Re: [PATCH v6 4/8] KVM: x86/pmu: Snapshot host IA32_PERF_CAPABILITIES in kvm_host

[Mi, Dapeng]

Reviewed-by: Dapeng Mi <dapeng1.mi@xxxxxxxxxxxxxxx>


On 6/30/2026 7:19 AM, Zide Chen wrote:
> From: Mingwei Zhang <mizhang@xxxxxxxxxx>
>
> Cache the unadulterated snapshot of perf_capabilities so that KVM can
> compare guest vPMU capabilities against raw hardware capabilities.
>
> For example, if the host supports PERF_METRICS but it is not configured
> for the guest, KVM can use it to determine that RDPMC accesses must be
> intercepted.
>
> Signed-off-by: Mingwei Zhang <mizhang@xxxxxxxxxx>
> Signed-off-by: Zide Chen <zide.chen@xxxxxxxxx>
> ---
> v5: new patch.
> ---
>  arch/x86/include/asm/kvm_host.h | 1 +
>  arch/x86/kvm/vmx/vmx.c          | 8 ++------
>  arch/x86/kvm/x86.c              | 4 ++++
>  3 files changed, 7 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index dc9e4e8bfc07..80f638588bf7 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -347,6 +347,7 @@ struct kvm_host_values {
>  	u64 xss;
>  	u64 s_cet;
>  	u64 arch_capabilities;
> +	u64 perf_capabilities;
>  };
>  extern struct kvm_host_values kvm_host;
>  
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index aded7039bd3e..b736b9ff965b 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -8050,14 +8050,10 @@ void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
>  static __init u64 vmx_get_perf_capabilities(void)
>  {
>  	u64 perf_cap = PERF_CAP_FW_WRITES;
> -	u64 host_perf_cap = 0;
>  
>  	if (!enable_pmu)
>  		return 0;
>  
> -	if (boot_cpu_has(X86_FEATURE_PDCM))
> -		rdmsrq(MSR_IA32_PERF_CAPABILITIES, host_perf_cap);
> -
>  	if (!cpu_feature_enabled(X86_FEATURE_ARCH_LBR) &&
>  	    !enable_mediated_pmu) {
>  		x86_perf_get_lbr(&vmx_lbr_caps);
> @@ -8070,11 +8066,11 @@ static __init u64 vmx_get_perf_capabilities(void)
>  		if (!vmx_lbr_caps.has_callstack)
>  			memset(&vmx_lbr_caps, 0, sizeof(vmx_lbr_caps));
>  		else if (vmx_lbr_caps.nr)
> -			perf_cap |= host_perf_cap & PERF_CAP_LBR_FMT;
> +			perf_cap |= kvm_host.perf_capabilities & PERF_CAP_LBR_FMT;
>  	}
>  
>  	if (vmx_pebs_supported()) {
> -		perf_cap |= host_perf_cap & PERF_CAP_PEBS_MASK;
> +		perf_cap |= kvm_host.perf_capabilities & PERF_CAP_PEBS_MASK;
>  
>  		/*
>  		 * Disallow adaptive PEBS as it is functionally broken, can be
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 8dbc0fa302a8..8e775855f9be 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -7032,6 +7032,10 @@ int kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
>  	if (boot_cpu_has(X86_FEATURE_ARCH_CAPABILITIES))
>  		rdmsrq(MSR_IA32_ARCH_CAPABILITIES, kvm_host.arch_capabilities);
>  
> +	if (boot_cpu_has(X86_FEATURE_PDCM))
> +		rdmsrq_safe(MSR_IA32_PERF_CAPABILITIES,
> +			    &kvm_host.perf_capabilities);
> +
>  	WARN_ON_ONCE(kvm_nr_uret_msrs);
>  
>  	r = ops->hardware_setup();