing information in fw_cfg isn't necessarily
> bad, but please document it explicitly as a
>   hotplug_cpu_selector => cpu_hardware_id
> mapping, so people won't use "CPU index" as a generic identifier
> elsewhere.
> 
> -- 
> Eduardo


^ permalink raw reply	[flat|nested] 31+ messages in thread

* Re: [RFC 3/3] acpi: cpuhp: add CPHP_GET_CPU_ID_CMD command
  2019-10-09 13:22 ` [RFC 3/3] acpi: cpuhp: add CPHP_GET_CPU_ID_CMD command Igor Mammedov
@ 2019-10-10 14:56   ` Laszlo Ersek
  2019-10-10 15:06     ` Michael S. Tsirkin
  0 siblings, 1 reply; 31+ messages in thread
From: Laszlo Ersek @ 2019-10-10 14:56 UTC (permalink / raw)
  To: Igor Mammedov, qemu-devel
  Cc: Eduardo Habkost, Michael S. Tsirkin, Gerd Hoffmann,
	Paolo Bonzini, Philippe Mathieu-Daudé,
	Richard Henderson

On 10/09/19 15:22, Igor Mammedov wrote:
> Extend CPU hotplug interface to return architecture specific
> identifier for current CPU (in case of x86, it's APIC ID).
> 
> Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> ---
> TODO:
>   * cripple it to behave old way on old machine types so that
>     new firmware started on new QEMU won't see a difference
>     when migrated to an old QEMU (i.e. QEMU that doesn't support
>     this command)
> ---
>  docs/specs/acpi_cpu_hotplug.txt | 10 +++++++++-
>  hw/acpi/cpu.c                   | 15 +++++++++++++++
>  hw/acpi/trace-events            |  1 +
>  3 files changed, 25 insertions(+), 1 deletion(-)
> 
> diff --git a/docs/specs/acpi_cpu_hotplug.txt b/docs/specs/acpi_cpu_hotplug.txt
> index 43c5a193f0..0438678249 100644
> --- a/docs/specs/acpi_cpu_hotplug.txt
> +++ b/docs/specs/acpi_cpu_hotplug.txt
> @@ -32,7 +32,9 @@ Register block size:
>  
>  read access:
>      offset:
> -    [0x0-0x3] reserved
> +    [0x0-0x3] Command data 2: (DWORD access)
> +              upper 32 bit of 'Command data' if returned data value is 64 bit.
> +              in case of error or unsupported command reads is 0x0

How about

    [0x0] Command data 2: (DWORD access, little endian)
          If the "CPU selector" value last stored by the guest refers to
          an impossible CPU, then 0.
          Otherwise, if the "Command field" value last stored by the
          guest differs from 3, then 0.
          Otherwise, the most significant 32 bits of the selected CPU's
          architecture specific ID.

    [0x8] Command data: (DWORD access, little endian)
          If the "CPU selector" value last stored by the guest refers to
          an impossible CPU, then 0.
          Otherwise,
          - if the "Command field" value last stored by the guest is 0,
            then the selector of the currently selected CPU;
          - if the "Command field" value last stored by the guest is 3,
            then the least significant 32 bits of the selected CPU's
            architecture specific ID;
          - otherwise, 0.

>      [0x4] CPU device status fields: (1 byte access)
>          bits:
>             0: Device is enabled and may be used by guest
> @@ -87,6 +89,8 @@ write access:
>                2: stores value into OST status register, triggers
>                   ACPI_DEVICE_OST QMP event from QEMU to external applications
>                   with current values of OST event and status registers.
> +              3: OSPM reads architecture specific value identifying CPU
> +                 (x86: APIC ID)
>              other values: reserved
>  

Seems OK.

>  Selecting CPU device beyond possible range has no effect on platform:
> @@ -115,3 +119,7 @@ Typical usecases:
>       5.2 if 'Command data' register has not changed, there is not CPU
>           corresponding to iterator value and the last valid iterator value
>           equals to 'max_cpus' + 1
> +   - Get architecture specific id for a CPU
> +     1. pick a CPU to read from using 'CPU selector' register
> +     2. write 0x3 int0 'Command field' register
> +     3. read architecture specific id from 'Command data' register

Looks good, except for:

- typo: "int0"

- in step 3, we should reference 'Command data 2' as well.


In fact, in
<http://mid.mail-archive.com/2b10ca48-c734-4f41-9521-136c44060812@redhat.com>,
I wrote, for the "Get a cpu with pending event" use case:

> 1. Store 0x0 to the 'CPU selector' register.
> 2. Store 0x0 to the 'Command field' register.
> 3. Read the 'CPU device status fields' register.
> 4. If both bit#1 and bit#2 are clear in the value read, there is no
>    CPU with a pending event.
> 5. Otherwise, read the 'Command data' register. The value read is the
>    selector of the CPU with the pending event (which is already
>    selected).

and your steps #2 and #3, for getting the arch specific ID, can be
directly appended as steps 6. and 7.!


> diff --git a/hw/acpi/cpu.c b/hw/acpi/cpu.c
> index 87f30a31d7..701542d860 100644
> --- a/hw/acpi/cpu.c
> +++ b/hw/acpi/cpu.c
> @@ -12,11 +12,13 @@
>  #define ACPI_CPU_FLAGS_OFFSET_RW 4
>  #define ACPI_CPU_CMD_OFFSET_WR 5
>  #define ACPI_CPU_CMD_DATA_OFFSET_RW 8
> +#define ACPI_CPU_CMD_DATA2_OFFSET_RW 0
>  
>  enum {
>      CPHP_GET_NEXT_CPU_WITH_EVENT_CMD = 0,
>      CPHP_OST_EVENT_CMD = 1,
>      CPHP_OST_STATUS_CMD = 2,
> +    CPHP_GET_CPU_ID_CMD = 3,
>      CPHP_CMD_MAX
>  };
>  
> @@ -74,11 +76,24 @@ static uint64_t cpu_hotplug_rd(void *opaque, hwaddr addr, unsigned size)
>          case CPHP_GET_NEXT_CPU_WITH_EVENT_CMD:
>             val = cpu_st->selector;
>             break;
> +        case CPHP_GET_CPU_ID_CMD:
> +           val = cpu_to_le64(cdev->arch_id) & 0xFFFFFFFF;
> +           break;
>          default:
>             break;
>          }
>          trace_cpuhp_acpi_read_cmd_data(cpu_st->selector, val);
>          break;
> +    case ACPI_CPU_CMD_DATA2_OFFSET_RW:
> +        switch (cpu_st->command) {
> +        case CPHP_GET_CPU_ID_CMD:
> +           val = cpu_to_le64(cdev->arch_id) >> 32;
> +           break;
> +        default:
> +           break;
> +        }
> +        trace_cpuhp_acpi_read_cmd_data2(cpu_st->selector, val);
> +        break;
>      default:
>          break;
>      }
> diff --git a/hw/acpi/trace-events b/hw/acpi/trace-events
> index 96b8273297..afbc77de1c 100644
> --- a/hw/acpi/trace-events
> +++ b/hw/acpi/trace-events
> @@ -23,6 +23,7 @@ cpuhp_acpi_read_flags(uint32_t idx, uint8_t flags) "idx[0x%"PRIx32"] flags: 0x%"
>  cpuhp_acpi_write_idx(uint32_t idx) "set active cpu idx: 0x%"PRIx32
>  cpuhp_acpi_write_cmd(uint32_t idx, uint8_t cmd) "idx[0x%"PRIx32"] cmd: 0x%"PRIx8
>  cpuhp_acpi_read_cmd_data(uint32_t idx, uint32_t data) "idx[0x%"PRIx32"] data: 0x%"PRIx32
> +cpuhp_acpi_read_cmd_data2(uint32_t idx, uint32_t data) "idx[0x%"PRIx32"] data: 0x%"PRIx32
>  cpuhp_acpi_cpu_has_events(uint32_t idx, bool ins, bool rm) "idx[0x%"PRIx32"] inserting: %d, removing: %d"
>  cpuhp_acpi_clear_inserting_evt(uint32_t idx) "idx[0x%"PRIx32"]"
>  cpuhp_acpi_clear_remove_evt(uint32_t idx) "idx[0x%"PRIx32"]"
> 

Looks plausible to me, thanks (discounting the TODO item).

Right now, I can't offer testing for patch#3 (I'm quite far from the
point where I'll be actually looking for a hotplugged CPU :) ), but
based on the docs patches #1 and #2, and my proposed updates, I can
rework my "possible CPU count detection" in OVMF.

Do I need to check in OVMF specifically whether the "modern" CPU hotplug
register block is available? Can you tell me what the oldest machine
types are that support the modern interface?

Hmm... Commit abd49bc2ed2f ("docs: update ACPI CPU hotplug spec with new
protocol", 2016-06-24) seems relevant. First released in v2.7.0. I think
I should detect whether this interface is available.

Can I use the following sequence to detect whether the interface is
available?

1. Store 0x0 to command register.
2. Store 0x0 to selector register.
3. Read 'command data' register.
4. If value read is 0, the interface is available.

(Because I assume that unmapped IO ports read as all-bits-one. Is that
right?)

BTW, can I dynamically detect support for the GET_CPU_ID command too?
I'm thinking, when I enumerate / count all possible CPUs, I can at once
fetch the arch IDs for all of them. If I only get zeros from the command
data registers, across all CPUs, in response to GET_CPU_ID, then the
command is not available.

Thanks
Laszlo


^ permalink raw reply	[flat|nested] 31+ messages in thread

* Re: [RFC 3/3] acpi: cpuhp: add CPHP_GET_CPU_ID_CMD command
  2019-10-10 14:56   ` Laszlo Ersek
@ 2019-10-10 15:06     ` Michael S. Tsirkin
  2019-10-10 17:23       ` Igor Mammedov
                         ` (2 more replies)
  0 siblings, 3 replies; 31+ messages in thread
From: Michael S. Tsirkin @ 2019-10-10 15:06 UTC (permalink / raw)
  To: Laszlo Ersek
  Cc: Eduardo Habkost, qemu-devel, Gerd Hoffmann, Paolo Bonzini,
	Igor Mammedov, Philippe Mathieu-Daudé,
	Richard Henderson

On Thu, Oct 10, 2019 at 04:56:18PM +0200, Laszlo Ersek wrote:
> On 10/09/19 15:22, Igor Mammedov wrote:
> > Extend CPU hotplug interface to return architecture specific
> > identifier for current CPU (in case of x86, it's APIC ID).
> > 
> > Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> > ---
> > TODO:
> >   * cripple it to behave old way on old machine types so that
> >     new firmware started on new QEMU won't see a difference
> >     when migrated to an old QEMU (i.e. QEMU that doesn't support
> >     this command)
> > ---
> >  docs/specs/acpi_cpu_hotplug.txt | 10 +++++++++-
> >  hw/acpi/cpu.c                   | 15 +++++++++++++++
> >  hw/acpi/trace-events            |  1 +
> >  3 files changed, 25 insertions(+), 1 deletion(-)
> > 
> > diff --git a/docs/specs/acpi_cpu_hotplug.txt b/docs/specs/acpi_cpu_hotplug.txt
> > index 43c5a193f0..0438678249 100644
> > --- a/docs/specs/acpi_cpu_hotplug.txt
> > +++ b/docs/specs/acpi_cpu_hotplug.txt
> > @@ -32,7 +32,9 @@ Register block size:
> >  
> >  read access:
> >      offset:
> > -    [0x0-0x3] reserved
> > +    [0x0-0x3] Command data 2: (DWORD access)
> > +              upper 32 bit of 'Command data' if returned data value is 64 bit.
> > +              in case of error or unsupported command reads is 0x0
> 
> How about
> 
>     [0x0] Command data 2: (DWORD access, little endian)
>           If the "CPU selector" value last stored by the guest refers to
>           an impossible CPU, then 0.
>           Otherwise, if the "Command field" value last stored by the
>           guest differs from 3, then 0.
>           Otherwise, the most significant 32 bits of the selected CPU's
>           architecture specific ID.
> 
>     [0x8] Command data: (DWORD access, little endian)
>           If the "CPU selector" value last stored by the guest refers to
>           an impossible CPU, then 0.
>           Otherwise,
>           - if the "Command field" value last stored by the guest is 0,
>             then the selector of the currently selected CPU;
>           - if the "Command field" value last stored by the guest is 3,
>             then the least significant 32 bits of the selected CPU's
>             architecture specific ID;
>           - otherwise, 0.
> 
> >      [0x4] CPU device status fields: (1 byte access)
> >          bits:
> >             0: Device is enabled and may be used by guest
> > @@ -87,6 +89,8 @@ write access:
> >                2: stores value into OST status register, triggers
> >                   ACPI_DEVICE_OST QMP event from QEMU to external applications
> >                   with current values of OST event and status registers.
> > +              3: OSPM reads architecture specific value identifying CPU
> > +                 (x86: APIC ID)
> >              other values: reserved
> >  
> 
> Seems OK.
> 
> >  Selecting CPU device beyond possible range has no effect on platform:
> > @@ -115,3 +119,7 @@ Typical usecases:
> >       5.2 if 'Command data' register has not changed, there is not CPU
> >           corresponding to iterator value and the last valid iterator value
> >           equals to 'max_cpus' + 1
> > +   - Get architecture specific id for a CPU
> > +     1. pick a CPU to read from using 'CPU selector' register
> > +     2. write 0x3 int0 'Command field' register
> > +     3. read architecture specific id from 'Command data' register
> 
> Looks good, except for:
> 
> - typo: "int0"
> 
> - in step 3, we should reference 'Command data 2' as well.
> 
> 
> In fact, in
> <http://mid.mail-archive.com/2b10ca48-c734-4f41-9521-136c44060812@redhat.com>,
> I wrote, for the "Get a cpu with pending event" use case:
> 
> > 1. Store 0x0 to the 'CPU selector' register.
> > 2. Store 0x0 to the 'Command field' register.
> > 3. Read the 'CPU device status fields' register.
> > 4. If both bit#1 and bit#2 are clear in the value read, there is no
> >    CPU with a pending event.
> > 5. Otherwise, read the 'Command data' register. The value read is the
> >    selector of the CPU with the pending event (which is already
> >    selected).
> 
> and your steps #2 and #3, for getting the arch specific ID, can be
> directly appended as steps 6. and 7.!
> 
> 
> > diff --git a/hw/acpi/cpu.c b/hw/acpi/cpu.c
> > index 87f30a31d7..701542d860 100644
> > --- a/hw/acpi/cpu.c
> > +++ b/hw/acpi/cpu.c
> > @@ -12,11 +12,13 @@
> >  #define ACPI_CPU_FLAGS_OFFSET_RW 4
> >  #define ACPI_CPU_CMD_OFFSET_WR 5
> >  #define ACPI_CPU_CMD_DATA_OFFSET_RW 8
> > +#define ACPI_CPU_CMD_DATA2_OFFSET_RW 0
> >  
> >  enum {
> >      CPHP_GET_NEXT_CPU_WITH_EVENT_CMD = 0,
> >      CPHP_OST_EVENT_CMD = 1,
> >      CPHP_OST_STATUS_CMD = 2,
> > +    CPHP_GET_CPU_ID_CMD = 3,
> >      CPHP_CMD_MAX
> >  };
> >  
> > @@ -74,11 +76,24 @@ static uint64_t cpu_hotplug_rd(void *opaque, hwaddr addr, unsigned size)
> >          case CPHP_GET_NEXT_CPU_WITH_EVENT_CMD:
> >             val = cpu_st->selector;
> >             break;
> > +        case CPHP_GET_CPU_ID_CMD:
> > +           val = cpu_to_le64(cdev->arch_id) & 0xFFFFFFFF;
> > +           break;
> >          default:
> >             break;
> >          }
> >          trace_cpuhp_acpi_read_cmd_data(cpu_st->selector, val);
> >          break;
> > +    case ACPI_CPU_CMD_DATA2_OFFSET_RW:
> > +        switch (cpu_st->command) {
> > +        case CPHP_GET_CPU_ID_CMD:
> > +           val = cpu_to_le64(cdev->arch_id) >> 32;
> > +           break;
> > +        default:
> > +           break;
> > +        }
> > +        trace_cpuhp_acpi_read_cmd_data2(cpu_st->selector, val);
> > +        break;
> >      default:
> >          break;
> >      }
> > diff --git a/hw/acpi/trace-events b/hw/acpi/trace-events
> > index 96b8273297..afbc77de1c 100644
> > --- a/hw/acpi/trace-events
> > +++ b/hw/acpi/trace-events
> > @@ -23,6 +23,7 @@ cpuhp_acpi_read_flags(uint32_t idx, uint8_t flags) "idx[0x%"PRIx32"] flags: 0x%"
> >  cpuhp_acpi_write_idx(uint32_t idx) "set active cpu idx: 0x%"PRIx32
> >  cpuhp_acpi_write_cmd(uint32_t idx, uint8_t cmd) "idx[0x%"PRIx32"] cmd: 0x%"PRIx8
> >  cpuhp_acpi_read_cmd_data(uint32_t idx, uint32_t data) "idx[0x%"PRIx32"] data: 0x%"PRIx32
> > +cpuhp_acpi_read_cmd_data2(uint32_t idx, uint32_t data) "idx[0x%"PRIx32"] data: 0x%"PRIx32
> >  cpuhp_acpi_cpu_has_events(uint32_t idx, bool ins, bool rm) "idx[0x%"PRIx32"] inserting: %d, removing: %d"
> >  cpuhp_acpi_clear_inserting_evt(uint32_t idx) "idx[0x%"PRIx32"]"
> >  cpuhp_acpi_clear_remove_evt(uint32_t idx) "idx[0x%"PRIx32"]"
> > 
> 
> Looks plausible to me, thanks (discounting the TODO item).
> 
> Right now, I can't offer testing for patch#3 (I'm quite far from the
> point where I'll be actually looking for a hotplugged CPU :) ), but
> based on the docs patches #1 and #2, and my proposed updates, I can
> rework my "possible CPU count detection" in OVMF.
> 
> Do I need to check in OVMF specifically whether the "modern" CPU hotplug
> register block is available? Can you tell me what the oldest machine
> types are that support the modern interface?
> 
> Hmm... Commit abd49bc2ed2f ("docs: update ACPI CPU hotplug spec with new
> protocol", 2016-06-24) seems relevant. First released in v2.7.0. I think
> I should detect whether this interface is available.
> 
> Can I use the following sequence to detect whether the interface is
> available?
> 
> 1. Store 0x0 to command register.
> 2. Store 0x0 to selector register.
> 3. Read 'command data' register.
> 4. If value read is 0, the interface is available.
> 
> (Because I assume that unmapped IO ports read as all-bits-one. Is that
> right?)
> 
> BTW, can I dynamically detect support for the GET_CPU_ID command too?
> I'm thinking, when I enumerate / count all possible CPUs, I can at once
> fetch the arch IDs for all of them. If I only get zeros from the command
> data registers, across all CPUs, in response to GET_CPU_ID, then the
> command is not available.
> 
> Thanks
> Laszlo

Laszlo, won't we need to add topology info anyway?
if yes then this patch is just a stopgap, so let's do
fw cfg and be done with it?

-- 
MST


^ permalink raw reply	[flat|nested] 31+ messages in thread

* Re: [RFC 0/3] acpi: cphp: add CPHP_GET_CPU_ID_CMD command to cpu hotplug MMIO interface
  2019-10-10 13:59     ` Michael S. Tsirkin
@ 2019-10-10 15:57       ` Igor Mammedov
  2019-10-10 18:15         ` Michael S. Tsirkin
                           ` (2 more replies)
  0 siblings, 3 replies; 31+ messages in thread
From: Igor Mammedov @ 2019-10-10 15:57 UTC (permalink / raw)
  To: Michael S. Tsirkin
  Cc: Eduardo Habkost, Laszlo Ersek, qemu-devel, Gerd Hoffmann,
	Paolo Bonzini, Philippe Mathieu-Daudé,
	Richard Henderson

On Thu, 10 Oct 2019 09:59:42 -0400
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Thu, Oct 10, 2019 at 03:39:12PM +0200, Igor Mammedov wrote:
> > On Thu, 10 Oct 2019 05:56:55 -0400
> > "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > 
> > > On Wed, Oct 09, 2019 at 09:22:49AM -0400, Igor Mammedov wrote:
> > > > As an alternative to passing to firmware topology info via new fwcfg files
> > > > so it could recreate APIC IDs based on it and order CPUs are enumerated,
> > > > 
> > > > extend CPU hotplug interface to return APIC ID as response to the new command
> > > > CPHP_GET_CPU_ID_CMD.  
> > > 
> > > One big piece missing here is motivation:
> > I thought the only willing reader was Laszlo (who is aware of context)
> > so I skipped on details and confused others :/
> > 
> > > Who's going to use this interface?
> > In current state it's for firmware, since ACPI tables can cheat
> > by having APIC IDs statically built in.
> > 
> > If we were creating CPU objects in ACPI dynamically
> > we would be using this command as well.
> 
> I'm not sure how it's even possible to create devices dynamically. Well
> I guess it's possible with LoadTable. Is this what you had in
> mind?

Yep. I even played this shiny toy and I can say it's very tempting one.
On the  other side, even problem of legacy OSes not working with it aside,
it's hard to debug and reproduce compared to static tables.
So from maintaining pov I dislike it enough to be against it.


> > It would save
> > us quite a bit space in ACPI blob but it would be a pain
> > to debug and diagnose problems in ACPI tables, so I'd rather
> > stay with static CPU descriptions in ACPI tables for the sake
> > of maintenance.
> > > So far CPU hotplug was used by the ACPI, so we didn't
> > > really commit to a fixed interface too strongly.
> > > 
> > > Is this a replacement to Laszlo's fw cfg interface?
> > > If yes is the idea that OVMF going to depend on CPU hotplug directly then?
> > > It does not depend on it now, does it?
> > It doesn't, but then it doesn't support cpu hotplug,
> > OVMF(SMM) needs to cooperate with QEMU "and" ACPI tables to perform
> > the task and using the same interface/code path between all involved
> > parties makes the task easier with the least amount of duplicated
> > interfaces and more robust.
> > 
> > Re-implementing alternative interface for firmware (fwcfg or what not)
> > would work as well, but it's only question of time when ACPI and
> > this new interface disagree on how world works and process falls
> > apart.
> 
> Then we should consider switching acpi to use fw cfg.
> Or build another interface that can scale.

Could be an option, it would be a pain to write a driver in AML for fwcfg access though
(I've looked at possibility to access fwcfg from AML about a year ago and gave up.
I'm definitely not volunteering for the second attempt and can't even give an estimate
it it's viable approach).

But what scaling issue you are talking about, exactly?
With current CPU hotplug interface we can handle upto UNIT32_MAX cpus, and extend
interface without need to increase IO window we are using now.

Granted IO access it not fastest compared to fwcfg in DMA mode, but we already
doing stop machine when switching to SMM which is orders of magnitude slower.
Consensus was to compromise on speed of CPU hotplug versus more complex and more
problematic unicast SMM mode in OVMF (can't find a particular email but we have discussed
it with Laszlo already, when I considered ways to optimize hotplug speed)

> > > If answers to all of the above is yes, then I don't really like it: it
> > > is better to keep all paravirt stuff in one place, namely in fw cfg.
> > Lets discuss, what cpu hotplug fwcfg interface could look like in 
> >  [PATCH 3/4] hw/i386: add facility to expose CPU topology over  fw-cfg
> > mail thread and clarify (dis)likes with concrete reasons.
> > 
> > So far I managed to convince myself that we ought to reuse
> > and extend current CPU hotplug interface with firmware features,
> > to endup with consolidated cpu hotplug process without
> > introducing duplicate ABIs, but I could be wrong so
> > lets see if fwcfg will be the better approach.
> > 
> >  
> > > > CC: Laszlo Ersek <lersek@redhat.com>
> > > > CC: Eduardo Habkost <ehabkost@redhat.com>
> > > > CC: "Michael S. Tsirkin" <mst@redhat.com>
> > > > CC: Gerd Hoffmann <kraxel@redhat.com>
> > > > CC: Paolo Bonzini <pbonzini@redhat.com>
> > > > CC: Philippe Mathieu-Daudé <philmd@redhat.com>
> > > > CC: Richard Henderson <rth@twiddle.net>
> > > >  
> > > > Igor Mammedov (3):
> > > >   acpi: cpuhp: fix 'Command data' description is spec
> > > >   acpi: cpuhp: add typical usecases into spec
> > > >   acpi: cpuhp: add CPHP_GET_CPU_ID_CMD command
> > > > 
> > > >  docs/specs/acpi_cpu_hotplug.txt | 37 ++++++++++++++++++++++++++++++---
> > > >  hw/acpi/cpu.c                   | 15 +++++++++++++
> > > >  hw/acpi/trace-events            |  1 +
> > > >  3 files changed, 50 insertions(+), 3 deletions(-)
> > > > 
> > > > -- 
> > > > 2.18.1  
> > > 
> 



^ permalink raw reply	[flat|nested] 31+ messages in thread

* Re: [RFC 0/3] acpi: cphp: add CPHP_GET_CPU_ID_CMD command to cpu hotplug MMIO interface
  2019-10-10 14:16     ` Eduardo Habkost
  2019-10-10 14:49       ` Michael S. Tsirkin
@ 2019-10-10 17:09       ` Igor Mammedov
  1 sibling, 0 replies; 31+ messages in thread
From: Igor Mammedov @ 2019-10-10 17:09 UTC (permalink / raw)
  To: Eduardo Habkost
  Cc: Michael S. Tsirkin, Laszlo Ersek, qemu-devel, Gerd Hoffmann,
	Paolo Bonzini, Philippe Mathieu-Daudé,
	Richard Henderson

On Thu, 10 Oct 2019 11:16:52 -0300
Eduardo Habkost <ehabkost@redhat.com> wrote:

> On Thu, Oct 10, 2019 at 03:39:12PM +0200, Igor Mammedov wrote:
> > On Thu, 10 Oct 2019 05:56:55 -0400
> > "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > 
> > > On Wed, Oct 09, 2019 at 09:22:49AM -0400, Igor Mammedov wrote:
> > > > As an alternative to passing to firmware topology info via new fwcfg files
> > > > so it could recreate APIC IDs based on it and order CPUs are enumerated,
> > > > 
> > > > extend CPU hotplug interface to return APIC ID as response to the new command
> > > > CPHP_GET_CPU_ID_CMD.  
> > > 
> > > One big piece missing here is motivation:
> > I thought the only willing reader was Laszlo (who is aware of context)
> > so I skipped on details and confused others :/
> > 
> > > Who's going to use this interface?
> > In current state it's for firmware, since ACPI tables can cheat
> > by having APIC IDs statically built in.
> > 
> > If we were creating CPU objects in ACPI dynamically
> > we would be using this command as well. It would save
> > us quite a bit space in ACPI blob but it would be a pain
> > to debug and diagnose problems in ACPI tables, so I'd rather
> > stay with static CPU descriptions in ACPI tables for the sake
> > of maintenance.
> > 
> > > So far CPU hotplug was used by the ACPI, so we didn't
> > > really commit to a fixed interface too strongly.
> > > 
> > > Is this a replacement to Laszlo's fw cfg interface?
> > > If yes is the idea that OVMF going to depend on CPU hotplug directly then?
> > > It does not depend on it now, does it?
> > It doesn't, but then it doesn't support cpu hotplug,
> > OVMF(SMM) needs to cooperate with QEMU "and" ACPI tables to perform
> > the task and using the same interface/code path between all involved
> > parties makes the task easier with the least amount of duplicated
> > interfaces and more robust.
> > 
> > Re-implementing alternative interface for firmware (fwcfg or what not)
> > would work as well, but it's only question of time when ACPI and
> > this new interface disagree on how world works and process falls
> > apart.
> > 
> > > If answers to all of the above is yes, then I don't really like it: it
> > > is better to keep all paravirt stuff in one place, namely in fw cfg.
> > Lets discuss, what cpu hotplug fwcfg interface could look like in 
> >  [PATCH 3/4] hw/i386: add facility to expose CPU topology over  fw-cfg
> > mail thread and clarify (dis)likes with concrete reasons.
> > 
> > So far I managed to convince myself that we ought to reuse
> > and extend current CPU hotplug interface with firmware features,
> > to endup with consolidated cpu hotplug process without
> > introducing duplicate ABIs, but I could be wrong so
> > lets see if fwcfg will be the better approach.
> > 
> 
> I was more inclined towards the approach in this patch, because I
> see it as just a bug fix in the CPU hotplug interface (which
> should have been using the hardware CPU identifier as the CPU
> selector since the beginning).
> 
> Providing the missing information in fw_cfg isn't necessarily
> bad, but please document it explicitly as a
>   hotplug_cpu_selector => cpu_hardware_id
> mapping, so people won't use "CPU index" as a generic identifier
> elsewhere.

Currently cpu_selector is UID (or whatever you'd like to name it)
for a CPU instance in ACPI tables. It just happens to be non sparse
range [0..max_cpus) and was just a convenient way to make up IDs
and handle them on hw side (requires simple array).

Sure I'll document it as such to avoid mis-understanding,
plus a bunch of other fixes to the spec.



^ permalink raw reply	[flat|nested] 31+ messages in thread

* Re: [RFC 3/3] acpi: cpuhp: add CPHP_GET_CPU_ID_CMD command
  2019-10-10 15:06     ` Michael S. Tsirkin
@ 2019-10-10 17:23       ` Igor Mammedov
  2019-10-10 17:53       ` Laszlo Ersek
  2019-10-10 19:26       ` Eduardo Habkost
  2 siblings, 0 replies; 31+ messages in thread
From: Igor Mammedov @ 2019-10-10 17:23 UTC (permalink / raw)
  To: Michael S. Tsirkin
  Cc: Eduardo Habkost, Laszlo Ersek, qemu-devel, Gerd Hoffmann,
	Paolo Bonzini, Philippe Mathieu-Daudé,
	Richard Henderson

On Thu, 10 Oct 2019 11:06:29 -0400
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Thu, Oct 10, 2019 at 04:56:18PM +0200, Laszlo Ersek wrote:
> > On 10/09/19 15:22, Igor Mammedov wrote:
> > > Extend CPU hotplug interface to return architecture specific
> > > identifier for current CPU (in case of x86, it's APIC ID).
> > > 
> > > Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> > > ---
> > > TODO:
> > >   * cripple it to behave old way on old machine types so that
> > >     new firmware started on new QEMU won't see a difference
> > >     when migrated to an old QEMU (i.e. QEMU that doesn't support
> > >     this command)
> > > ---
> > >  docs/specs/acpi_cpu_hotplug.txt | 10 +++++++++-
> > >  hw/acpi/cpu.c                   | 15 +++++++++++++++
> > >  hw/acpi/trace-events            |  1 +
> > >  3 files changed, 25 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/docs/specs/acpi_cpu_hotplug.txt b/docs/specs/acpi_cpu_hotplug.txt
> > > index 43c5a193f0..0438678249 100644
> > > --- a/docs/specs/acpi_cpu_hotplug.txt
> > > +++ b/docs/specs/acpi_cpu_hotplug.txt
> > > @@ -32,7 +32,9 @@ Register block size:
> > >  
> > >  read access:
> > >      offset:
> > > -    [0x0-0x3] reserved
> > > +    [0x0-0x3] Command data 2: (DWORD access)
> > > +              upper 32 bit of 'Command data' if returned data value is 64 bit.
> > > +              in case of error or unsupported command reads is 0x0
> > 
> > How about
> > 
> >     [0x0] Command data 2: (DWORD access, little endian)
> >           If the "CPU selector" value last stored by the guest refers to
> >           an impossible CPU, then 0.
> >           Otherwise, if the "Command field" value last stored by the
> >           guest differs from 3, then 0.
> >           Otherwise, the most significant 32 bits of the selected CPU's
> >           architecture specific ID.
> > 
> >     [0x8] Command data: (DWORD access, little endian)
> >           If the "CPU selector" value last stored by the guest refers to
> >           an impossible CPU, then 0.
> >           Otherwise,
> >           - if the "Command field" value last stored by the guest is 0,
> >             then the selector of the currently selected CPU;
> >           - if the "Command field" value last stored by the guest is 3,
> >             then the least significant 32 bits of the selected CPU's
> >             architecture specific ID;
> >           - otherwise, 0.
> > 
> > >      [0x4] CPU device status fields: (1 byte access)
> > >          bits:
> > >             0: Device is enabled and may be used by guest
> > > @@ -87,6 +89,8 @@ write access:
> > >                2: stores value into OST status register, triggers
> > >                   ACPI_DEVICE_OST QMP event from QEMU to external applications
> > >                   with current values of OST event and status registers.
> > > +              3: OSPM reads architecture specific value identifying CPU
> > > +                 (x86: APIC ID)
> > >              other values: reserved
> > >  
> > 
> > Seems OK.
> > 
> > >  Selecting CPU device beyond possible range has no effect on platform:
> > > @@ -115,3 +119,7 @@ Typical usecases:
> > >       5.2 if 'Command data' register has not changed, there is not CPU
> > >           corresponding to iterator value and the last valid iterator value
> > >           equals to 'max_cpus' + 1
> > > +   - Get architecture specific id for a CPU
> > > +     1. pick a CPU to read from using 'CPU selector' register
> > > +     2. write 0x3 int0 'Command field' register
> > > +     3. read architecture specific id from 'Command data' register
> > 
> > Looks good, except for:
> > 
> > - typo: "int0"
> > 
> > - in step 3, we should reference 'Command data 2' as well.
> > 
> > 
> > In fact, in
> > <http://mid.mail-archive.com/2b10ca48-c734-4f41-9521-136c44060812@redhat.com>,
> > I wrote, for the "Get a cpu with pending event" use case:
> > 
> > > 1. Store 0x0 to the 'CPU selector' register.
> > > 2. Store 0x0 to the 'Command field' register.
> > > 3. Read the 'CPU device status fields' register.
> > > 4. If both bit#1 and bit#2 are clear in the value read, there is no
> > >    CPU with a pending event.
> > > 5. Otherwise, read the 'Command data' register. The value read is the
> > >    selector of the CPU with the pending event (which is already
> > >    selected).
> > 
> > and your steps #2 and #3, for getting the arch specific ID, can be
> > directly appended as steps 6. and 7.!
> > 
> > 
> > > diff --git a/hw/acpi/cpu.c b/hw/acpi/cpu.c
> > > index 87f30a31d7..701542d860 100644
> > > --- a/hw/acpi/cpu.c
> > > +++ b/hw/acpi/cpu.c
> > > @@ -12,11 +12,13 @@
> > >  #define ACPI_CPU_FLAGS_OFFSET_RW 4
> > >  #define ACPI_CPU_CMD_OFFSET_WR 5
> > >  #define ACPI_CPU_CMD_DATA_OFFSET_RW 8
> > > +#define ACPI_CPU_CMD_DATA2_OFFSET_RW 0
> > >  
> > >  enum {
> > >      CPHP_GET_NEXT_CPU_WITH_EVENT_CMD = 0,
> > >      CPHP_OST_EVENT_CMD = 1,
> > >      CPHP_OST_STATUS_CMD = 2,
> > > +    CPHP_GET_CPU_ID_CMD = 3,
> > >      CPHP_CMD_MAX
> > >  };
> > >  
> > > @@ -74,11 +76,24 @@ static uint64_t cpu_hotplug_rd(void *opaque, hwaddr addr, unsigned size)
> > >          case CPHP_GET_NEXT_CPU_WITH_EVENT_CMD:
> > >             val = cpu_st->selector;
> > >             break;
> > > +        case CPHP_GET_CPU_ID_CMD:
> > > +           val = cpu_to_le64(cdev->arch_id) & 0xFFFFFFFF;
> > > +           break;
> > >          default:
> > >             break;
> > >          }
> > >          trace_cpuhp_acpi_read_cmd_data(cpu_st->selector, val);
> > >          break;
> > > +    case ACPI_CPU_CMD_DATA2_OFFSET_RW:
> > > +        switch (cpu_st->command) {
> > > +        case CPHP_GET_CPU_ID_CMD:
> > > +           val = cpu_to_le64(cdev->arch_id) >> 32;
> > > +           break;
> > > +        default:
> > > +           break;
> > > +        }
> > > +        trace_cpuhp_acpi_read_cmd_data2(cpu_st->selector, val);
> > > +        break;
> > >      default:
> > >          break;
> > >      }
> > > diff --git a/hw/acpi/trace-events b/hw/acpi/trace-events
> > > index 96b8273297..afbc77de1c 100644
> > > --- a/hw/acpi/trace-events
> > > +++ b/hw/acpi/trace-events
> > > @@ -23,6 +23,7 @@ cpuhp_acpi_read_flags(uint32_t idx, uint8_t flags) "idx[0x%"PRIx32"] flags: 0x%"
> > >  cpuhp_acpi_write_idx(uint32_t idx) "set active cpu idx: 0x%"PRIx32
> > >  cpuhp_acpi_write_cmd(uint32_t idx, uint8_t cmd) "idx[0x%"PRIx32"] cmd: 0x%"PRIx8
> > >  cpuhp_acpi_read_cmd_data(uint32_t idx, uint32_t data) "idx[0x%"PRIx32"] data: 0x%"PRIx32
> > > +cpuhp_acpi_read_cmd_data2(uint32_t idx, uint32_t data) "idx[0x%"PRIx32"] data: 0x%"PRIx32
> > >  cpuhp_acpi_cpu_has_events(uint32_t idx, bool ins, bool rm) "idx[0x%"PRIx32"] inserting: %d, removing: %d"
> > >  cpuhp_acpi_clear_inserting_evt(uint32_t idx) "idx[0x%"PRIx32"]"
> > >  cpuhp_acpi_clear_remove_evt(uint32_t idx) "idx[0x%"PRIx32"]"
> > > 
> > 
> > Looks plausible to me, thanks (discounting the TODO item).
> > 
> > Right now, I can't offer testing for patch#3 (I'm quite far from the
> > point where I'll be actually looking for a hotplugged CPU :) ), but
> > based on the docs patches #1 and #2, and my proposed updates, I can
> > rework my "possible CPU count detection" in OVMF.
> > 
> > Do I need to check in OVMF specifically whether the "modern" CPU hotplug
> > register block is available? Can you tell me what the oldest machine
> > types are that support the modern interface?
> > 
> > Hmm... Commit abd49bc2ed2f ("docs: update ACPI CPU hotplug spec with new
> > protocol", 2016-06-24) seems relevant. First released in v2.7.0. I think
> > I should detect whether this interface is available.
> > 
> > Can I use the following sequence to detect whether the interface is
> > available?
> > 
> > 1. Store 0x0 to command register.
> > 2. Store 0x0 to selector register.
> > 3. Read 'command data' register.
> > 4. If value read is 0, the interface is available.
> > 
> > (Because I assume that unmapped IO ports read as all-bits-one. Is that
> > right?)
> > 
> > BTW, can I dynamically detect support for the GET_CPU_ID command too?
> > I'm thinking, when I enumerate / count all possible CPUs, I can at once
> > fetch the arch IDs for all of them. If I only get zeros from the command
> > data registers, across all CPUs, in response to GET_CPU_ID, then the
> > command is not available.
> > 
> > Thanks
> > Laszlo
> 
> Laszlo, won't we need to add topology info anyway?
> if yes then this patch is just a stopgap, so let's do
> fw cfg and be done with it?
answer to your question and suggestion is in following thread
  "Re: [RFC 3/3] acpi: cpuhp: add CPHP_GET_CPU_ID_CMD command"

"
In fact, it seems like OVMF will have to synthesize the new
(hot-plugged) VCPU's *APIC-ID* from the following information sources:

- the topology information described above (die / core / thread counts), and

- the "modern" CPU hotplug interface (docs/specs/acpi_cpu_hotplug.txt).
"


^ permalink raw reply	[flat|nested] 31+ messages in thread

* Re: [RFC 3/3] acpi: cpuhp: add CPHP_GET_CPU_ID_CMD command
  2019-10-10 15:06     ` Michael S. Tsirkin
  2019-10-10 17:23       ` Igor Mammedov
@ 2019-10-10 17:53       ` Laszlo Ersek
  2019-10-10 19:26       ` Eduardo Habkost
  2 siblings, 0 replies; 31+ messages in thread
From: Laszlo Ersek @ 2019-10-10 17:53 UTC (permalink / raw)
  To: Michael S. Tsirkin
  Cc: Eduardo Habkost, qemu-devel, Gerd Hoffmann, Paolo Bonzini,
	Igor Mammedov, Philippe Mathieu-Daudé,
	Richard Henderson

On 10/10/19 17:06, Michael S. Tsirkin wrote:
> On Thu, Oct 10, 2019 at 04:56:18PM +0200, Laszlo Ersek wrote:
>> On 10/09/19 15:22, Igor Mammedov wrote:
>>> Extend CPU hotplug interface to return architecture specific
>>> identifier for current CPU (in case of x86, it's APIC ID).
>>>
>>> Signed-off-by: Igor Mammedov <imammedo@redhat.com>
>>> ---
>>> TODO:
>>>   * cripple it to behave old way on old machine types so that
>>>     new firmware started on new QEMU won't see a difference
>>>     when migrated to an old QEMU (i.e. QEMU that doesn't support
>>>     this command)
>>> ---
>>>  docs/specs/acpi_cpu_hotplug.txt | 10 +++++++++-
>>>  hw/acpi/cpu.c                   | 15 +++++++++++++++
>>>  hw/acpi/trace-events            |  1 +
>>>  3 files changed, 25 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/docs/specs/acpi_cpu_hotplug.txt b/docs/specs/acpi_cpu_hotplug.txt
>>> index 43c5a193f0..0438678249 100644
>>> --- a/docs/specs/acpi_cpu_hotplug.txt
>>> +++ b/docs/specs/acpi_cpu_hotplug.txt
>>> @@ -32,7 +32,9 @@ Register block size:
>>>  
>>>  read access:
>>>      offset:
>>> -    [0x0-0x3] reserved
>>> +    [0x0-0x3] Command data 2: (DWORD access)
>>> +              upper 32 bit of 'Command data' if returned data value is 64 bit.
>>> +              in case of error or unsupported command reads is 0x0
>>
>> How about
>>
>>     [0x0] Command data 2: (DWORD access, little endian)
>>           If the "CPU selector" value last stored by the guest refers to
>>           an impossible CPU, then 0.
>>           Otherwise, if the "Command field" value last stored by the
>>           guest differs from 3, then 0.
>>           Otherwise, the most significant 32 bits of the selected CPU's
>>           architecture specific ID.
>>
>>     [0x8] Command data: (DWORD access, little endian)
>>           If the "CPU selector" value last stored by the guest refers to
>>           an impossible CPU, then 0.
>>           Otherwise,
>>           - if the "Command field" value last stored by the guest is 0,
>>             then the selector of the currently selected CPU;
>>           - if the "Command field" value last stored by the guest is 3,
>>             then the least significant 32 bits of the selected CPU's
>>             architecture specific ID;
>>           - otherwise, 0.
>>
>>>      [0x4] CPU device status fields: (1 byte access)
>>>          bits:
>>>             0: Device is enabled and may be used by guest
>>> @@ -87,6 +89,8 @@ write access:
>>>                2: stores value into OST status register, triggers
>>>                   ACPI_DEVICE_OST QMP event from QEMU to external applications
>>>                   with current values of OST event and status registers.
>>> +              3: OSPM reads architecture specific value identifying CPU
>>> +                 (x86: APIC ID)
>>>              other values: reserved
>>>  
>>
>> Seems OK.
>>
>>>  Selecting CPU device beyond possible range has no effect on platform:
>>> @@ -115,3 +119,7 @@ Typical usecases:
>>>       5.2 if 'Command data' register has not changed, there is not CPU
>>>           corresponding to iterator value and the last valid iterator value
>>>           equals to 'max_cpus' + 1
>>> +   - Get architecture specific id for a CPU
>>> +     1. pick a CPU to read from using 'CPU selector' register
>>> +     2. write 0x3 int0 'Command field' register
>>> +     3. read architecture specific id from 'Command data' register
>>
>> Looks good, except for:
>>
>> - typo: "int0"
>>
>> - in step 3, we should reference 'Command data 2' as well.
>>
>>
>> In fact, in
>> <http://mid.mail-archive.com/2b10ca48-c734-4f41-9521-136c44060812@redhat.com>,
>> I wrote, for the "Get a cpu with pending event" use case:
>>
>>> 1. Store 0x0 to the 'CPU selector' register.
>>> 2. Store 0x0 to the 'Command field' register.
>>> 3. Read the 'CPU device status fields' register.
>>> 4. If both bit#1 and bit#2 are clear in the value read, there is no
>>>    CPU with a pending event.
>>> 5. Otherwise, read the 'Command data' register. The value read is the
>>>    selector of the CPU with the pending event (which is already
>>>    selected).
>>
>> and your steps #2 and #3, for getting the arch specific ID, can be
>> directly appended as steps 6. and 7.!
>>
>>
>>> diff --git a/hw/acpi/cpu.c b/hw/acpi/cpu.c
>>> index 87f30a31d7..701542d860 100644
>>> --- a/hw/acpi/cpu.c
>>> +++ b/hw/acpi/cpu.c
>>> @@ -12,11 +12,13 @@
>>>  #define ACPI_CPU_FLAGS_OFFSET_RW 4
>>>  #define ACPI_CPU_CMD_OFFSET_WR 5
>>>  #define ACPI_CPU_CMD_DATA_OFFSET_RW 8
>>> +#define ACPI_CPU_CMD_DATA2_OFFSET_RW 0
>>>  
>>>  enum {
>>>      CPHP_GET_NEXT_CPU_WITH_EVENT_CMD = 0,
>>>      CPHP_OST_EVENT_CMD = 1,
>>>      CPHP_OST_STATUS_CMD = 2,
>>> +    CPHP_GET_CPU_ID_CMD = 3,
>>>      CPHP_CMD_MAX
>>>  };
>>>  
>>> @@ -74,11 +76,24 @@ static uint64_t cpu_hotplug_rd(void *opaque, hwaddr addr, unsigned size)
>>>          case CPHP_GET_NEXT_CPU_WITH_EVENT_CMD:
>>>             val = cpu_st->selector;
>>>             break;
>>> +        case CPHP_GET_CPU_ID_CMD:
>>> +           val = cpu_to_le64(cdev->arch_id) & 0xFFFFFFFF;
>>> +           break;
>>>          default:
>>>             break;
>>>          }
>>>          trace_cpuhp_acpi_read_cmd_data(cpu_st->selector, val);
>>>          break;
>>> +    case ACPI_CPU_CMD_DATA2_OFFSET_RW:
>>> +        switch (cpu_st->command) {
>>> +        case CPHP_GET_CPU_ID_CMD:
>>> +           val = cpu_to_le64(cdev->arch_id) >> 32;
>>> +           break;
>>> +        default:
>>> +           break;
>>> +        }
>>> +        trace_cpuhp_acpi_read_cmd_data2(cpu_st->selector, val);
>>> +        break;
>>>      default:
>>>          break;
>>>      }
>>> diff --git a/hw/acpi/trace-events b/hw/acpi/trace-events
>>> index 96b8273297..afbc77de1c 100644
>>> --- a/hw/acpi/trace-events
>>> +++ b/hw/acpi/trace-events
>>> @@ -23,6 +23,7 @@ cpuhp_acpi_read_flags(uint32_t idx, uint8_t flags) "idx[0x%"PRIx32"] flags: 0x%"
>>>  cpuhp_acpi_write_idx(uint32_t idx) "set active cpu idx: 0x%"PRIx32
>>>  cpuhp_acpi_write_cmd(uint32_t idx, uint8_t cmd) "idx[0x%"PRIx32"] cmd: 0x%"PRIx8
>>>  cpuhp_acpi_read_cmd_data(uint32_t idx, uint32_t data) "idx[0x%"PRIx32"] data: 0x%"PRIx32
>>> +cpuhp_acpi_read_cmd_data2(uint32_t idx, uint32_t data) "idx[0x%"PRIx32"] data: 0x%"PRIx32
>>>  cpuhp_acpi_cpu_has_events(uint32_t idx, bool ins, bool rm) "idx[0x%"PRIx32"] inserting: %d, removing: %d"
>>>  cpuhp_acpi_clear_inserting_evt(uint32_t idx) "idx[0x%"PRIx32"]"
>>>  cpuhp_acpi_clear_remove_evt(uint32_t idx) "idx[0x%"PRIx32"]"
>>>
>>
>> Looks plausible to me, thanks (discounting the TODO item).
>>
>> Right now, I can't offer testing for patch#3 (I'm quite far from the
>> point where I'll be actually looking for a hotplugged CPU :) ), but
>> based on the docs patches #1 and #2, and my proposed updates, I can
>> rework my "possible CPU count detection" in OVMF.
>>
>> Do I need to check in OVMF specifically whether the "modern" CPU hotplug
>> register block is available? Can you tell me what the oldest machine
>> types are that support the modern interface?
>>
>> Hmm... Commit abd49bc2ed2f ("docs: update ACPI CPU hotplug spec with new
>> protocol", 2016-06-24) seems relevant. First released in v2.7.0. I think
>> I should detect whether this interface is available.
>>
>> Can I use the following sequence to detect whether the interface is
>> available?
>>
>> 1. Store 0x0 to command register.
>> 2. Store 0x0 to selector register.
>> 3. Read 'command data' register.
>> 4. If value read is 0, the interface is available.
>>
>> (Because I assume that unmapped IO ports read as all-bits-one. Is that
>> right?)
>>
>> BTW, can I dynamically detect support for the GET_CPU_ID command too?
>> I'm thinking, when I enumerate / count all possible CPUs, I can at once
>> fetch the arch IDs for all of them. If I only get zeros from the command
>> data registers, across all CPUs, in response to GET_CPU_ID, then the
>> command is not available.
>>
>> Thanks
>> Laszlo
> 
> Laszlo, won't we need to add topology info anyway?
> if yes then this patch is just a stopgap, so let's do
> fw cfg and be done with it?

No, CPU topology is not relevant *per se* for this OVMF feature (CPU
hotplug with SMM).

CPU topology is only interesting as a building block.

What OVMF really needs, for the hotplug SMI handler, is the ability to:

- learn a QEMU-specific unique identifier for the CPU that has just been
hot-plugged,

- translate said QEMU-specific unique CPU identifier to the new CPU's
APIC-ID.

So how do we get there?

- If the hotplug register block (the hotplug hardware interface) lets me
fetch the APIC-ID for the new CPU immediately, in the hotplug SMI
handler, that's best (most convenient).

- If the QEMU-specific unique identifier fetched from the register block
is a sequential CPU index (or "selector"), then I *could* use the CPU
topology as a building block, for constructing the APIC-ID myself. I
would decompose the CPU index into thread / core / die / socket offsets,
based on the topology, and re-compose those offsets into an APIC-ID.

- If the QEMU-specific unique identifier fetched from the register block
is a sequential CPU index (or "selector"), then I could *also* use a
complete mapping table (index to APIC-ID), fetched earlier from fw_cfg.
I would use the CPU index as a subscript into that table.

What really matters for OVMF is the APIC-ID of the just-plugged CPU.

I definitely have to access the CPU hotplug register block in the SMI
handler, because that's where I can get *any kind* of identifier for the
new CPU. (Unless we want to implement a dedicated CPU hotplug controller
for OVMF, that is.)

Thanks
Laszlo


^ permalink raw reply	[flat|nested] 31+ messages in thread

* Re: [RFC 0/3] acpi: cphp: add CPHP_GET_CPU_ID_CMD command to cpu hotplug MMIO interface
  2019-10-10 15:57       ` Igor Mammedov
@ 2019-10-10 18:15         ` Michael S. Tsirkin
  2019-10-11  7:41           ` Laszlo Ersek
  2019-10-10 19:20         ` Eduardo Habkost
  2019-10-11  6:54         ` Laszlo Ersek
  2 siblings, 1 reply; 31+ messages in thread
From: Michael S. Tsirkin @ 2019-10-10 18:15 UTC (permalink / raw)
  To: Igor Mammedov
  Cc: Eduardo Habkost, Laszlo Ersek, qemu-devel, Gerd Hoffmann,
	Paolo Bonzini, Philippe Mathieu-Daudé,
	Richard Henderson

On Thu, Oct 10, 2019 at 05:57:54PM +0200, Igor Mammedov wrote:
> > Then we should consider switching acpi to use fw cfg.
> > Or build another interface that can scale.
> 
> Could be an option, it would be a pain to write a driver in AML for fwcfg access though
> (I've looked at possibility to access fwcfg from AML about a year ago and gave up.
> I'm definitely not volunteering for the second attempt and can't even give an estimate
> it it's viable approach).
> 
> But what scaling issue you are talking about, exactly?

Just this: each new thing we add is an ad-hoc data structure with
manually maintained backwards compatibility and no built-in discovery.

fw cfg has built-in discovery and we've finally managed to
handle compatibility reasonably well.

PV is already very problematic.  Spreading PV code all over the place
like this is a very bad idea.  For you CPU hotplug is something that you
keep in mind first of all, but someone bringing up a new platform
already has a steep hill to climb.  Adding tons of custom firmware is
not helping things.

-- 
MST


^ permalink raw reply	[flat|nested] 31+ messages in thread

* Re: [RFC 0/3] acpi: cphp: add CPHP_GET_CPU_ID_CMD command to cpu hotplug MMIO interface
  2019-10-10 15:57       ` Igor Mammedov
  2019-10-10 18:15         ` Michael S. Tsirkin
@ 2019-10-10 19:20         ` Eduardo Habkost
  2019-10-11  8:01           ` Laszlo Ersek
  2019-10-11 10:47           ` Igor Mammedov
  2019-10-11  6:54         ` Laszlo Ersek
  2 siblings, 2 replies; 31+ messages in thread
From: Eduardo Habkost @ 2019-10-10 19:20 UTC (permalink / raw)
  To: Igor Mammedov
  Cc: Michael S. Tsirkin, Laszlo Ersek, qemu-devel, Gerd Hoffmann,
	Paolo Bonzini, Philippe Mathieu-Daudé,
	Richard Henderson

On Thu, Oct 10, 2019 at 05:57:54PM +0200, Igor Mammedov wrote:
> On Thu, 10 Oct 2019 09:59:42 -0400
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > On Thu, Oct 10, 2019 at 03:39:12PM +0200, Igor Mammedov wrote:
> > > On Thu, 10 Oct 2019 05:56:55 -0400
> > > "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > > 
> > > > On Wed, Oct 09, 2019 at 09:22:49AM -0400, Igor Mammedov wrote:
> > > > > As an alternative to passing to firmware topology info via new fwcfg files
> > > > > so it could recreate APIC IDs based on it and order CPUs are enumerated,
> > > > > 
> > > > > extend CPU hotplug interface to return APIC ID as response to the new command
> > > > > CPHP_GET_CPU_ID_CMD.  
> > > > 
> > > > One big piece missing here is motivation:
> > > I thought the only willing reader was Laszlo (who is aware of context)
> > > so I skipped on details and confused others :/
> > > 
> > > > Who's going to use this interface?
> > > In current state it's for firmware, since ACPI tables can cheat
> > > by having APIC IDs statically built in.
> > > 
> > > If we were creating CPU objects in ACPI dynamically
> > > we would be using this command as well.
> > 
> > I'm not sure how it's even possible to create devices dynamically. Well
> > I guess it's possible with LoadTable. Is this what you had in
> > mind?
> 
> Yep. I even played this shiny toy and I can say it's very tempting one.
> On the  other side, even problem of legacy OSes not working with it aside,
> it's hard to debug and reproduce compared to static tables.
> So from maintaining pov I dislike it enough to be against it.
> 
> 
> > > It would save
> > > us quite a bit space in ACPI blob but it would be a pain
> > > to debug and diagnose problems in ACPI tables, so I'd rather
> > > stay with static CPU descriptions in ACPI tables for the sake
> > > of maintenance.
> > > > So far CPU hotplug was used by the ACPI, so we didn't
> > > > really commit to a fixed interface too strongly.
> > > > 
> > > > Is this a replacement to Laszlo's fw cfg interface?
> > > > If yes is the idea that OVMF going to depend on CPU hotplug directly then?
> > > > It does not depend on it now, does it?
> > > It doesn't, but then it doesn't support cpu hotplug,
> > > OVMF(SMM) needs to cooperate with QEMU "and" ACPI tables to perform
> > > the task and using the same interface/code path between all involved
> > > parties makes the task easier with the least amount of duplicated
> > > interfaces and more robust.
> > > 
> > > Re-implementing alternative interface for firmware (fwcfg or what not)
> > > would work as well, but it's only question of time when ACPI and
> > > this new interface disagree on how world works and process falls
> > > apart.
> > 
> > Then we should consider switching acpi to use fw cfg.
> > Or build another interface that can scale.
> 
> Could be an option, it would be a pain to write a driver in AML for fwcfg access though
> (I've looked at possibility to access fwcfg from AML about a year ago and gave up.
> I'm definitely not volunteering for the second attempt and can't even give an estimate
> it it's viable approach).
> 
> But what scaling issue you are talking about, exactly?
> With current CPU hotplug interface we can handle upto UNIT32_MAX cpus, and extend
> interface without need to increase IO window we are using now.
> 
> Granted IO access it not fastest compared to fwcfg in DMA mode, but we already
> doing stop machine when switching to SMM which is orders of magnitude slower.
> Consensus was to compromise on speed of CPU hotplug versus more complex and more
> problematic unicast SMM mode in OVMF (can't find a particular email but we have discussed
> it with Laszlo already, when I considered ways to optimize hotplug speed)

If we were designing the interface from the ground up, I would
agree with Michael.  But I don't see why we would reimplement
everything from scratch now, if just providing the
cpu_selector => cpu_hardware_id mapping to firmware is enough to
make the existing interface work.

If somebody is really unhappy with the current interface and
wants to implement a new purely fw_cfg-based one (and write the
corresponding ACPI code), they would be welcome.  I just don't
see why we should spend our time doing that now.

-- 
Eduardo


^ permalink raw reply	[flat|nested] 31+ messages in thread

* Re: [RFC 3/3] acpi: cpuhp: add CPHP_GET_CPU_ID_CMD command
  2019-10-10 15:06     ` Michael S. Tsirkin
  2019-10-10 17:23       ` Igor Mammedov
  2019-10-10 17:53       ` Laszlo Ersek
@ 2019-10-10 19:26       ` Eduardo Habkost
  2019-10-11  8:07         ` Laszlo Ersek
  2 siblings, 1 reply; 31+ messages in thread
From: Eduardo Habkost @ 2019-10-10 19:26 UTC (permalink / raw)
  To: Michael S. Tsirkin
  Cc: Laszlo Ersek, qemu-devel, Gerd Hoffmann, Paolo Bonzini,
	Igor Mammedov, Philippe Mathieu-Daudé,
	Richard Henderson

On Thu, Oct 10, 2019 at 11:06:29AM -0400, Michael S. Tsirkin wrote:
> On Thu, Oct 10, 2019 at 04:56:18PM +0200, Laszlo Ersek wrote:
> > On 10/09/19 15:22, Igor Mammedov wrote:
> > > Extend CPU hotplug interface to return architecture specific
> > > identifier for current CPU (in case of x86, it's APIC ID).
> > > 
> > > Signed-off-by: Igor Mammedov <imammedo@redhat.com>
> > > ---
> > > TODO:
> > >   * cripple it to behave old way on old machine types so that
> > >     new firmware started on new QEMU won't see a difference
> > >     when migrated to an old QEMU (i.e. QEMU that doesn't support
> > >     this command)
> > > ---
> > >  docs/specs/acpi_cpu_hotplug.txt | 10 +++++++++-
> > >  hw/acpi/cpu.c                   | 15 +++++++++++++++
> > >  hw/acpi/trace-events            |  1 +
> > >  3 files changed, 25 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/docs/specs/acpi_cpu_hotplug.txt b/docs/specs/acpi_cpu_hotplug.txt
> > > index 43c5a193f0..0438678249 100644
> > > --- a/docs/specs/acpi_cpu_hotplug.txt
> > > +++ b/docs/specs/acpi_cpu_hotplug.txt
> > > @@ -32,7 +32,9 @@ Register block size:
> > >  
> > >  read access:
> > >      offset:
> > > -    [0x0-0x3] reserved
> > > +    [0x0-0x3] Command data 2: (DWORD access)
> > > +              upper 32 bit of 'Command data' if returned data value is 64 bit.
> > > +              in case of error or unsupported command reads is 0x0
> > 
> > How about
> > 
> >     [0x0] Command data 2: (DWORD access, little endian)
> >           If the "CPU selector" value last stored by the guest refers to
> >           an impossible CPU, then 0.
> >           Otherwise, if the "Command field" value last stored by the
> >           guest differs from 3, then 0.
> >           Otherwise, the most significant 32 bits of the selected CPU's
> >           architecture specific ID.
> > 
> >     [0x8] Command data: (DWORD access, little endian)
> >           If the "CPU selector" value last stored by the guest refers to
> >           an impossible CPU, then 0.
> >           Otherwise,
> >           - if the "Command field" value last stored by the guest is 0,
> >             then the selector of the currently selected CPU;
> >           - if the "Command field" value last stored by the guest is 3,
> >             then the least significant 32 bits of the selected CPU's
> >             architecture specific ID;
> >           - otherwise, 0.
> > 
> > >      [0x4] CPU device status fields: (1 byte access)
> > >          bits:
> > >             0: Device is enabled and may be used by guest
> > > @@ -87,6 +89,8 @@ write access:
> > >                2: stores value into OST status register, triggers
> > >                   ACPI_DEVICE_OST QMP event from QEMU to external applications
> > >                   with current values of OST event and status registers.
> > > +              3: OSPM reads architecture specific value identifying CPU
> > > +                 (x86: APIC ID)
> > >              other values: reserved
> > >  
> > 
> > Seems OK.
> > 
> > >  Selecting CPU device beyond possible range has no effect on platform:
> > > @@ -115,3 +119,7 @@ Typical usecases:
> > >       5.2 if 'Command data' register has not changed, there is not CPU
> > >           corresponding to iterator value and the last valid iterator value
> > >           equals to 'max_cpus' + 1
> > > +   - Get architecture specific id for a CPU
> > > +     1. pick a CPU to read from using 'CPU selector' register
> > > +     2. write 0x3 int0 'Command field' register
> > > +     3. read architecture specific id from 'Command data' register
> > 
> > Looks good, except for:
> > 
> > - typo: "int0"
> > 
> > - in step 3, we should reference 'Command data 2' as well.
> > 
> > 
> > In fact, in
> > <http://mid.mail-archive.com/2b10ca48-c734-4f41-9521-136c44060812@redhat.com>,
> > I wrote, for the "Get a cpu with pending event" use case:
> > 
> > > 1. Store 0x0 to the 'CPU selector' register.
> > > 2. Store 0x0 to the 'Command field' register.
> > > 3. Read the 'CPU device status fields' register.
> > > 4. If both bit#1 and bit#2 are clear in the value read, there is no
> > >    CPU with a pending event.
> > > 5. Otherwise, read the 'Command data' register. The value read is the
> > >    selector of the CPU with the pending event (which is already
> > >    selected).
> > 
> > and your steps #2 and #3, for getting the arch specific ID, can be
> > directly appended as steps 6. and 7.!
> > 
> > 
> > > diff --git a/hw/acpi/cpu.c b/hw/acpi/cpu.c
> > > index 87f30a31d7..701542d860 100644
> > > --- a/hw/acpi/cpu.c
> > > +++ b/hw/acpi/cpu.c
> > > @@ -12,11 +12,13 @@
> > >  #define ACPI_CPU_FLAGS_OFFSET_RW 4
> > >  #define ACPI_CPU_CMD_OFFSET_WR 5
> > >  #define ACPI_CPU_CMD_DATA_OFFSET_RW 8
> > > +#define ACPI_CPU_CMD_DATA2_OFFSET_RW 0
> > >  
> > >  enum {
> > >      CPHP_GET_NEXT_CPU_WITH_EVENT_CMD = 0,
> > >      CPHP_OST_EVENT_CMD = 1,
> > >      CPHP_OST_STATUS_CMD = 2,
> > > +    CPHP_GET_CPU_ID_CMD = 3,
> > >      CPHP_CMD_MAX
> > >  };
> > >  
> > > @@ -74,11 +76,24 @@ static uint64_t cpu_hotplug_rd(void *opaque, hwaddr addr, unsigned size)
> > >          case CPHP_GET_NEXT_CPU_WITH_EVENT_CMD:
> > >             val = cpu_st->selector;
> > >             break;
> > > +        case CPHP_GET_CPU_ID_CMD:
> > > +           val = cpu_to_le64(cdev->arch_id) & 0xFFFFFFFF;
> > > +           break;
> > >          default:
> > >             break;
> > >          }
> > >          trace_cpuhp_acpi_read_cmd_data(cpu_st->selector, val);
> > >          break;
> > > +    case ACPI_CPU_CMD_DATA2_OFFSET_RW:
> > > +        switch (cpu_st->command) {
> > > +        case CPHP_GET_CPU_ID_CMD:
> > > +           val = cpu_to_le64(cdev->arch_id) >> 32;
> > > +           break;
> > > +        default:
> > > +           break;
> > > +        }
> > > +        trace_cpuhp_acpi_read_cmd_data2(cpu_st->selector, val);
> > > +        break;
> > >      default:
> > >          break;
> > >      }
> > > diff --git a/hw/acpi/trace-events b/hw/acpi/trace-events
> > > index 96b8273297..afbc77de1c 100644
> > > --- a/hw/acpi/trace-events
> > > +++ b/hw/acpi/trace-events
> > > @@ -23,6 +23,7 @@ cpuhp_acpi_read_flags(uint32_t idx, uint8_t flags) "idx[0x%"PRIx32"] flags: 0x%"
> > >  cpuhp_acpi_write_idx(uint32_t idx) "set active cpu idx: 0x%"PRIx32
> > >  cpuhp_acpi_write_cmd(uint32_t idx, uint8_t cmd) "idx[0x%"PRIx32"] cmd: 0x%"PRIx8
> > >  cpuhp_acpi_read_cmd_data(uint32_t idx, uint32_t data) "idx[0x%"PRIx32"] data: 0x%"PRIx32
> > > +cpuhp_acpi_read_cmd_data2(uint32_t idx, uint32_t data) "idx[0x%"PRIx32"] data: 0x%"PRIx32
> > >  cpuhp_acpi_cpu_has_events(uint32_t idx, bool ins, bool rm) "idx[0x%"PRIx32"] inserting: %d, removing: %d"
> > >  cpuhp_acpi_clear_inserting_evt(uint32_t idx) "idx[0x%"PRIx32"]"
> > >  cpuhp_acpi_clear_remove_evt(uint32_t idx) "idx[0x%"PRIx32"]"
> > > 
> > 
> > Looks plausible to me, thanks (discounting the TODO item).
> > 
> > Right now, I can't offer testing for patch#3 (I'm quite far from the
> > point where I'll be actually looking for a hotplugged CPU :) ), but
> > based on the docs patches #1 and #2, and my proposed updates, I can
> > rework my "possible CPU count detection" in OVMF.
> > 
> > Do I need to check in OVMF specifically whether the "modern" CPU hotplug
> > register block is available? Can you tell me what the oldest machine
> > types are that support the modern interface?
> > 
> > Hmm... Commit abd49bc2ed2f ("docs: update ACPI CPU hotplug spec with new
> > protocol", 2016-06-24) seems relevant. First released in v2.7.0. I think
> > I should detect whether this interface is available.
> > 
> > Can I use the following sequence to detect whether the interface is
> > available?
> > 
> > 1. Store 0x0 to command register.
> > 2. Store 0x0 to selector register.
> > 3. Read 'command data' register.
> > 4. If value read is 0, the interface is available.
> > 
> > (Because I assume that unmapped IO ports read as all-bits-one. Is that
> > right?)
> > 
> > BTW, can I dynamically detect support for the GET_CPU_ID command too?
> > I'm thinking, when I enumerate / count all possible CPUs, I can at once
> > fetch the arch IDs for all of them. If I only get zeros from the command
> > data registers, across all CPUs, in response to GET_CPU_ID, then the
> > command is not available.
> > 
> > Thanks
> > Laszlo
> 
> Laszlo, won't we need to add topology info anyway?
> if yes then this patch is just a stopgap, so let's do
> fw cfg and be done with it?

Topology info is already available on CPUID.

-- 
Eduardo


^ permalink raw reply	[flat|nested] 31+ messages in thread

* Re: [RFC 0/3] acpi: cphp: add CPHP_GET_CPU_ID_CMD command to cpu hotplug MMIO interface
  2019-10-10 15:57       ` Igor Mammedov
  2019-10-10 18:15         ` Michael S. Tsirkin
  2019-10-10 19:20         ` Eduardo Habkost
@ 2019-10-11  6:54         ` Laszlo Ersek
  2 siblings, 0 replies; 31+ messages in thread
From: Laszlo Ersek @ 2019-10-11  6:54 UTC (permalink / raw)
  To: Igor Mammedov, Michael S. Tsirkin
  Cc: Eduardo Habkost, qemu-devel, Gerd Hoffmann, Paolo Bonzini,
	Philippe Mathieu-Daudé,
	Richard Henderson

On 10/10/19 17:57, Igor Mammedov wrote:
> On Thu, 10 Oct 2019 09:59:42 -0400
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
>> On Thu, Oct 10, 2019 at 03:39:12PM +0200, Igor Mammedov wrote:
>>> On Thu, 10 Oct 2019 05:56:55 -0400
>>> "Michael S. Tsirkin" <mst@redhat.com> wrote:
>>>
>>>> On Wed, Oct 09, 2019 at 09:22:49AM -0400, Igor Mammedov wrote:
>>>>> As an alternative to passing to firmware topology info via new fwcfg files
>>>>> so it could recreate APIC IDs based on it and order CPUs are enumerated,
>>>>>
>>>>> extend CPU hotplug interface to return APIC ID as response to the new command
>>>>> CPHP_GET_CPU_ID_CMD.  
>>>>
>>>> One big piece missing here is motivation:
>>> I thought the only willing reader was Laszlo (who is aware of context)
>>> so I skipped on details and confused others :/
>>>
>>>> Who's going to use this interface?
>>> In current state it's for firmware, since ACPI tables can cheat
>>> by having APIC IDs statically built in.
>>>
>>> If we were creating CPU objects in ACPI dynamically
>>> we would be using this command as well.
>>
>> I'm not sure how it's even possible to create devices dynamically. Well
>> I guess it's possible with LoadTable. Is this what you had in
>> mind?
> 
> Yep. I even played this shiny toy and I can say it's very tempting one.
> On the  other side, even problem of legacy OSes not working with it aside,
> it's hard to debug and reproduce compared to static tables.
> So from maintaining pov I dislike it enough to be against it.
> 
> 
>>> It would save
>>> us quite a bit space in ACPI blob but it would be a pain
>>> to debug and diagnose problems in ACPI tables, so I'd rather
>>> stay with static CPU descriptions in ACPI tables for the sake
>>> of maintenance.
>>>> So far CPU hotplug was used by the ACPI, so we didn't
>>>> really commit to a fixed interface too strongly.
>>>>
>>>> Is this a replacement to Laszlo's fw cfg interface?
>>>> If yes is the idea that OVMF going to depend on CPU hotplug directly then?
>>>> It does not depend on it now, does it?
>>> It doesn't, but then it doesn't support cpu hotplug,
>>> OVMF(SMM) needs to cooperate with QEMU "and" ACPI tables to perform
>>> the task and using the same interface/code path between all involved
>>> parties makes the task easier with the least amount of duplicated
>>> interfaces and more robust.
>>>
>>> Re-implementing alternative interface for firmware (fwcfg or what not)
>>> would work as well, but it's only question of time when ACPI and
>>> this new interface disagree on how world works and process falls
>>> apart.
>>
>> Then we should consider switching acpi to use fw cfg.
>> Or build another interface that can scale.
> 
> Could be an option, it would be a pain to write a driver in AML for fwcfg access though
> (I've looked at possibility to access fwcfg from AML about a year ago and gave up.
> I'm definitely not volunteering for the second attempt and can't even give an estimate
> it it's viable approach).
> 
> But what scaling issue you are talking about, exactly?
> With current CPU hotplug interface we can handle upto UNIT32_MAX cpus, and extend
> interface without need to increase IO window we are using now.
> 
> Granted IO access it not fastest compared to fwcfg in DMA mode, but we already
> doing stop machine when switching to SMM which is orders of magnitude slower.
> Consensus was to compromise on speed of CPU hotplug versus more complex and more
> problematic unicast SMM mode in OVMF (can't find a particular email but we have discussed
> it with Laszlo already, when I considered ways to optimize hotplug speed)

Right, the speed of handling a CPU hotplug event is basically
irrelevant, whereas broadcast SMI (in response to writing IO port 0xB2)
is really important.

Thanks
Laszlo


^ permalink raw reply	[flat|nested] 31+ messages in thread

* Re: [RFC 0/3] acpi: cphp: add CPHP_GET_CPU_ID_CMD command to cpu hotplug MMIO interface
  2019-10-10 18:15         ` Michael S. Tsirkin
@ 2019-10-11  7:41           ` Laszlo Ersek
  0 siblings, 0 replies; 31+ messages in thread
From: Laszlo Ersek @ 2019-10-11  7:41 UTC (permalink / raw)
  To: Michael S. Tsirkin, Igor Mammedov
  Cc: Eduardo Habkost, qemu-devel, Gerd Hoffmann, Paolo Bonzini,
	Philippe Mathieu-Daudé,
	Richard Henderson

On 10/10/19 20:15, Michael S. Tsirkin wrote:
> On Thu, Oct 10, 2019 at 05:57:54PM +0200, Igor Mammedov wrote:
>>> Then we should consider switching acpi to use fw cfg.
>>> Or build another interface that can scale.
>>
>> Could be an option, it would be a pain to write a driver in AML for fwcfg access though
>> (I've looked at possibility to access fwcfg from AML about a year ago and gave up.
>> I'm definitely not volunteering for the second attempt and can't even give an estimate
>> it it's viable approach).
>>
>> But what scaling issue you are talking about, exactly?
> 
> Just this: each new thing we add is an ad-hoc data structure with
> manually maintained backwards compatibility and no built-in discovery.
> 
> fw cfg has built-in discovery and we've finally managed to
> handle compatibility reasonably well.
> 
> PV is already very problematic.  Spreading PV code all over the place
> like this is a very bad idea.  For you CPU hotplug is something that you
> keep in mind first of all, but someone bringing up a new platform
> already has a steep hill to climb.  Adding tons of custom firmware is
> not helping things.
> 

- Custom firmware is unfortunately unavoidable in this case. SMM is a
privilege barrier between the firmware and the OS. The firmware needs to
care because to OS could use a hotplugged CPU to attack SMM, unless
hardware and firmware co-operate to prevent that. This whole ordeal aims
to prevent such an attack.

If you remove SMM from the picture (build OVMF without it), there is no
such privilege barrier between fw and OS, the firmware doesn't need to
know anything about VCPU hotplug events, and hotplug already works.


- Custom hardware is also expected. On physical platforms, there is a
dedicated BMC / RAS thingy that e.g. provides the APIC-ID to firmware,
when a CPU is hotplugged. One factor that makes this feature difficult
is that edk2 does not contain any code driving such hardware; it only
contains firmware code for *consuming* what the BMC / RAS thingy
produces. We're trying to work backwards from that. We could implement a
brand new hotplug controller device model, but it would be entirely PV
(= it would be our own design). Reusing the CPU hotplug register block
isn't worse, in that regard. This is my understanding anyway.


- Using fw_cfg *DMA* in an SMI handler (at OS runtime) could be
problematic. Especially if you consider SEV guests. For DMA transfers,
memory has to be decrypted and encrypted, which involves page table
manipulation.

In the firmware, this is handled with a SEV-specific
EDKII_IOMMU_PROTOCOL implementation. The fw_cfg library (transparently
to the caller) utilizes the IOMMU protocol, for dealing with DMA in SEV
guests.

Such protocols are boot-time only; they are not runtime protocols, let
alone SMM protocols. They are not available to SMI handlers.

Raw IO ports are extremely attractive in this regard: they don't depend
on page tables (SMM has its own set of page tables), they just work
under SEV out of the box. (The only exception is [REP] INS*/OUTS*, as
those depend on memory, not just registers, but no such instructions are
expected for the hotplug register block.)

- IO-port based fw_cfg might be suitable for the hotplug SMI handler,
yes. Assuming it's really only the firmware that needs information from
QEMU in the hotplug SMI handler, and not the other way around. (IO-port
based fw_cfg doesn't support writes.)

Now, when considering IO-port based fw_cfg in the SMI handler, we should
at least mention the risk of interfering with the Linux guest's own
fw_cfg driver. I've *never* been happy with anything non-firmware
accessing fw_cfg (it says "*firmware* config" in the name!), so I
wouldn't care much in practice. It's just that we should be aware that
there is a chance for entanglement. (The SMI handler cannot restore the
previous fw_cfg state, when it finishes up.)

At least on a theoretical level, the ACPI CPU hotplug register block is
nicer in this regard: the OS doesn't know about it at all (it is
accessed only through QEMU-generated AML code), and the plan is for the
same AML code to closely coordinate with the firmware. Namely, the AML
would raise the hotplug SMI.

Thanks
Laszlo


^ permalink raw reply	[flat|nested] 31+ messages in thread

* Re: [RFC 0/3] acpi: cphp: add CPHP_GET_CPU_ID_CMD command to cpu hotplug MMIO interface
  2019-10-10 19:20         ` Eduardo Habkost
@ 2019-10-11  8:01           ` Laszlo Ersek
  2019-10-11 13:00             ` Michael S. Tsirkin
  2019-10-11 10:47           ` Igor Mammedov
  1 sibling, 1 reply; 31+ messages in thread
From: Laszlo Ersek @ 2019-10-11  8:01 UTC (permalink / raw)
  To: Eduardo Habkost, Igor Mammedov
  Cc: Michael S. Tsirkin, qemu-devel, Gerd Hoffmann, Paolo Bonzini,
	Philippe Mathieu-Daudé,
	Richard Henderson

On 10/10/19 21:20, Eduardo Habkost wrote:
> On Thu, Oct 10, 2019 at 05:57:54PM +0200, Igor Mammedov wrote:
>> On Thu, 10 Oct 2019 09:59:42 -0400
>> "Michael S. Tsirkin" <mst@redhat.com> wrote:
>>
>>> On Thu, Oct 10, 2019 at 03:39:12PM +0200, Igor Mammedov wrote:
>>>> On Thu, 10 Oct 2019 05:56:55 -0400
>>>> "Michael S. Tsirkin" <mst@redhat.com> wrote:
>>>>
>>>>> On Wed, Oct 09, 2019 at 09:22:49AM -0400, Igor Mammedov wrote:
>>>>>> As an alternative to passing to firmware topology info via new fwcfg files
>>>>>> so it could recreate APIC IDs based on it and order CPUs are enumerated,
>>>>>>
>>>>>> extend CPU hotplug interface to return APIC ID as response to the new command
>>>>>> CPHP_GET_CPU_ID_CMD.  
>>>>>
>>>>> One big piece missing here is motivation:
>>>> I thought the only willing reader was Laszlo (who is aware of context)
>>>> so I skipped on details and confused others :/
>>>>
>>>>> Who's going to use this interface?
>>>> In current state it's for firmware, since ACPI tables can cheat
>>>> by having APIC IDs statically built in.
>>>>
>>>> If we were creating CPU objects in ACPI dynamically
>>>> we would be using this command as well.
>>>
>>> I'm not sure how it's even possible to create devices dynamically. Well
>>> I guess it's possible with LoadTable. Is this what you had in
>>> mind?
>>
>> Yep. I even played this shiny toy and I can say it's very tempting one.
>> On the  other side, even problem of legacy OSes not working with it aside,
>> it's hard to debug and reproduce compared to static tables.
>> So from maintaining pov I dislike it enough to be against it.
>>
>>
>>>> It would save
>>>> us quite a bit space in ACPI blob but it would be a pain
>>>> to debug and diagnose problems in ACPI tables, so I'd rather
>>>> stay with static CPU descriptions in ACPI tables for the sake
>>>> of maintenance.
>>>>> So far CPU hotplug was used by the ACPI, so we didn't
>>>>> really commit to a fixed interface too strongly.
>>>>>
>>>>> Is this a replacement to Laszlo's fw cfg interface?
>>>>> If yes is the idea that OVMF going to depend on CPU hotplug directly then?
>>>>> It does not depend on it now, does it?
>>>> It doesn't, but then it doesn't support cpu hotplug,
>>>> OVMF(SMM) needs to cooperate with QEMU "and" ACPI tables to perform
>>>> the task and using the same interface/code path between all involved
>>>> parties makes the task easier with the least amount of duplicated
>>>> interfaces and more robust.
>>>>
>>>> Re-implementing alternative interface for firmware (fwcfg or what not)
>>>> would work as well, but it's only question of time when ACPI and
>>>> this new interface disagree on how world works and process falls
>>>> apart.
>>>
>>> Then we should consider switching acpi to use fw cfg.
>>> Or build another interface that can scale.
>>
>> Could be an option, it would be a pain to write a driver in AML for fwcfg access though
>> (I've looked at possibility to access fwcfg from AML about a year ago and gave up.
>> I'm definitely not volunteering for the second attempt and can't even give an estimate
>> it it's viable approach).
>>
>> But what scaling issue you are talking about, exactly?
>> With current CPU hotplug interface we can handle upto UNIT32_MAX cpus, and extend
>> interface without need to increase IO window we are using now.
>>
>> Granted IO access it not fastest compared to fwcfg in DMA mode, but we already
>> doing stop machine when switching to SMM which is orders of magnitude slower.
>> Consensus was to compromise on speed of CPU hotplug versus more complex and more
>> problematic unicast SMM mode in OVMF (can't find a particular email but we have discussed
>> it with Laszlo already, when I considered ways to optimize hotplug speed)
> 
> If we were designing the interface from the ground up, I would
> agree with Michael.  But I don't see why we would reimplement
> everything from scratch now, if just providing the
> cpu_selector => cpu_hardware_id mapping to firmware is enough to
> make the existing interface work.
> 
> If somebody is really unhappy with the current interface and
> wants to implement a new purely fw_cfg-based one (and write the
> corresponding ACPI code), they would be welcome.

Let me re-iterate the difficulties quickly:

- DMA-based fw_cfg is troublesome in SEV guests (do you want to mess
with page table entries in AML methods? or pre-allocate an always
decrypted opregion? how large?)

- IO port based fw_cfg does not support writes (and I reckon that, when
the *OS* handles a hotplug event, it does have to talk back to QEMU)

- the CPU hotplug AML would have to arbitrate with Linux's own fw_cfg
driver (which exposes fw_cfg files to userspace, yay! /s)

In the phys world, CPU hotplug takes dedicated RAS hardware. Shoehorning
CPU hotplug into *firmware* config, when in two use cases [*], the
firmware shouldn't even know about CPU hotplug, feels messy.

[*] being (a) SeaBIOS, and (b) OVMF built without SMM

> I just don't see why we should spend our time doing that now.

I have to agree, we're already spread thin.

... I must admit: I didn't expect this, but now I've grown to *prefer*
the CPU hotplug register block!

Laszlo


^ permalink raw reply	[flat|nested] 31+ messages in thread

* Re: [RFC 3/3] acpi: cpuhp: add CPHP_GET_CPU_ID_CMD command
  2019-10-10 19:26       ` Eduardo Habkost
@ 2019-10-11  8:07         ` Laszlo Ersek
  0 siblings, 0 replies; 31+ messages in thread
From: Laszlo Ersek @ 2019-10-11  8:07 UTC (permalink / raw)
  To: Eduardo Habkost, Michael S. Tsirkin
  Cc: qemu-devel, Gerd Hoffmann, Paolo Bonzini, Igor Mammedov,
	Philippe Mathieu-Daudé,
	Richard Henderson

On 10/10/19 21:26, Eduardo Habkost wrote:

> Topology info is already available on CPUID.

Independently of everything else, thanks for pointing this out.

The edk2 library called "LocalApicLib" has two relevant functions:

> /**
>   Get Package ID/Core ID/Thread ID of a processor.
>
>   The algorithm assumes the target system has symmetry across physical
>   package  boundaries with respect to the number of logical processors
>   per package,  number of cores per package.
>
>   @param[in]  InitialApicId  Initial APIC ID of the target logical processor.
>   @param[out]  Package       Returns the processor package ID.
>   @param[out]  Core          Returns the processor core ID.
>   @param[out]  Thread        Returns the processor thread ID.
> **/
> VOID
> EFIAPI
> GetProcessorLocationByApicId (
>   IN  UINT32  InitialApicId,
>   OUT UINT32  *Package  OPTIONAL,
>   OUT UINT32  *Core    OPTIONAL,
>   OUT UINT32  *Thread  OPTIONAL
>   );
>
> /**
>   Get Package ID/Module ID/Tile ID/Die ID/Core ID/Thread ID of a processor.
>
>   The algorithm assumes the target system has symmetry across physical
>   package boundaries with respect to the number of threads per core, number of
>   cores per module, number of modules per tile, number of tiles per die, number
>   of dies per package.
>
>   @param[in]   InitialApicId Initial APIC ID of the target logical processor.
>   @param[out]  Package       Returns the processor package ID.
>   @param[out]  Die           Returns the processor die ID.
>   @param[out]  Tile          Returns the processor tile ID.
>   @param[out]  Module        Returns the processor module ID.
>   @param[out]  Core          Returns the processor core ID.
>   @param[out]  Thread        Returns the processor thread ID.
> **/
> VOID
> EFIAPI
> GetProcessorLocation2ByApicId (
>   IN  UINT32  InitialApicId,
>   OUT UINT32  *Package  OPTIONAL,
>   OUT UINT32  *Die      OPTIONAL,
>   OUT UINT32  *Tile     OPTIONAL,
>   OUT UINT32  *Module   OPTIONAL,
>   OUT UINT32  *Core     OPTIONAL,
>   OUT UINT32  *Thread   OPTIONAL
>   );

They are implemented with heavy CPUID usage.

So... just give me the APIC-ID. That's the primary key in edk2 for identifying
x86 processors.

Thanks
Laszlo


^ permalink raw reply	[flat|nested] 31+ messages in thread

* Re: [RFC 0/3] acpi: cphp: add CPHP_GET_CPU_ID_CMD command to cpu hotplug MMIO interface
  2019-10-10 19:20         ` Eduardo Habkost
  2019-10-11  8:01           ` Laszlo Ersek
@ 2019-10-11 10:47           ` Igor Mammedov
  1 sibling, 0 replies; 31+ messages in thread
From: Igor Mammedov @ 2019-10-11 10:47 UTC (permalink / raw)
  To: Eduardo Habkost
  Cc: Michael S. Tsirkin, Laszlo Ersek, qemu-devel, Gerd Hoffmann,
	Paolo Bonzini, Philippe Mathieu-Daudé,
	Richard Henderson

On Thu, 10 Oct 2019 16:20:39 -0300
Eduardo Habkost <ehabkost@redhat.com> wrote:

> On Thu, Oct 10, 2019 at 05:57:54PM +0200, Igor Mammedov wrote:
> > On Thu, 10 Oct 2019 09:59:42 -0400
> > "Michael S. Tsirkin" <mst@redhat.com> wrote:
> >   
> > > On Thu, Oct 10, 2019 at 03:39:12PM +0200, Igor Mammedov wrote:  
> > > > On Thu, 10 Oct 2019 05:56:55 -0400
> > > > "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > > >   
> > > > > On Wed, Oct 09, 2019 at 09:22:49AM -0400, Igor Mammedov wrote:  
> > > > > > As an alternative to passing to firmware topology info via new fwcfg files
> > > > > > so it could recreate APIC IDs based on it and order CPUs are enumerated,
> > > > > > 
> > > > > > extend CPU hotplug interface to return APIC ID as response to the new command
> > > > > > CPHP_GET_CPU_ID_CMD.    
> > > > > 
> > > > > One big piece missing here is motivation:  
> > > > I thought the only willing reader was Laszlo (who is aware of context)
> > > > so I skipped on details and confused others :/
> > > >   
> > > > > Who's going to use this interface?  
> > > > In current state it's for firmware, since ACPI tables can cheat
> > > > by having APIC IDs statically built in.
> > > > 
> > > > If we were creating CPU objects in ACPI dynamically
> > > > we would be using this command as well.  
> > > 
> > > I'm not sure how it's even possible to create devices dynamically. Well
> > > I guess it's possible with LoadTable. Is this what you had in
> > > mind?  
> > 
> > Yep. I even played this shiny toy and I can say it's very tempting one.
> > On the  other side, even problem of legacy OSes not working with it aside,
> > it's hard to debug and reproduce compared to static tables.
> > So from maintaining pov I dislike it enough to be against it.
> > 
> >   
> > > > It would save
> > > > us quite a bit space in ACPI blob but it would be a pain
> > > > to debug and diagnose problems in ACPI tables, so I'd rather
> > > > stay with static CPU descriptions in ACPI tables for the sake
> > > > of maintenance.  
> > > > > So far CPU hotplug was used by the ACPI, so we didn't
> > > > > really commit to a fixed interface too strongly.
> > > > > 
> > > > > Is this a replacement to Laszlo's fw cfg interface?
> > > > > If yes is the idea that OVMF going to depend on CPU hotplug directly then?
> > > > > It does not depend on it now, does it?  
> > > > It doesn't, but then it doesn't support cpu hotplug,
> > > > OVMF(SMM) needs to cooperate with QEMU "and" ACPI tables to perform
> > > > the task and using the same interface/code path between all involved
> > > > parties makes the task easier with the least amount of duplicated
> > > > interfaces and more robust.
> > > > 
> > > > Re-implementing alternative interface for firmware (fwcfg or what not)
> > > > would work as well, but it's only question of time when ACPI and
> > > > this new interface disagree on how world works and process falls
> > > > apart.  
> > > 
> > > Then we should consider switching acpi to use fw cfg.
> > > Or build another interface that can scale.  
> > 
> > Could be an option, it would be a pain to write a driver in AML for fwcfg access though
> > (I've looked at possibility to access fwcfg from AML about a year ago and gave up.
> > I'm definitely not volunteering for the second attempt and can't even give an estimate
> > it it's viable approach).
> > 
> > But what scaling issue you are talking about, exactly?
> > With current CPU hotplug interface we can handle upto UNIT32_MAX cpus, and extend
> > interface without need to increase IO window we are using now.
> > 
> > Granted IO access it not fastest compared to fwcfg in DMA mode, but we already
> > doing stop machine when switching to SMM which is orders of magnitude slower.
> > Consensus was to compromise on speed of CPU hotplug versus more complex and more
> > problematic unicast SMM mode in OVMF (can't find a particular email but we have discussed
> > it with Laszlo already, when I considered ways to optimize hotplug speed)  
> 
> If we were designing the interface from the ground up, I would
> agree with Michael.  But I don't see why we would reimplement
> everything from scratch now, if just providing the
> cpu_selector => cpu_hardware_id mapping to firmware is enough to
> make the existing interface work.
> 
> If somebody is really unhappy with the current interface and
> wants to implement a new purely fw_cfg-based one (and write the
> corresponding ACPI code), they would be welcome.  I just don't
> see why we should spend our time doing that now.

Right, we can give fwcfg a shot next time we try to allocate
new register block for a new PV interface, assuming it suits
interface requirements.


^ permalink raw reply	[flat|nested] 31+ messages in thread

* Re: [RFC 0/3] acpi: cphp: add CPHP_GET_CPU_ID_CMD command to cpu hotplug MMIO interface
  2019-10-11  8:01           ` Laszlo Ersek
@ 2019-10-11 13:00             ` Michael S. Tsirkin
  2019-10-11 16:13               ` Laszlo Ersek
  0 siblings, 1 reply; 31+ messages in thread
From: Michael S. Tsirkin @ 2019-10-11 13:00 UTC (permalink / raw)
  To: Laszlo Ersek
  Cc: Eduardo Habkost, qemu-devel, Gerd Hoffmann, Paolo Bonzini,
	Igor Mammedov, Philippe Mathieu-Daudé,
	Richard Henderson

On Fri, Oct 11, 2019 at 10:01:42AM +0200, Laszlo Ersek wrote:
> On 10/10/19 21:20, Eduardo Habkost wrote:
> > On Thu, Oct 10, 2019 at 05:57:54PM +0200, Igor Mammedov wrote:
> >> On Thu, 10 Oct 2019 09:59:42 -0400
> >> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> >>
> >>> On Thu, Oct 10, 2019 at 03:39:12PM +0200, Igor Mammedov wrote:
> >>>> On Thu, 10 Oct 2019 05:56:55 -0400
> >>>> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> >>>>
> >>>>> On Wed, Oct 09, 2019 at 09:22:49AM -0400, Igor Mammedov wrote:
> >>>>>> As an alternative to passing to firmware topology info via new fwcfg files
> >>>>>> so it could recreate APIC IDs based on it and order CPUs are enumerated,
> >>>>>>
> >>>>>> extend CPU hotplug interface to return APIC ID as response to the new command
> >>>>>> CPHP_GET_CPU_ID_CMD.  
> >>>>>
> >>>>> One big piece missing here is motivation:
> >>>> I thought the only willing reader was Laszlo (who is aware of context)
> >>>> so I skipped on details and confused others :/
> >>>>
> >>>>> Who's going to use this interface?
> >>>> In current state it's for firmware, since ACPI tables can cheat
> >>>> by having APIC IDs statically built in.
> >>>>
> >>>> If we were creating CPU objects in ACPI dynamically
> >>>> we would be using this command as well.
> >>>
> >>> I'm not sure how it's even possible to create devices dynamically. Well
> >>> I guess it's possible with LoadTable. Is this what you had in
> >>> mind?
> >>
> >> Yep. I even played this shiny toy and I can say it's very tempting one.
> >> On the  other side, even problem of legacy OSes not working with it aside,
> >> it's hard to debug and reproduce compared to static tables.
> >> So from maintaining pov I dislike it enough to be against it.
> >>
> >>
> >>>> It would save
> >>>> us quite a bit space in ACPI blob but it would be a pain
> >>>> to debug and diagnose problems in ACPI tables, so I'd rather
> >>>> stay with static CPU descriptions in ACPI tables for the sake
> >>>> of maintenance.
> >>>>> So far CPU hotplug was used by the ACPI, so we didn't
> >>>>> really commit to a fixed interface too strongly.
> >>>>>
> >>>>> Is this a replacement to Laszlo's fw cfg interface?
> >>>>> If yes is the idea that OVMF going to depend on CPU hotplug directly then?
> >>>>> It does not depend on it now, does it?
> >>>> It doesn't, but then it doesn't support cpu hotplug,
> >>>> OVMF(SMM) needs to cooperate with QEMU "and" ACPI tables to perform
> >>>> the task and using the same interface/code path between all involved
> >>>> parties makes the task easier with the least amount of duplicated
> >>>> interfaces and more robust.
> >>>>
> >>>> Re-implementing alternative interface for firmware (fwcfg or what not)
> >>>> would work as well, but it's only question of time when ACPI and
> >>>> this new interface disagree on how world works and process falls
> >>>> apart.
> >>>
> >>> Then we should consider switching acpi to use fw cfg.
> >>> Or build another interface that can scale.
> >>
> >> Could be an option, it would be a pain to write a driver in AML for fwcfg access though
> >> (I've looked at possibility to access fwcfg from AML about a year ago and gave up.
> >> I'm definitely not volunteering for the second attempt and can't even give an estimate
> >> it it's viable approach).
> >>
> >> But what scaling issue you are talking about, exactly?
> >> With current CPU hotplug interface we can handle upto UNIT32_MAX cpus, and extend
> >> interface without need to increase IO window we are using now.
> >>
> >> Granted IO access it not fastest compared to fwcfg in DMA mode, but we already
> >> doing stop machine when switching to SMM which is orders of magnitude slower.
> >> Consensus was to compromise on speed of CPU hotplug versus more complex and more
> >> problematic unicast SMM mode in OVMF (can't find a particular email but we have discussed
> >> it with Laszlo already, when I considered ways to optimize hotplug speed)
> > 
> > If we were designing the interface from the ground up, I would
> > agree with Michael.  But I don't see why we would reimplement
> > everything from scratch now, if just providing the
> > cpu_selector => cpu_hardware_id mapping to firmware is enough to
> > make the existing interface work.
> > 
> > If somebody is really unhappy with the current interface and
> > wants to implement a new purely fw_cfg-based one (and write the
> > corresponding ACPI code), they would be welcome.
> 
> Let me re-iterate the difficulties quickly:
> 
> - DMA-based fw_cfg is troublesome in SEV guests (do you want to mess
> with page table entries in AML methods? or pre-allocate an always
> decrypted opregion? how large?)
> 
> - IO port based fw_cfg does not support writes (and I reckon that, when
> the *OS* handles a hotplug event, it does have to talk back to QEMU)
> 
> - the CPU hotplug AML would have to arbitrate with Linux's own fw_cfg
> driver (which exposes fw_cfg files to userspace, yay! /s)
> 
> In the phys world, CPU hotplug takes dedicated RAS hardware. Shoehorning
> CPU hotplug into *firmware* config, when in two use cases [*], the
> firmware shouldn't even know about CPU hotplug, feels messy.
> 
> [*] being (a) SeaBIOS, and (b) OVMF built without SMM

I agree. So ACPI should use a dedicated interface.

> > I just don't see why we should spend our time doing that now.
> 
> I have to agree, we're already spread thin.
> 
> ... I must admit: I didn't expect this, but now I've grown to *prefer*
> the CPU hotplug register block!
> 
> Laszlo

OK, send an ack then.

-- 
MST


^ permalink raw reply	[flat|nested] 31+ messages in thread

* Re: [RFC 0/3] acpi: cphp: add CPHP_GET_CPU_ID_CMD command to cpu hotplug MMIO interface
  2019-10-11 13:00             ` Michael S. Tsirkin
@ 2019-10-11 16:13               ` Laszlo Ersek
  0 siblings, 0 replies; 31+ messages in thread
From: Laszlo Ersek @ 2019-10-11 16:13 UTC (permalink / raw)
  To: Michael S. Tsirkin, Igor Mammedov
  Cc: Eduardo Habkost, qemu-devel, Gerd Hoffmann, Paolo Bonzini,
	Philippe Mathieu-Daudé,
	Richard Henderson

On 10/11/19 15:00, Michael S. Tsirkin wrote:
> On Fri, Oct 11, 2019 at 10:01:42AM +0200, Laszlo Ersek wrote:

[...]

>> ... I must admit: I didn't expect this, but now I've grown to *prefer*
>> the CPU hotplug register block!
> 
> OK, send an ack then.

This RFC isn't mature enough for an ACK, but I like the direction, and
I've given ample feedback. I'm looking forward to v1.

When Igor posts v1, I plan to first write firmware code for deriving
"max_cpus" through the register block. For that, I only need the docs to
reflect reality *closely* (I've posted my own suggestions for the docs);
plus "max_cpus" is something I can put to use immediately.

Regarding the CPHP_GET_CPU_ID_CMD feature, I'll have to test that from
within the SMI handler. Thus, until my "final" ACK, it's going to take a
while. I'm OK if the QEMU patch set remains pending on the list meanwhile.

Igor -- can you please answer my questions in this thread? (Part of my
feedback has been questions.)

Thanks!
Laszlo



^ permalink raw reply	[flat|nested] 31+ messages in thread

end of thread, back to index

Thread overview: 31+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2019-10-09 13:22 [RFC 0/3] acpi: cphp: add CPHP_GET_CPU_ID_CMD command to cpu hotplug MMIO interface Igor Mammedov
2019-10-09 13:22 ` [RFC 1/3] acpi: cpuhp: fix 'Command data' description is spec Igor Mammedov
2019-10-10 12:33   ` Laszlo Ersek
2019-10-10 13:31   ` Laszlo Ersek
2019-10-10 13:36     ` Laszlo Ersek
2019-10-09 13:22 ` [RFC 2/3] acpi: cpuhp: add typical usecases into spec Igor Mammedov
2019-10-10 13:04   ` Laszlo Ersek
2019-10-10 13:15     ` Laszlo Ersek
2019-10-10 14:13   ` Laszlo Ersek
2019-10-09 13:22 ` [RFC 3/3] acpi: cpuhp: add CPHP_GET_CPU_ID_CMD command Igor Mammedov
2019-10-10 14:56   ` Laszlo Ersek
2019-10-10 15:06     ` Michael S. Tsirkin
2019-10-10 17:23       ` Igor Mammedov
2019-10-10 17:53       ` Laszlo Ersek
2019-10-10 19:26       ` Eduardo Habkost
2019-10-11  8:07         ` Laszlo Ersek
2019-10-10  9:56 ` [RFC 0/3] acpi: cphp: add CPHP_GET_CPU_ID_CMD command to cpu hotplug MMIO interface Michael S. Tsirkin
2019-10-10 13:39   ` Igor Mammedov
2019-10-10 13:59     ` Michael S. Tsirkin
2019-10-10 15:57       ` Igor Mammedov
2019-10-10 18:15         ` Michael S. Tsirkin
2019-10-11  7:41           ` Laszlo Ersek
2019-10-10 19:20         ` Eduardo Habkost
2019-10-11  8:01           ` Laszlo Ersek
2019-10-11 13:00             ` Michael S. Tsirkin
2019-10-11 16:13               ` Laszlo Ersek
2019-10-11 10:47           ` Igor Mammedov
2019-10-11  6:54         ` Laszlo Ersek
2019-10-10 14:16     ` Eduardo Habkost
2019-10-10 14:49       ` Michael S. Tsirkin
2019-10-10 17:09       ` Igor Mammedov

QEMU-Devel Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/qemu-devel/0 qemu-devel/git/0.git
	git clone --mirror https://lore.kernel.org/qemu-devel/1 qemu-devel/git/1.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 qemu-devel qemu-devel/ https://lore.kernel.org/qemu-devel \
		qemu-devel@nongnu.org qemu-devel@archiver.kernel.org
	public-inbox-index qemu-devel

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.nongnu.qemu-devel


AGPL code for this site: git clone https://public-inbox.org/ public-inbox