linux-kernel - Re: [PATCH v4 20/38] KVM: x86/pmu: Check if mediated vPMU can intercept rdpmc

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Message-ID: <aCUzKp1uhMsn-g_u@google.com>
Date: Wed, 14 May 2025 17:19:54 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Mingwei Zhang <mizhang@...gle.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, Namhyung Kim <namhyung@...nel.org>, 
	Paolo Bonzini <pbonzini@...hat.com>, 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>, Liang@...gle.com, 
	Kan <kan.liang@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>, 
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org, 
	kvm@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	Yongwei Ma <yongwei.ma@...el.com>, Xiong Zhang <xiong.y.zhang@...ux.intel.com>, 
	Dapeng Mi <dapeng1.mi@...ux.intel.com>, Jim Mattson <jmattson@...gle.com>, 
	Sandipan Das <sandipan.das@....com>, Zide Chen <zide.chen@...el.com>, 
	Eranian Stephane <eranian@...gle.com>, Shukla Manali <Manali.Shukla@....com>, 
	Nikunj Dadhania <nikunj.dadhania@....com>
Subject: Re: [PATCH v4 20/38] KVM: x86/pmu: Check if mediated vPMU can
 intercept rdpmc

The shortlog is wildly inaccurate.  KVM is not simply checking, KVM is actively
disabling RDPMC interception.  *That* needs to be the focus of the shortlog and
changelog.

> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> index 92c742ead663..6ad71752be4b 100644
> --- a/arch/x86/kvm/pmu.c
> +++ b/arch/x86/kvm/pmu.c
> @@ -604,6 +604,40 @@ int kvm_pmu_rdpmc(struct kvm_vcpu *vcpu, unsigned idx, u64 *data)
>  	return 0;
>  }
>  
> +inline bool kvm_rdpmc_in_guest(struct kvm_vcpu *vcpu)

Strongly prefer kvm_need_rdpmc_intercept(), e.g. to follow vmx_need_pf_intercept(),
and because it makes the users more obviously correct.  The "in_guest" terminology
from kvm_{hlt,mwait,pause,cstate}_in_guest() isn't great, but at least in those
flows it's not awful because they are very direct reflections of knobs that control
interception, whereas this helper is making a variety of runtime checks.

> +{
> +	struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
> +
> +	if (!kvm_mediated_pmu_enabled(vcpu))
> +		return false;
> +
> +	/*
> +	 * VMware allows access to these Pseduo-PMCs even when read via RDPMC
> +	 * in Ring3 when CR4.PCE=0.
> +	 */
> +	if (enable_vmware_backdoor)
> +		return false;
> +
> +	/*
> +	 * FIXME: In theory, perf metrics is always combined with fixed
> +	 *	  counter 3. it's fair enough to compare the guest and host
> +	 *	  fixed counter number and don't need to check perf metrics
> +	 *	  explicitly. However kvm_pmu_cap.num_counters_fixed is limited
> +	 *	  KVM_MAX_NR_FIXED_COUNTERS (3) as fixed counter 3 is not
> +	 *	  supported now. perf metrics is still needed to be checked
> +	 *	  explicitly here. Once fixed counter 3 is supported, the perf
> +	 *	  metrics checking can be removed.
> +	 */

And then what happens when hardware supported fixed counter #4?  KVM has the same
problem, and we can't check for features that KVM doesn't know about.

The entire problem is that this code is checking for *KVM* support, but what the
guest can see and access needs to be checked against *hardware* support.  Handling
that is simple, just take a snapshot of the host PMU capabilities before KVM
generates kvm_pmu_cap, and use the unadulterated snapshot here (and everywhere
else with similar checks).

> +	return pmu->nr_arch_gp_counters == kvm_pmu_cap.num_counters_gp &&
> +	       pmu->nr_arch_fixed_counters == kvm_pmu_cap.num_counters_fixed &&
> +	       vcpu_has_perf_metrics(vcpu) == kvm_host_has_perf_metrics() &&
> +	       pmu->counter_bitmask[KVM_PMC_GP] ==
> +				(BIT_ULL(kvm_pmu_cap.bit_width_gp) - 1) &&
> +	       pmu->counter_bitmask[KVM_PMC_FIXED] ==
> +				(BIT_ULL(kvm_pmu_cap.bit_width_fixed) - 1);
> +}
> @@ -212,6 +212,18 @@ static void amd_pmu_refresh(struct kvm_vcpu *vcpu)
>  	bitmap_set(pmu->all_valid_pmc_idx, 0, pmu->nr_arch_gp_counters);
>  }
>  
> +static void amd_pmu_refresh(struct kvm_vcpu *vcpu)
> +{
> +	struct vcpu_svm *svm = to_svm(vcpu);
> +
> +	__amd_pmu_refresh(vcpu);

To better communicate the roles of the two paths to refresh():

	amd_pmu_refresh_capabilities(vcpu);

	amd_pmu_refresh_controls(vcpu);

Ditto for Intel.