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.
