linux-kernel - Re: [PATCH V5 05/10] KVM: x86/pmu: Disable vPMU if the minimum num of counters isn't met

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Date:   Mon, 10 Apr 2023 22:36:23 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH V5 05/10] KVM: x86/pmu: Disable vPMU if the minimum num of
 counters isn't met

On Mon, Apr 10, 2023 at 3:51 AM Like Xu <like.xu.linux@...il.com> wrote:
>
> From: Like Xu <likexu@...cent.com>
>
> Disable PMU support when running on AMD and perf reports fewer than four
> general purpose counters. All AMD PMUs must define at least four counters
> due to AMD's legacy architecture hardcoding the number of counters
> without providing a way to enumerate the number of counters to software,
> e.g. from AMD's APM:
>
>  The legacy architecture defines four performance counters (PerfCtrn)
>  and corresponding event-select registers (PerfEvtSeln).
>
> Virtualizing fewer than four counters can lead to guest instability as
> software expects four counters to be available.

I'm confused. Isn't zero less than four?

> Suggested-by: Sean Christopherson <seanjc@...gle.com>
> Signed-off-by: Like Xu <likexu@...cent.com>
> ---
>  arch/x86/kvm/pmu.h | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/arch/x86/kvm/pmu.h b/arch/x86/kvm/pmu.h
> index dd7c7d4ffe3b..002b527360f4 100644
> --- a/arch/x86/kvm/pmu.h
> +++ b/arch/x86/kvm/pmu.h
> @@ -182,6 +182,9 @@ static inline void kvm_init_pmu_capability(const struct kvm_pmu_ops *pmu_ops)
>                         enable_pmu = false;
>         }
>
> +       if (!is_intel && kvm_pmu_cap.num_counters_gp < AMD64_NUM_COUNTERS)
> +               enable_pmu = false;
> +
>         if (!enable_pmu) {
>                 memset(&kvm_pmu_cap, 0, sizeof(kvm_pmu_cap));
>                 return;
> --
> 2.40.0
>