Re: [kvm-unit-tests PATCH] x86: Add tests for Guest Processor Event Based Sampling (PEBS)

Sean Christopherson
2022-07-27T22:42:18Z

On Thu, Jul 21, 2022, Like Xu wrote:
> +union perf_capabilities {
> + struct {
> + u64 lbr_format:6;
> + u64 pebs_trap:1;
> + u64 pebs_arch_reg:1;
> + u64 pebs_format:4;
> + u64 smm_freeze:1;
> + /*
> + * PMU supports separate counter range for writing
> + * values > 32bit.
> + */
> + u64 full_width_write:1;
> + u64 pebs_baseline:1;
> + u64 perf_metrics:1;
> + u64 pebs_output_pt_available:1;
> + u64 anythread_deprecated:1;
> + };
> + u64 capabilities;
> +};
> +
> +union cpuid10_eax {
> + struct {
> + unsigned int version_id:8;
> + unsigned int num_counters:8;
> + unsigned int bit_width:8;
> + unsigned int mask_length:8;
> + } split;
> + unsigned int full;
> +} pmu_eax;
> +
> +union cpuid10_edx {
> + struct {
> + unsigned int num_counters_fixed:5;
> + unsigned int bit_width_fixed:8;
> + unsigned int reserved:19;
> + } split;
> + unsigned int full;
> +} pmu_edx;

The generic unions are hopefully unnecessary now that helpers are provided by
lib/x86/processor.h, e.g. for pmu_version().

I would prefer to have similar helpers instead of "union perf_capabilities",
but it's not a sticking point if helpers a signifiantly more painful to use.

> + if (!is_intel() || (pmu_eax.split.version_id < 2) ||
> + !(perf.capabilities & PERF_CAP_PEBS_FORMAT) ||
> + (rdmsr(MSR_IA32_MISC_ENABLE) & MSR_IA32_MISC_ENABLE_PEBS_UNAVAIL)) {

Split these up, it's really, really annoying to have to guess which one of the
four checks failed.

> + report_skip("This platform doesn't support guest PEBS.");
> + return 0;

This needs be be "return report_summary()", otherwise the test says pass when it
didn't do anyting:

TESTNAME=pmu_pebs TIMEOUT=90s ACCEL=kvm ./x86/run x86/pmu_pebs.flat -smp 1 -cpu host,migratable=no
PASS pmu_pebs

wait a second...

SKIP: This platform doesn't support guest PEBS.

E.g. (though if KUT can provide more information on why PERF_CAP_PEBS_FORMAT
may not be advertised, e.g. requires ICX+?, that would be nice to have)

if (!is_intel()) {
report_skip("PEBS is only supported on Intel CPUs");
return report_summary();
}
if (pmu_version() < 2) {
report_skip("Architectural PMU not available");
return report_summary();
}
if (!(perf.capabilities & PERF_CAP_PEBS_FORMAT)) {
report_skip("PEBS not enumerated in PERF_CAPABILITIES");
return report_summary();
}
if (rdmsr(MSR_IA32_MISC_ENABLE) & MSR_IA32_MISC_ENABLE_PEBS_UNAVAIL) {
report_skip("PEBS unavailable according to MISC_ENABLE");
return report_summary();
}