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

[Zhao Liu <zhao1.liu@intel.com>]

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).

(I'll help go through Dongli's v3 soon.)

Thank you both,
Zhao