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From:   Jim Mattson <jmattson@google.com>
Date:   Wed, 7 Sep 2022 09:33:13 -0700
Message-ID: <CALMp9eSPfxPunKW-K6LLPxsXdaeezKU=2G9Sdh7FS6VGb3goFA@mail.gmail.com>
Subject: Re: [PATCH v2 1/3] KVM: x86/pmu: Stop adding speculative Intel GP
 PMCs that don't exist yet
To:     Like Xu <like.xu.linux@gmail.com>
Cc:     Sean Christopherson <seanjc@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Vitaly Kuznetsov <vkuznets@redhat.com>
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On Wed, Sep 7, 2022 at 3:48 AM Like Xu <like.xu.linux@gmail.com> wrote:
>
> From: Like Xu <likexu@tencent.com>
>
> The Intel April 2022 SDM - Table 2-2. IA-32 Architectural MSRs adds
> a new architectural IA32_OVERCLOCKING_STATUS msr (0x195), plus the
> presence of IA32_CORE_CAPABILITIES (0xCF), the theoretical effective
> maximum value of the Intel GP PMCs is 14 (0xCF - 0xC1) instead of 18.
>
> But the conclusion of this speculation "14" is very fragile and can
> easily be overturned once Intel declares another meaningful arch msr
> in the above reserved range, and even worse, Intel probably put PMCs
> 8-15 in a completely different range of MSR indices.

The last clause is just conjecture.

> A conservative proposal would be to stop at the maximum number of Intel
> GP PMCs supported today. Also subsequent changes would limit both AMD
> and Intel on the number of GP counter supported by KVM.
>
> There are some boxes like Intel P4 may indeed have 18 counters, but
> those counters are in a completely different msr address range and do
> not strictly adhere to the Intel Arch PMU specification, and will not
> be supported by KVM in the near future.

The P4 PMU isn't virtualized by KVM today, is it?

>
> Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
> Suggested-by: Jim Mattson <jmattson@google.com>
> Signed-off-by: Like Xu <likexu@tencent.com>

Please put the "Fixes" tag back. You convinced me that it should be there.

Reviewed-by: Jim Mattson <jmattson@google.com>