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

[Sean Christopherson <seanjc@google.com>]

On Tue, Jul 21, 2026, Luka Absandze wrote:
> On 2026-07-21 09:47, Sean Christopherson wrote:
> > 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;
> >         }
> 
> I believe this is fine, but perhaps the tolerance itself could be a knob

Ya, I was envisioning a knob.

> with <0 meaning no reprogram?

Eh, I don't see a need to assign special meaning to <0.  Make it a u32 (or u64),
and then a large value will provide the same behavior in practice, without needing
special handling in KVM.

> While the contention on this path will be much lower, we are still
> introducing more steal time, and the added accuracy is probably not worth
> even that for guests who really only care for the rate of change, rather than
> exact instruction counts.