Re: [PATCH v6 8/8] KVM: selftests: Add PERF_METRICS and fixed counter 3 tests

[Jim Mattson]

On Mon, Jun 29, 2026 at 7:36 PM Mi, Dapeng <dapeng1.mi@xxxxxxxxxxxxxxx> wrote:
>
>
> On 6/30/2026 7:19 AM, Zide Chen wrote:
> > Add a test case to exercise IA32_PERF_METRICS, i.e. architectural
> > support for Topdown (TMA) Level 1 metrics, enumerated by
> > IA32_PERF_CAPABILITIES[15].
> >
> > Only check for non-zero metrics, as they are derived and depend on
> > the workload, CPU model, and host scheduling, making precise
> > expectations fragile.
> >
> > Extend the PMU selftest to cover Intel fixed counter 3 by bumping
> > MAX_NR_FIXED_COUNTERS to 4 and validating basic functionality.
> >
> > Signed-off-by: Zide Chen <zide.chen@xxxxxxxxx>
> > ---
> > ...
> > +static void __guest_test_perf_metrics(void)
> > +{
> > +     int retiring, bad_spec, fe_bound, be_bound, sum;
> > +     u64 global_ctrl, metrics;
> > +
> > +     if ((guest_get_pmu_version() < 2) ||    /* Does guest have GLOBAL_CTRL? */
> > +         !this_cpu_has(X86_FEATURE_PDCM) ||
> > +         !(rdmsr(MSR_IA32_PERF_CAPABILITIES) & PERF_CAP_PERF_METRICS))
> > +             return;
> > +
> > +     wrmsr(MSR_CORE_PERF_GLOBAL_CTRL, 0);
> > +     wrmsr(MSR_CORE_PERF_FIXED_CTR3, 0);
> > +     wrmsr(MSR_PERF_METRICS, 0);
> > +
> > +     /* Enable fixed ctr3 (TOPDOWN.SLOTS) and PERF_METRICS. */
> > +     wrmsr(MSR_CORE_PERF_FIXED_CTR_CTRL, FIXED_PMC_CTRL(3, FIXED_PMC_KERNEL));
> > +     global_ctrl = FIXED_PMC_GLOBAL_CTRL_ENABLE(3) |
> > +                   PERF_METRICS_GLOBAL_CTRL_ENABLE;
> > +
> > +     GUEST_RUN_PAYLOAD(MSR_CORE_PERF_GLOBAL_CTRL, global_ctrl, "");
> > +
> > +     /* Check test results. */
> > +     metrics = rdmsr(MSR_PERF_METRICS);
>
> Could we use rdpmc instead of rdmsr here? rdpmc is a preferred way to read
> counter value.

This is in-guest code, so the unintercepted RDMSR will be much faster
than the emulated RDPMC.

Should we rethink that preference, or add hardware support for
selective RDPMC intercepts?