Re: [RFC PATCH 0/2] KVM: x86/pmu: Let userspace disable SW accounting of emulated instructions

[David Woodhouse]

On Tue, 2026-07-21 at 09:47 -0700, Sean Christopherson wrote:
> On Tue, Jul 21, 2026, David Woodhouse wrote:
> > On Mon, 2026-07-20 at 19:22 +0000, Luka Absandze wrote:
> > > On hosts that run guests with an *emulated* vPMU -- guest PMU MSR
> > > accesses trap into KVM and each guest counter is backed by a host
> > > perf_event -- the per-emulated-instruction PMU accounting added by commit
> > > 9cd803d496e7 ("KVM: x86: Update vPMCs when retiring instructions") is
> > > pathologically expensive.
> > 
> > Given that the guest PMU MSR accesses are trapped... is there a reason
> > we can't fold a *separate* count of emulated instructions into the
> > value we return to the guest?
> > 
> > I guess the overflow would come late... but isn't that tolerable? 
> 
> Like ignoring emulated instructions entirely, it probably depends on the guest.
> 
> > And we could probably impose a limit on how large the addend could be — even
> > if we only reprogram the PMU once every 100 times, that's still a win?
> 
> Hmm, yeah, that'd be better than ignoring emulated instruction entirely, and
> would give userspace far better control over how shoddy KVM's emulation is.  Off
> the cuff, the change would be fairly trivial too?  E.g.
> 
> diff --git arch/x86/kvm/pmu.c arch/x86/kvm/pmu.c
> index 7f777049d328..c5f436c6f83f 100644
> --- arch/x86/kvm/pmu.c
> +++ arch/x86/kvm/pmu.c
> @@ -1084,7 +1084,8 @@ static void kvm_pmu_incr_counter(struct kvm_pmc *pmc)
>          */
>         if (!kvm_vcpu_has_mediated_pmu(vcpu)) {
>                 pmc->emulated_counter++;
> -               kvm_pmu_request_counter_reprogram(pmc);
> +               if (pmc->emulated_counter > pmu_something_something_tolerance)
> +                       kvm_pmu_request_counter_reprogram(pmc);
>                 return;
>         }
> 

We also want to add the same delta when the the guest reads the
counter?

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