linux-kernel - Re: [PATCH v2 5/6] KVM: x86: Update vPMCs when retiring instructions

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Date:   Wed, 8 Dec 2021 20:33:00 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Like Xu <likexu@...cent.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2 5/6] KVM: x86: Update vPMCs when retiring instructions

On Mon, Nov 29, 2021 at 11:42 PM Like Xu <like.xu.linux@...il.com> wrote:
>
> From: Like Xu <likexu@...cent.com>
>
> When KVM retires a guest instruction through emulation, increment any
> vPMCs that are configured to monitor "instructions retired," and
> update the sample period of those counters so that they will overflow
> at the right time.
>
> Signed-off-by: Eric Hankland <ehankland@...gle.com>
> [jmattson:
>   - Split the code to increment "branch instructions retired" into a
>     separate commit.
>   - Added 'static' to kvm_pmu_incr_counter() definition.
>   - Modified kvm_pmu_incr_counter() to check pmc->perf_event->state ==
>     PERF_EVENT_STATE_ACTIVE.
> ]
> Fixes: f5132b01386b ("KVM: Expose a version 2 architectural PMU to a guests")
> Signed-off-by: Jim Mattson <jmattson@...gle.com>
> [likexu:
>   - Drop checks for pmc->perf_event or event state or event type
>   - Increase a counter once its umask bits and the first 8 select bits are matched
>   - Rewrite kvm_pmu_incr_counter() with a less invasive approach to the host perf;
>   - Rename kvm_pmu_record_event to kvm_pmu_trigger_event;
>   - Add counter enable and CPL check for kvm_pmu_trigger_event();
> ]
> Cc: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Like Xu <likexu@...cent.com>
> ---

> +void kvm_pmu_trigger_event(struct kvm_vcpu *vcpu, u64 perf_hw_id)
> +{
> +       struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
> +       struct kvm_pmc *pmc;
> +       int i;
> +
> +       for_each_set_bit(i, pmu->all_valid_pmc_idx, X86_PMC_IDX_MAX) {
> +               pmc = kvm_x86_ops.pmu_ops->pmc_idx_to_pmc(pmu, i);
> +
> +               if (!pmc || !pmc_is_enabled(pmc) || !pmc_speculative_in_use(pmc))
> +                       continue;
> +
> +               /* Ignore checks for edge detect, pin control, invert and CMASK bits */

I don't understand how we can ignore these checks. Doesn't that
violate the architectural specification?

> +               if (eventsel_match_perf_hw_id(pmc, perf_hw_id) && cpl_is_matched(pmc))
> +                       kvm_pmu_incr_counter(pmc);
> +       }
> +}
> +EXPORT_SYMBOL_GPL(kvm_pmu_trigger_event);
> +