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

[Jim Mattson]

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.

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
>