linux-kernel - Re: [PATCH 4/4] KVM: x86/cpuid: Add AMD CPUID ExtPerfMonAndDbg leaf 0x80000022

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Date:   Thu, 8 Sep 2022 16:14:54 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Sandipan Das <sandipan.das@....com>
Cc:     Like Xu <like.xu.linux@...il.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] KVM: x86/cpuid: Add AMD CPUID ExtPerfMonAndDbg leaf 0x80000022

On Wed, Sep 7, 2022 at 11:00 PM Sandipan Das <sandipan.das@....com> wrote:
> This is the suggested method for detecting the number of counters:
>
>   If CPUID Fn8000_0022_EAX[PerfMonV2] is set, then use the new interface in
>   CPUID Fn8000_0022_EBX to determine the number of counters.
>
>   Else if CPUID Fn8000_0001_ECX[PerfCtrExtCore] is set, then six counters
>   are available.
>
>   Else, four legacy counters are available.
>
> There will be an APM update that will have this information in the
> "Detecting Hardware Support for Performance Counters" section.

Nonetheless, for compatibility with old software, Fn8000_0022_EBX can
never report less than four counters (or six, if
Fn8000_0001_ECX[PerfCtrExtCore] is set).