Re: [PATCH v6 11/20] KVM: selftests: Test Intel PMU architectural events on fixed counters

[Sean Christopherson]

On Sat, Nov 04, 2023, Jim Mattson wrote:
> On Fri, Nov 3, 2023 at 5:03 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> >  static void guest_test_arch_event(uint8_t idx)
> >  {
> >         const struct {
> >                 struct kvm_x86_pmu_feature gp_event;
> > +               struct kvm_x86_pmu_feature fixed_event;
> >         } intel_event_to_feature[] = {
> > -               [INTEL_ARCH_CPU_CYCLES]            = { X86_PMU_FEATURE_CPU_CYCLES },
> > -               [INTEL_ARCH_INSTRUCTIONS_RETIRED]  = { X86_PMU_FEATURE_INSNS_RETIRED },
> > -               [INTEL_ARCH_REFERENCE_CYCLES]      = { X86_PMU_FEATURE_REFERENCE_CYCLES },
> > -               [INTEL_ARCH_LLC_REFERENCES]        = { X86_PMU_FEATURE_LLC_REFERENCES },
> > -               [INTEL_ARCH_LLC_MISSES]            = { X86_PMU_FEATURE_LLC_MISSES },
> > -               [INTEL_ARCH_BRANCHES_RETIRED]      = { X86_PMU_FEATURE_BRANCH_INSNS_RETIRED },
> > -               [INTEL_ARCH_BRANCHES_MISPREDICTED] = { X86_PMU_FEATURE_BRANCHES_MISPREDICTED },
> > +               [INTEL_ARCH_CPU_CYCLES]            = { X86_PMU_FEATURE_CPU_CYCLES, X86_PMU_FEATURE_CPU_CYCLES_FIXED },
> > +               [INTEL_ARCH_INSTRUCTIONS_RETIRED]  = { X86_PMU_FEATURE_INSNS_RETIRED, X86_PMU_FEATURE_INSNS_RETIRED_FIXED },
> > +               /*
> > +                * Note, the fixed counter for reference cycles is NOT the same
> > +                * as the general purpose architectural event (because the GP
> > +                * event is garbage).  The fixed counter explicitly counts at
> > +                * the same frequency as the TSC, whereas the GP event counts
> > +                * at a fixed, but uarch specific, frequency.  Bundle them here
> > +                * for simplicity.
> > +                */
> 
> Implementation-specific is not necessarily garbage, though it would be
> nice if there was a way to query the frequency rather than calibrating
> against another clock.

Heh, I'll drop the editorial commentry, though I still think an architectural event
with implementation-specific behavior is garbage :-)