linux-kernel - Re: [PATCH v2] KVM: x86/pmu: Fix emulation on Intel counters' bit width

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Date:   Tue, 28 Mar 2023 11:20:34 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Sean Christopherson <seanjc@...gle.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] KVM: x86/pmu: Fix emulation on Intel counters' bit
 width

On 3/28/23 11:16, Like Xu wrote:
> 
> 
> If IA32_PERF_CAPABILITIES.FW_WRITE[bit 13] =1, each IA32_PMCi is 
> accompanied by a
> corresponding alias address starting at 4C1H for IA32_A_PMC0.
> 
> The bit width of the performance monitoring counters is specified in 
> CPUID.0AH:EAX[23:16].
> If IA32_A_PMCi is present, the 64-bit input value (EDX:EAX) of WRMSR to 
> IA32_A_PMCi will cause
> IA32_PMCi to be updated by:
> 
>      COUNTERWIDTH =
>          CPUID.0AH:EAX[23:16] bit width of the performance monitoring 
> counter
>      IA32_PMCi[COUNTERWIDTH-1:32] := EDX[COUNTERWIDTH-33:0]);
>      IA32_PMCi[31:0] := EAX[31:0];
>      EDX[63:COUNTERWIDTH] are reserved
> 
> ---
> 
> Some might argue that this is all talking about GP counters, not
> fixed counters. In fact, the full-width write hw behaviour is
> presumed to do the same thing for all counters.
But the above behavior, and the #GP, is only true for IA32_A_PMCi (the 
full-witdh MSR).  Did I understand correctly that the behavior for fixed 
counters is changed without introducing an alias MSR?

Paolo