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

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Message-ID: <27ad4a47-d39f-4210-898c-b55a3e28de67@linux.intel.com>
Date: Thu, 15 May 2025 11:23:07 +0800
From: "Mi, Dapeng" <dapeng1.mi@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.com>,
 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>,
 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

On 5/15/2025 8:19 AM, Sean Christopherson wrote:
> 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.

Sure.

>
>> 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.

Sure.

>
>> +{
>> +	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).

Yes. That's correct. Whether disabling intercept should check against  HW 
instead of KVM PMU capability since host perf subsystem may hide some PMU
features.

>
>> +	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.

Sure.