Re: [edk2-devel] [Qemu-devel] [PATCH 1/2] q35: implement 128K SMRAM at default SMBASE address

[Laszlo Ersek <lersek@redhat.com>]

On 10/04/19 13:31, Igor Mammedov wrote:
> On Tue, 1 Oct 2019 20:03:20 +0200
> "Laszlo Ersek" <lersek@redhat.com> wrote:

>> (1) What values to use.

> SeaBIOS writes 0x00 into command port, but it seems that's taken by
> EFI_SMM_COMMUNICATION_PROTOCOL. So we can use the next unused value
> (lets say 0x4). We probably don't have to use status port or 
> EFI_SMM_COMMUNICATION_PROTOCOL, since the value of written into 0xB2
> is sufficient to distinguish hotplug event.

Thanks. Can you please write a QEMU patch for the ACPI generator such
that hotplugging a VCPU writes value 4 to IO port 0xB2?

That will allow me to experiment with OVMF.

(I can experiment with some other parts in edk2 even before that.)

>> (2) How the parameters are passed.
>>
>>
>> (2a) For the new CPU, the SMI remains pending, until it gets an
>> INIT-SIPI-SIPI from one of the previously plugged CPUs (most likely, the
>> BSP). At that point, the new CPU will execute the "initial SMI handler
>> for hotplugged CPUs", at the default SMBASE.
>>
>> That's a routine we'll have to write in assembly, from zero. In this
>> routine, we can read back IO ports 0xB2 and 0xB3. And QEMU will be happy
>> to provide the values last written (see apm_ioport_readb() in
>> "hw/isa/apm.c"). So we can receive the values in this routine. Alright.
> 
> Potentially we can can avoid writing custom SMI handler,
> what do you think about following workflow:
> 
> on system boot after initial CPUs relocation, firmware set NOP SMI handler
> at default SMBASE.
> Then as reaction to GPE triggered SMI (on cpu hotplug), after SMI rendezvous,
> a host cpu reads IO port 0xB2 and does hotplugged CPUs enumeration.
> 
>   a) assuming we allow hotplug only in case of negotiated SMI broadcast
>      host CPU shoots down all in-flight INIT/SIPI/SIPI for hotpugged CPUs
>      to avoid race within relocation handler.

How is that "shootdown" possible?

>  After that host CPU in loop
> 
>   b) it prepares/initializes necessary CPU structures for a hotplugged
>      CPU if necessary and replaces NOP SMI handler with the relocation
>      SMI handler that is used during system boot.
>      
>   c) a host CPU sends NOP INIT/SIPI/SIPI to the hotplugged CPU
> 
>   d) the woken up hotplugged CPU, jumps to default SMBASE and
>      executes hotplug relocation handler.
> 
>   e) after the hotplugged CPU  is relocated and if there are more
>      hotplugged CPUs, a host CPU repeats b-d steps for the next
>      hotplugged CPU.
> 
>   f) after all CPUs are relocated, restore NOP SMI handler at default
>      SMBASE.
> 

Thanks
Laszlo