linux-kernel - Re: [PATCH 4/6] KVM: x86/pmu: [De]activate HG_ONLY PMCs at SVME changes and nested transitions

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Message-ID: <aXJWkIw0oSzOmxLS@google.com>
Date: Thu, 22 Jan 2026 08:55:44 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Jim Mattson <jmattson@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, Peter Zijlstra <peterz@...radead.org>, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, Namhyung Kim <namhyung@...nel.org>, 
	Mark Rutland <mark.rutland@....com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, 
	Ian Rogers <irogers@...gle.com>, Adrian Hunter <adrian.hunter@...el.com>, 
	James Clark <james.clark@...aro.org>, Shuah Khan <shuah@...nel.org>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org, 
	linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 4/6] KVM: x86/pmu: [De]activate HG_ONLY PMCs at SVME
 changes and nested transitions

On Wed, Jan 21, 2026, Jim Mattson wrote:
> diff --git a/arch/x86/include/asm/kvm-x86-pmu-ops.h b/arch/x86/include/asm/kvm-x86-pmu-ops.h
> index f0aa6996811f..7b32796213a0 100644
> --- a/arch/x86/include/asm/kvm-x86-pmu-ops.h
> +++ b/arch/x86/include/asm/kvm-x86-pmu-ops.h
> @@ -26,6 +26,7 @@ KVM_X86_PMU_OP_OPTIONAL(cleanup)
>  KVM_X86_PMU_OP_OPTIONAL(write_global_ctrl)
>  KVM_X86_PMU_OP(mediated_load)
>  KVM_X86_PMU_OP(mediated_put)
> +KVM_X86_PMU_OP_OPTIONAL(set_pmc_eventsel_hw_enable)
>  
>  #undef KVM_X86_PMU_OP
>  #undef KVM_X86_PMU_OP_OPTIONAL
> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> index 833ee2ecd43f..1541c201285b 100644
> --- a/arch/x86/kvm/pmu.c
> +++ b/arch/x86/kvm/pmu.c
> @@ -1142,6 +1142,13 @@ void kvm_pmu_branch_retired(struct kvm_vcpu *vcpu)
>  }
>  EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_pmu_branch_retired);
>  
> +void kvm_pmu_set_pmc_eventsel_hw_enable(struct kvm_vcpu *vcpu,
> +				       unsigned long *bitmap, bool enable)
> +{
> +	kvm_pmu_call(set_pmc_eventsel_hw_enable)(vcpu, bitmap, enable);
> +}
> +EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_pmu_set_pmc_eventsel_hw_enable);

Why bounce through a PMU op just to go from nested.c to pmu.c?  AFAICT, common
x86 code never calls kvm_pmu_set_pmc_eventsel_hw_enable(), just wire up calls
directly to amd_pmu_refresh_host_guest_eventsels().

> @@ -1054,6 +1055,11 @@ int nested_svm_vmrun(struct kvm_vcpu *vcpu)
>  	if (enter_svm_guest_mode(vcpu, vmcb12_gpa, vmcb12, true))
>  		goto out_exit_err;
>  
> +	kvm_pmu_set_pmc_eventsel_hw_enable(vcpu,
> +		vcpu_to_pmu(vcpu)->pmc_hostonly, false);
> +	kvm_pmu_set_pmc_eventsel_hw_enable(vcpu,
> +		vcpu_to_pmu(vcpu)->pmc_guestonly, true);
> +
>  	if (nested_svm_merge_msrpm(vcpu))
>  		goto out;
>  
> @@ -1137,6 +1143,10 @@ int nested_svm_vmexit(struct vcpu_svm *svm)
>  
>  	/* Exit Guest-Mode */
>  	leave_guest_mode(vcpu);
> +	kvm_pmu_set_pmc_eventsel_hw_enable(vcpu,
> +		vcpu_to_pmu(vcpu)->pmc_hostonly, true);
> +	kvm_pmu_set_pmc_eventsel_hw_enable(vcpu,
> +		vcpu_to_pmu(vcpu)->pmc_guestonly, false);
>  	svm->nested.vmcb12_gpa = 0;
>  	WARN_ON_ONCE(svm->nested.nested_run_pending);

I don't think these are the right places to hook.  Shouldn't KVM update the
event selectors on _all_ transitions, whether they're architectural or not?  E.g.
by wrapping {enter,leave}_guest_mode()?

static void svm_enter_guest_mode(struct kvm_vcpu *vcpu)
{
	enter_guest_mode(vcpu);
	amd_pmu_refresh_host_guest_eventsels(vcpu);
}

static void svm_leave_guest_mode(struct kvm_vcpu *vcpu)
{
	leave_guest_mode(vcpu);
	amd_pmu_refresh_host_guest_eventsels(vcpu);
}