Re: [PATCH v6 5/8] KVM: x86/pmu: Support PERF_METRICS MSR in mediated vPMU

[Jim Mattson]

On Mon, Jun 29, 2026 at 4:28 PM Zide Chen <zide.chen@xxxxxxxxx> wrote:
>
> From: Dapeng Mi <dapeng1.mi@xxxxxxxxxxxxxxx>
>
> Bit 15 in IA32_PERF_CAPABILITIES indicates that the CPU provides
> built-in support for Topdown Microarchitecture Analysis (TMA) L1
> metrics via the IA32_PERF_METRICS MSR.
>
> Expose this capability only when mediated vPMU is enabled, as emulating
> IA32_PERF_METRICS in the legacy vPMU model is impractical.
>
> Pass IA32_PERF_METRICS through to the guest only when mediated vPMU is
> enabled and bit 15 is set in guest IA32_PERF_CAPABILITIES.  Allow
> kvm_pmu_{get,set}_msr() to handle this MSR for host accesses.
>
> Save and restore this MSR on host/guest PMU context switches so that
> host PMU activity does not clobber the guest value, and guest state
> is not leaked into the host.
>
> Signed-off-by: Dapeng Mi <dapeng1.mi@xxxxxxxxxxxxxxx>
> Signed-off-by: Zide Chen <zide.chen@xxxxxxxxx>
> ---
> v5:
> - Remove host_initiated check in set/get MSR handlers.
> v4:
> - Remove WARN_ON_ONCE() and simply reject the guest accesses by checking
>   host_initiated. (Sashiko)
> - Passthru MSR_PERF_METRICS only if has_mediated_pmu is true. (Sashiko)
> - Remove the redundant !! in vcpu_has_perf_metrics().
> v3:
> - Replace WARN_ON() with WARN_ON_ONCE(). (Dapeng)
> - Add comments to explain why we don't validate writes on PERF_METRICS.
> ---
>  arch/x86/include/asm/kvm_host.h   |  1 +
>  arch/x86/include/asm/msr-index.h  |  1 +
>  arch/x86/include/asm/perf_event.h |  1 +
>  arch/x86/kvm/msrs.c               |  6 +++++-
>  arch/x86/kvm/pmu.h                |  5 +++++
>  arch/x86/kvm/vmx/pmu_intel.c      | 31 +++++++++++++++++++++++++++++++
>  arch/x86/kvm/vmx/vmx.c            |  7 +++++++
>  7 files changed, 51 insertions(+), 1 deletion(-)
>
> ...
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index b736b9ff965b..21eb4b339fa6 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -4273,6 +4273,10 @@ static void vmx_recalc_pmu_msr_intercepts(struct kvm_vcpu *vcpu)
>                                   MSR_TYPE_RW, intercept);
>         vmx_set_intercept_for_msr(vcpu, MSR_CORE_PERF_GLOBAL_OVF_CTRL,
>                                   MSR_TYPE_RW, intercept);
> +
> +       intercept = !has_mediated_pmu || !kvm_vcpu_has_perf_metrics(vcpu);
> +       vmx_set_intercept_for_msr(vcpu, MSR_PERF_METRICS,
> +                                 MSR_TYPE_RW, intercept);
>  }

Should we also update nested_vmx_merge_pmu_msr_bitmaps()?