linux-kernel - Re: [PATCH 3/3] KVM: x86/pmu: Drop redundant-clumsy-asymmetric PERFCTR

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Date:   Fri, 20 May 2022 16:32:55 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Jim Mattson <jmattson@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] KVM: x86/pmu: Drop redundant-clumsy-asymmetric
 PERFCTR_CORE MSRs handling

On 5/9/22 12:22, Like Xu wrote:
> In commit c51eb52b8f98 ("KVM: x86: Add support for AMD Core Perf Extension
> in guest"), the entry "case MSR_F15H_PERF_CTL0 ... MSR_F15H_PERF_CTR5 " is
> introduced asymmetrically into kvm_get_msr_common(), ignoring the set part.
> 
> The missing guest PERFCTR_CORE cpuid check from the above commit leads to
> the commit c28fa560c5bb ("KVM: x86/vPMU: Forbid reading from MSR_F15H_PERF
> MSRs when guest doesn't have X86_FEATURE_PERFCTR_CORE"), but it simply
> duplicates the default entry at the end of the switch statement explicitly.
> 
> Removing the PERFCTR_CORE MSRs entry in kvm_get_msr_common() thoroughly
> would be more maintainable, as we did for the same group of MSRs in the
> kvm_set_msr_common() at the very beginning when the feature was enabled.

The code and the commit message suggest that some guests are expecting a 
#GP, and complain if they don't get it.

Paolo