Re: [PATCH v2 1/6] KVM: x86/pmu: Setup pmc->eventsel for fixed PMCs

[Jim Mattson <jmattson@google.com>]

On Mon, Dec 6, 2021 at 10:08 PM Like Xu <like.xu.linux@gmail.com> wrote:
>
> On 7/12/2021 3:50 am, Jim Mattson wrote:
> > On Mon, Nov 29, 2021 at 11:42 PM Like Xu <like.xu.linux@gmail.com> wrote:
> >
> >> From: Like Xu <likexu@tencent.com>
> >>
> >> The current pmc->eventsel for fixed counter is underutilised. The
> >> pmc->eventsel can be setup for all known available fixed counters
> >> since we have mapping between fixed pmc index and
> >> the intel_arch_events array.
> >>
> >> Either gp or fixed counter, it will simplify the later checks for
> >> consistency between eventsel and perf_hw_id.
> >>
> >> Signed-off-by: Like Xu <likexu@tencent.com>
> >> ---
> >>   arch/x86/kvm/vmx/pmu_intel.c | 16 ++++++++++++++++
> >>   1 file changed, 16 insertions(+)
> >>
> >> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> >> index 1b7456b2177b..b7ab5fd03681 100644
> >> --- a/arch/x86/kvm/vmx/pmu_intel.c
> >> +++ b/arch/x86/kvm/vmx/pmu_intel.c
> >> @@ -459,6 +459,21 @@ static int intel_pmu_set_msr(struct kvm_vcpu *vcpu,
> >> struct msr_data *msr_info)
> >>          return 1;
> >>   }
> >>
> >> +static void setup_fixed_pmc_eventsel(struct kvm_pmu *pmu)
> >> +{
> >> +       size_t size = ARRAY_SIZE(fixed_pmc_events);
> >> +       struct kvm_pmc *pmc;
> >> +       u32 event;
> >> +       int i;
> >> +
> >> +       for (i = 0; i < pmu->nr_arch_fixed_counters; i++) {
> >> +               pmc = &pmu->fixed_counters[i];
> >> +               event = fixed_pmc_events[array_index_nospec(i, size)];
> >>
> >
> > How do we know that i < size? For example, Ice Lake supports 4
> > fixed counters, but fixed_pmc_events only has three entries.
>
> With the help of macro MAX_FIXED_COUNTERS,
> the fourth or more fixed counter is currently not supported in KVM.

Thanks for the hint. I see it now.

> If the user space sets a super set of CPUID supported by KVM,
> any pmu emulation failure is to be expected, right ?

Actually, I would expect a misconfigured VM to elicit an error. I
don't see the advantage of mis-emulating an unsupported configuration.
But maybe that's just me.

> Waiting for more comments from you on this patch set.

I'll try to get to them this week. Thanks for following up while I was
on holiday.

> >
> >
> >> +               pmc->eventsel = (intel_arch_events[event].unit_mask << 8) |
> >> +                       intel_arch_events[event].eventsel;
> >> +       }
> >> +}
> >> +
> >>
> >>
> >>
> >