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.
