Re: [PATCH] UefiCpuPkg/LocalApicLib: Exclude second SendIpi sequence on AMD processors.

[Paolo Bonzini <pbonzini@redhat.com>]

On 14/06/2018 07:39, Ni, Ruiyu wrote:
> 
> 
> Thanks/Ray
> 
>> -----Original Message-----
>> From: edk2-devel <edk2-devel-bounces@lists.01.org> On Behalf Of Paolo
>> Bonzini
>> Sent: Thursday, June 14, 2018 4:52 AM
>> To: Laszlo Ersek <lersek@redhat.com>; Leo Duran <leo.duran@amd.com>;
>> edk2-devel@lists.01.org
>> Cc: Justen, Jordan L <jordan.l.justen@intel.com>; Brijesh Singh
>> <brijesh.singh@amd.com>; Jeff Fan <jeff.fan@intel.com>; Gao, Liming
>> <liming.gao@intel.com>
>> Subject: Re: [edk2] [PATCH] UefiCpuPkg/LocalApicLib: Exclude second
>> SendIpi sequence on AMD processors.
>>
>> On 13/06/2018 22:49, Laszlo Ersek wrote:
>>> Hello Leo,
>>>
>>> On 06/13/18 22:11, Leo Duran wrote:
>>>> On AMD processors the second SendIpi in the SendInitSipiSipi and
>>>> SendInitSipiSipiAllExcludingSelf routines is not required, and may
>>>> cause undesired side-effects during MP initialization.
>>>>
>>>> This patch leverages the StandardSignatureIsAuthenticAMD check to
>>>> exclude the second SendIpi and its associated MicroSecondDelay (200).
>>>
>>> QEMU and KVM emulate some AMD processors too; of particular interest
>>> is the recent EPYC addition, I believe (for SME/SEV, minimally).
>>>
>>> Did you check whether the StandardSignatureIsAuthenticAMD() check
>>> applies to those QEMU VCPU models, and if so, whether omitting the
>>> second Startup IPI interferes with *V*CPU startup in OVMF guests? (In
>>> multiprocessing modules, such as CpuMpPei, CpuDxe, and
>>> PiSmmCpuDxeSmm.)
>>>
>>> Adding Brijesh, Paolo and Igor.
>>
>> Actually I would be surprised if any recent processor needs the INIT-SIPI-SIPI
>> (though I'm not sure what undesired side effects it might trigger).
> 
> Why the recent processors don't need INIT-SIPI-SIPI?
> I thought it is the only way to wake up processors when it's halted (HLT).

INIT-SIPI should be enough.  The second SIPI is just in case the first
one was missed.

Paolo