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

[Mi, Dapeng]

On 7/9/2026 8:35 PM, Jim Mattson wrote:
> 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?

Hmm, in current most cases, rdpmc and rdmsr should share consistent
interception configuration for PERF_METRICS except host and guest have
different counters bitmap.

Considering this is a test case, the test coverage should be more important
than the performance, there should be at least a place to call rdpmc
against the PERF_METRICS, otherwise, that path won't be validated.