Re: [PATCH v2 08/10] target/i386/kvm: reset AMD PMU registers during VM reset

[Ewan Hai <ewanhai-oc@zhaoxin.com>]

On 4/7/25 4:51 PM, Zhao Liu wrote:

> 
> On Tue, Apr 01, 2025 at 11:35:49AM +0800, Ewan Hai wrote:
>> Date: Tue, 1 Apr 2025 11:35:49 +0800
>> From: Ewan Hai <ewanhai-oc@zhaoxin.com>
>> Subject: Re: [PATCH v2 08/10] target/i386/kvm: reset AMD PMU registers
>>   during VM reset
>>
>>>> [2] As mentioned in [1], QEMU always sets the vCPU's vendor to match the host's
>>>> vendor
>>>> when acceleration (KVM or HVF) is enabled. Therefore, if users want to emulate a
>>>> Zhaoxin CPU on an Intel host, the vendor must be set manually.Furthermore,
>>>> should we display a warning to users who enable both vPMU and KVM acceleration
>>>> but do not manually set the guest vendor when it differs from the host vendor?
>>>
>>> Maybe not? Sometimes I emulate AMD on Intel host, while vendor is still the
>>> default :)
>>
>> Okay, handling this situation can be rather complex, so let's keep it
>> simple. I have added a dedicated function to capture the intended behavior
>> for potential future reference.
>>
>> Anyway, Thanks for taking Zhaoxin's situation into account, regardless.
>>
> 
> Thanks for your code example!!
> 
> Zhaoxin implements perfmon v2, so I think checking the vendor might be
> overly complicated. If a check is needed, it seems more reasonable to
> check the perfmon version rather than the vendor, similar to how avx10
> version is checked in x86_cpu_filter_features().
> 
> I understand Ewan's concern is that if an Intel guest requires a higher
> perfmon version that the Zhaoxin host doesn't support, there could be
> issues (although I think this situation doesn't currently exist in KVM-QEMU,
> one reason is QEMU uses the pmu_version in 0xa queried from KVM directly,
> which means QEMU currently doesn't support custom pmu_version).

Yeah, that's exactly what I was concerned about.
Thanks for clearing that up!

perfmon_version is a great idea --- I might add it as a property to the QEMU 
vCPU template in the future, so it can adjust based on user input and host support.
Can't promise a timeline yet, but it's definitely something I'll keep in mind.