linux-kernel - Re: [PATCH v2 2/3] KVM: x86/svm/pmu: Add AMD PerfMonV2 support

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Date:   Wed, 9 Nov 2022 14:51:55 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/3] KVM: x86/svm/pmu: Add AMD PerfMonV2 support

On Wed, Nov 09, 2022, Like Xu wrote:
> On 28/10/2022 6:47 am, Sean Christopherson wrote:
> > What happens if userspace sets X86_FEATURE_PERFCTR_CORE when its not supported?
> > E.g. will KVM be coerced into taking a #GP on a non-existent counter?
> 
> I'm getting a bit tired of this generic issue, what does KVM need to do when
> the KVM user space sets a capability that KVM doesn't support (like cpuid).
> Should it change the guest cpuid audibly or silently ? Should it report an
> error when the guest uses this unsupported capability at run time, or should
> it just let the KVM report an error when user space setting the cpuid ?

KVM should do nothing unless the bogus CPUID can be used to trigger unexpected
and/or unwanted behavior in the kernel, which is why I asked if KVM might end up
taking a #GP.