Re: [PATCH v5 08/13] KVM: selftests: Test Intel PMU architectural events on gp counters

Mingwei Zhang
2023-10-26T22:10:40Z

On Thu, Oct 26, 2023, Sean Christopherson wrote:
> On Thu, Oct 26, 2023, Mingwei Zhang wrote:
> > > +static bool pmu_is_intel_event_stable(uint8_t idx)
> > > +{
> > > + switch (idx) {
> > > + case INTEL_ARCH_CPU_CYCLES:
> > > + case INTEL_ARCH_INSTRUCTIONS_RETIRED:
> > > + case INTEL_ARCH_REFERENCE_CYCLES:
> > > + case INTEL_ARCH_BRANCHES_RETIRED:
> > > + return true;
> > > + default:
> > > + return false;
> > > + }
> > > +}
> >
> > Brief explanation on why other events are not stable please. Since there
> > are only a few architecture events, maybe listing all of them with
> > explanation in comments would work better.
>
> Heh, I've already rewritten this logic to make
>
>
> > > +
> > > +static void guest_measure_pmu_v1(struct kvm_x86_pmu_feature event,
> > > + uint32_t counter_msr, uint32_t nr_gp_counters)
> > > +{
> > > + uint8_t idx = event.f.bit;
> > > + unsigned int i;
> > > +
> > > + for (i = 0; i < nr_gp_counters; i++) {
> > > + wrmsr(counter_msr + i, 0);
> > > + wrmsr(MSR_P6_EVNTSEL0 + i, ARCH_PERFMON_EVENTSEL_OS |
> > > + ARCH_PERFMON_EVENTSEL_ENABLE | intel_pmu_arch_events[idx]);
> > > + __asm__ __volatile__("loop ." : "+c"((int){NUM_BRANCHES}));
> >
> > Some comment might be needed for readability. Abuptly inserting inline
> > assembly code in C destroys the readability.
> >
> > I wonder do we need add 'clobber' here for the above line, since it
> > takes away ecx?
>
> It's already there. You can't directly clobber a register that is used as an
> input constraint. The workaround is to make the register both an input and an
> output, hense the "+c" in the outputs section instead of just "c" in the inputs
> section. The extra bit of cleverness is to use an intermediate anonymous variable
> so that NUM_BRANCHES can effectively be passed in (#defines won't work as output
> constraints).
>
> > Also, I wonder if we need to disable IRQ here? This code might be
> > intercepted and resumed. If so, then the test will get a different
> > number?
>
> This is guest code, disabling IRQs is pointless. There are no guest virtual IRQs,
> guarding aginst host IRQs is impossible, unnecessary, and actualy undesirable,
> i.e. the guest vPMU shouldn't be counting host instructions and whatnot.
>
> > > +
> > > + if (pmu_is_intel_event_stable(idx))
> > > + GUEST_ASSERT_EQ(this_pmu_has(event), !!_rdpmc(i));
> >
> > Okay, just the counter value is non-zero means we pass the test ?!
>
> FWIW, I've updated
>
> > hmm, I wonder other than IRQ stuff, what else may affect the result? NMI
> > watchdog or what?
>
> This is the beauty of selftests. There _so_ simple that there are very few
> surprises. E.g. there are no events of any kind unless the test explicitly
> generates them. The downside is that doing anything complex in selftests requires
> writing a fair bit of code.

Understood, so we could support precise matching.
>
> > > +
> > > + wrmsr(MSR_P6_EVNTSEL0 + i, ARCH_PERFMON_EVENTSEL_OS |
> > > + !ARCH_PERFMON_EVENTSEL_ENABLE |
> > > + intel_pmu_arch_events[idx]);
> > > + wrmsr(counter_msr + i, 0);
> > > + __asm__ __volatile__("loop ." : "+c"((int){NUM_BRANCHES}));
> > ditto for readability. Please consider using a macro to avoid repeated
> > explanation.
>
> Heh, already did this too. Though I'm not entirely sure it's more readable. It's
> definitely more precise and featured :-)
>
Oh dear, this is challenging to my rusty inline assembly skills :)

> #define GUEST_MEASURE_EVENT(_msr, _value, clflush, FEP) \
> do { \
> __asm__ __volatile__("wrmsr\n\t" \
> clflush "\n\t" \
> "mfence\n\t" \
> "1: mov $" __stringify(NUM_BRANCHES) ", %%ecx\n\t" \
> FEP "loop .\n\t" \
> FEP "mov %%edi, %%ecx\n\t" \
> FEP "xor %%eax, %%eax\n\t" \
> FEP "xor %%edx, %%edx\n\t" \
> "wrmsr\n\t" \
> : "+c"((int){_msr}) \
isn't it NUM_BRANCHES?
> : "a"((uint32_t)_value), "d"(_value >> 32), \
> "D"(_msr) \
> ); \
> } while (0)
>

do we need this label '1:' in the above code? It does not seems to be
used anywhere within the code.

why is clflush needed here?
>
> > > +int main(int argc, char *argv[])
> > > +{
> > > + TEST_REQUIRE(get_kvm_param_bool("enable_pmu"));
> > > +
> > > + TEST_REQUIRE(host_cpu_is_intel);
> > > + TEST_REQUIRE(kvm_cpu_has_p(X86_PROPERTY_PMU_VERSION));
> > > + TEST_REQUIRE(kvm_cpu_property(X86_PROPERTY_PMU_VERSION) > 0);
> > > + TEST_REQUIRE(kvm_cpu_has(X86_FEATURE_PDCM));
> >
> > hmm, this means we cannot run this in nested if X86_FEATURE_PDCM is
> > missing. It only affects full-width counter, right?
>
> Ah, yeah, good call. It won't be too much trouble to have the test play nice
> with !PDCM.