linux-kernel - Re: [PATCH 1/3] KVM: x86: Refresh PMU after writes to MSR_IA32_PERF

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Date:   Wed, 3 Aug 2022 14:37:10 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH 1/3] KVM: x86: Refresh PMU after writes to
 MSR_IA32_PERF_CAPABILITIES

On Wed, Aug 03, 2022, Like Xu wrote:
> On 28/7/2022 7:34 am, Sean Christopherson wrote:
> > Refresh the PMU if userspace modifies MSR_IA32_PERF_CAPABILITIES.  KVM
> > consumes the vCPU's PERF_CAPABILITIES when enumerating PEBS support, but
> > relies on CPUID updates to refresh the PMU.  I.e. KVM will do the wrong
> > thing if userspace stuffs PERF_CAPABILITIES _after_ setting guest CPUID.
> > 
> > Opportunistically fix a curly-brace indentation.
> > 
> > Fixes: c59a1f106f5c ("KVM: x86/pmu: Add IA32_PEBS_ENABLE MSR emulation for extended PEBS")
> 
> Shouldn't it be: Fixes: 27461da31089 ("KVM: x86/pmu: Support full width counting") ?

Strictly speaking, I don't think so?  fw_writes_is_enabled() returns false if
guest CPUID doesn't have X86_FEATURE_PDCM, and AFAICT there are no other side
effects that are handled by intel_pmu_refresh().

> Now, all the dots have been connected. As punishment, I'd like to cook this
> patch set more with trackable tests so that you have more time for other
> things that are not housekeeping.

Let me post v2, I've already done all the work and testing.  If there's more to
be done, we can figure out next steps from there.