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

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Message-ID: <7e84bfb4-b052-4c31-a319-1ea2dd52ae54@gmail.com>
Date:   Tue, 28 Mar 2023 18:04:45 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.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 28/3/2023 5:20 pm, Paolo Bonzini wrote:
> 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
> 

If true, why introducing those alias MSRs ? My archaeological findings are:

a platform w/o full-witdh like Westmere (has 3-fixed counters already) is 
declared to
have a counter width (R:48, W:32) and its successor Sandy Bridge has (R:48 , W: 
32/48).

Thus I think the behaviour of the fixed counter has changed from there, and the 
alias GP MSRs
were introduced to keep the support on 32-bit writes on #GP counters (via 
original address).

[*] Intel® 64 and IA-32 Architectures Software Developer’s Manual Documentation 
Changes
(252046-030, January 2011) Table 30-18 Core PMU Comparison.