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

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Date:   Thu, 9 Dec 2021 16:44:54 +0800
From:   Like Xu <like.xu.linux@...il.com>
To:     Jim Mattson <jmattson@...gle.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 9/12/2021 12:33 pm, Jim Mattson wrote:
> 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?

OK, let's take a conservative approach in the V3.

> 
>> +               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);
>> +
>