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

[Like Xu]

On 7/12/2021 3:50 am, Jim Mattson wrote:
> On Mon, Nov 29, 2021 at 11:42 PM Like Xu <like.xu.linux@xxxxxxxxx> wrote:
>
> > From: Like Xu <likexu@xxxxxxxxxxx>
> >
> > 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@xxxxxxxxxxx>
> > ---
> >   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.

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

Waiting for more comments from you on this patch set.

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