Re: [PATCH v6 03/20] KVM: x86/pmu: Don't enumerate arch events KVM doesn't support

[Mi, Dapeng]

On 11/4/2023 8:41 PM, Jim Mattson wrote:
> On Fri, Nov 3, 2023 at 5:02 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> > Don't advertise support to userspace for architectural events that KVM
> > doesn't support, i.e. for "real" events that aren't listed in
> > intel_pmu_architectural_events.  On current hardware, this effectively
> > means "don't advertise support for Top Down Slots".
> NR_REAL_INTEL_ARCH_EVENTS is only used in intel_hw_event_available().
> As discussed (https://lore.kernel.org/kvm/ZUU12-TUR_1cj47u@xxxxxxxxxx/),
> intel_hw_event_available() should go away.
>
> Aside from mapping fixed counters to event selector and unit mask
> (fixed_pmc_events[]), KVM has no reason to know when a new
> architectural event is defined.


Since intel_hw_event_available() would be removed, it looks the enum 
intel_pmu_architectural_events and intel_arch_events[] array become 
useless. We can directly simply modify current fixed_pmc_events[] array 
and use it to store fixed counter events code and umask.


> The variable that this change "fixes" is only used to feed
> CPUID.0AH:EBX in KVM_GET_SUPPORTED_CPUID, and kvm_pmu_cap.events_mask
> is already constructed from what host perf advertises support for.
>
> > Mask off the associated "unavailable" bits, as said bits for undefined
> > events are reserved to zero.  Arguably the events _are_ defined, but from
> > a KVM perspective they might as well not exist, and there's absolutely no
> > reason to leave useless unavailable bits set.
> >
> > Fixes: a6c06ed1a60a ("KVM: Expose the architectural performance monitoring CPUID leaf")
> > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > ---
> >   arch/x86/kvm/vmx/pmu_intel.c | 9 +++++++++
> >   1 file changed, 9 insertions(+)
> >
> > diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> > index 3316fdea212a..8d545f84dc4a 100644
> > --- a/arch/x86/kvm/vmx/pmu_intel.c
> > +++ b/arch/x86/kvm/vmx/pmu_intel.c
> > @@ -73,6 +73,15 @@ static void intel_init_pmu_capability(void)
> >          int i;
> >
> >          /*
> > +        * Do not enumerate support for architectural events that KVM doesn't
> > +        * support.  Clear unsupported events "unavailable" bit as well, as
> > +        * architecturally such bits are reserved to zero.
> > +        */
> > +       kvm_pmu_cap.events_mask_len = min(kvm_pmu_cap.events_mask_len,
> > +                                         NR_REAL_INTEL_ARCH_EVENTS);
> > +       kvm_pmu_cap.events_mask &= GENMASK(kvm_pmu_cap.events_mask_len - 1, 0);
> > +
> > +        /*
> >           * Perf may (sadly) back a guest fixed counter with a general purpose
> >           * counter, and so KVM must hide fixed counters whose associated
> >           * architectural event are unsupported.  On real hardware, this should
> > --
> > 2.42.0.869.gea05f2083d-goog