Re: [PATCH 2/2] xen: cleanup unrealized flash devices

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 7/1/20 4:59 PM, Jason Andryuk wrote:
> On Wed, Jul 1, 2020 at 8:55 AM Philippe Mathieu-Daudé <philmd@redhat.com> wrote:
>> On 7/1/20 2:40 PM, Philippe Mathieu-Daudé wrote:
>>> On 7/1/20 2:25 PM, Jason Andryuk wrote:
>>>> On Wed, Jul 1, 2020 at 3:03 AM Paul Durrant <xadimgnik@gmail.com> wrote:
>>>>>
>>>>>> -----Original Message-----
>>>>>> From: Philippe Mathieu-Daudé <philmd@redhat.com>
>>>>>> Sent: 30 June 2020 18:27
>>>>>> To: paul@xen.org; xen-devel@lists.xenproject.org; qemu-devel@nongnu.org
>>>>>> Cc: 'Eduardo Habkost' <ehabkost@redhat.com>; 'Michael S. Tsirkin' <mst@redhat.com>; 'Paul Durrant'
>>>>>> <pdurrant@amazon.com>; 'Jason Andryuk' <jandryuk@gmail.com>; 'Paolo Bonzini' <pbonzini@redhat.com>;
>>>>>> 'Richard Henderson' <rth@twiddle.net>
>>>>>> Subject: Re: [PATCH 2/2] xen: cleanup unrealized flash devices
>>>>>>
>>>>>> On 6/30/20 5:44 PM, Paul Durrant wrote:
>>>>>>>> -----Original Message-----
>>>>>>>> From: Philippe Mathieu-Daudé <philmd@redhat.com>
>>>>>>>> Sent: 30 June 2020 16:26
>>>>>>>> To: Paul Durrant <paul@xen.org>; xen-devel@lists.xenproject.org; qemu-devel@nongnu.org
>>>>>>>> Cc: Eduardo Habkost <ehabkost@redhat.com>; Michael S. Tsirkin <mst@redhat.com>; Paul Durrant
>>>>>>>> <pdurrant@amazon.com>; Jason Andryuk <jandryuk@gmail.com>; Paolo Bonzini <pbonzini@redhat.com>;
>>>>>>>> Richard Henderson <rth@twiddle.net>
>>>>>>>> Subject: Re: [PATCH 2/2] xen: cleanup unrealized flash devices
>>>>>>>>
>>>>>>>> On 6/24/20 2:18 PM, Paul Durrant wrote:
>>>>>>>>> From: Paul Durrant <pdurrant@amazon.com>
>>>>>>>>>
>>>>>>>>> The generic pc_machine_initfn() calls pc_system_flash_create() which creates
>>>>>>>>> 'system.flash0' and 'system.flash1' devices. These devices are then realized
>>>>>>>>> by pc_system_flash_map() which is called from pc_system_firmware_init() which
>>>>>>>>> itself is called via pc_memory_init(). The latter however is not called when
>>>>>>>>> xen_enable() is true and hence the following assertion fails:
>>>>>>>>>
>>>>>>>>> qemu-system-i386: hw/core/qdev.c:439: qdev_assert_realized_properly:
>>>>>>>>> Assertion `dev->realized' failed
>>>>>>>>>
>>>>>>>>> These flash devices are unneeded when using Xen so this patch avoids the
>>>>>>>>> assertion by simply removing them using pc_system_flash_cleanup_unused().
>>>>>>>>>
>>>>>>>>> Reported-by: Jason Andryuk <jandryuk@gmail.com>
>>>>>>>>> Fixes: ebc29e1beab0 ("pc: Support firmware configuration with -blockdev")
>>>>>>>>> Signed-off-by: Paul Durrant <pdurrant@amazon.com>
>>>>>>>>> Tested-by: Jason Andryuk <jandryuk@gmail.com>
>>>>>>>>> ---
>>>>>>>>> Cc: Paolo Bonzini <pbonzini@redhat.com>
>>>>>>>>> Cc: Richard Henderson <rth@twiddle.net>
>>>>>>>>> Cc: Eduardo Habkost <ehabkost@redhat.com>
>>>>>>>>> Cc: "Michael S. Tsirkin" <mst@redhat.com>
>>>>>>>>> Cc: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
>>>>>>>>> ---
>>>>>>>>>  hw/i386/pc_piix.c    | 9 ++++++---
>>>>>>>>>  hw/i386/pc_sysfw.c   | 2 +-
>>>>>>>>>  include/hw/i386/pc.h | 1 +
>>>>>>>>>  3 files changed, 8 insertions(+), 4 deletions(-)
>>>>>>>>>
>>>>>>>>> diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
>>>>>>>>> index 1497d0e4ae..977d40afb8 100644
>>>>>>>>> --- a/hw/i386/pc_piix.c
>>>>>>>>> +++ b/hw/i386/pc_piix.c
>>>>>>>>> @@ -186,9 +186,12 @@ static void pc_init1(MachineState *machine,
>>>>>>>>>      if (!xen_enabled()) {
>>>>>>>>>          pc_memory_init(pcms, system_memory,
>>>>>>>>>                         rom_memory, &ram_memory);
>>>>>>>>> -    } else if (machine->kernel_filename != NULL) {
>>>>>>>>> -        /* For xen HVM direct kernel boot, load linux here */
>>>>>>>>> -        xen_load_linux(pcms);
>>>>>>>>> +    } else {
>>>>>>>>> +        pc_system_flash_cleanup_unused(pcms);
>>>>>>>>
>>>>>>>> TIL pc_system_flash_cleanup_unused().
>>>>>>>>
>>>>>>>> What about restricting at the source?
>>>>>>>>
>>>>>>>
>>>>>>> And leave the devices in place? They are not relevant for Xen, so why not clean up?
>>>>>>
>>>>>> No, I meant to not create them in the first place, instead of
>>>>>> create+destroy.
>>>>>>
>>>>>> Anyway what you did works, so I don't have any problem.
>>>>>
>>>>> IIUC Jason originally tried restricting creation but encountered a problem because xen_enabled() would always return false at that point, because machine creation occurs before accelerators are initialized.
>>>>
>>>> Correct.  Quoting my previous email:
>>>> """
>>>> Removing the call to pc_system_flash_create() from pc_machine_initfn()
>>>> lets QEMU startup and run a Xen HVM again.  xen_enabled() doesn't work
>>>> there since accelerators have not been initialized yes, I guess?
>>>
>>> Ah, I missed that. You pointed at the bug here :)
>>>
>>> I think pc_system_flash_create() shouldn't be called in init() but
>>> realize().
>>
>> Hmm this is a MachineClass, not qdev, so no realize().
>>
>> In softmmu/vl.c we have:
>>
>> 4152     configure_accelerators(argv[0]);
>> ....
>> 4327     if (!xen_enabled()) { // so xen_enable() is working
>> 4328         /* On 32-bit hosts, QEMU is limited by virtual address space */
>> 4329         if (ram_size > (2047 << 20) && HOST_LONG_BITS == 32) {
>> 4330             error_report("at most 2047 MB RAM can be simulated");
>> 4331             exit(1);
>> 4332         }
>> 4333     }
>> ....
>> 4348     machine_run_board_init(current_machine);
>>
>> which calls in hw/core/machine.c:
>>
>> 1089 void machine_run_board_init(MachineState *machine)
>> 1090 {
>> 1091     MachineClass *machine_class = MACHINE_GET_CLASS(machine);
>> ....
>> 1138     machine_class->init(machine);
>> 1139 }
>>
>>          -> pc_machine_class_init()
>>
>> This should come after:
>>
>>          -> pc_machine_initfn()
>>
>>             -> pc_system_flash_create(pcms)
> 
> Sorry, I'm not following the flow you want.

Well, I was simply trying to understand what was wrong, and
what we should fix so you don't have to create flash devices
then destroy them when using Xen.

I already said Paul patch is OK and gave my R-b to it.

> 
> The xen_enabled() call in vl.c may always fail.  Or at least in most
> common Xen usage.  Xen HVMs are started with machine xenfv and don't
> specify an accelerator.  You need to process the xenfv default machine
> opts to process "accel=xen".  Actually, I don't know how the
> accelerator initialization works, but xen_accel_class_init() needs to
> be called to set `ac->allowed = &xen_allowed`.  xen_allowed is the
> return value of xen_enabled() and the accelerator initialization must
> toggle it.

Since you are happy how this current works, I'm also fine with it,
I won't investigate further.

Regards,

Phil.