Re: [PATCH 2/2] KVM: x86/pmu: Remove vcpu_get_perf_capabilities()

Sean Christopherson
2024-01-24T15:52:03Z

On Wed, Jan 24, 2024, Mingwei Zhang wrote:
> Remove vcpu_get_perf_capabilities() helper and directly use the
> vcpu->arch.perf_capabilities which now contains the true value of
> IA32_PERF_CAPABILITIES if exposed to guest (and 0 otherwise). This should
> slightly improve performance by avoiding the runtime check of
> X86_FEATURE_PDCM.

I have a generic in-progress series[*] to more or less solve the performance woes
with guest_cpuid_has(). I would rather keep the current code, even though it's
somewhat odd, as it's possible there are setups that rely on KVM checking PDCM.
E.g. if userspace sets MSRs *after* CPUID and plugs in a non-zero PERF_CAPABILITES.

[*] https://lore.kernel.org/all/20231110235528.1561679-1-seanjc@google.com

> Signed-off-by: Mingwei Zhang <mizhang@google.com>
> ---
> arch/x86/kvm/vmx/pmu_intel.c | 16 ++++------------
> 1 file changed, 4 insertions(+), 12 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> index a6216c874729..7cbee2d16ed9 100644
> --- a/arch/x86/kvm/vmx/pmu_intel.c
> +++ b/arch/x86/kvm/vmx/pmu_intel.c
> @@ -158,17 +158,9 @@ static struct kvm_pmc *intel_rdpmc_ecx_to_pmc(struct kvm_vcpu *vcpu,
> return &counters[array_index_nospec(idx, num_counters)];
> }
>
> -static inline u64 vcpu_get_perf_capabilities(struct kvm_vcpu *vcpu)
> -{
> - if (!guest_cpuid_has(vcpu, X86_FEATURE_PDCM))
> - return 0;
> -
> - return vcpu->arch.perf_capabilities;
> -}
> -
> static inline bool fw_writes_is_enabled(struct kvm_vcpu *vcpu)
> {
> - return (vcpu_get_perf_capabilities(vcpu) & PMU_CAP_FW_WRITES) != 0;
> + return (vcpu->arch.perf_capabilities & PMU_CAP_FW_WRITES) != 0;
> }
>
> static inline struct kvm_pmc *get_fw_gp_pmc(struct kvm_pmu *pmu, u32 msr)
> @@ -207,13 +199,13 @@ static bool intel_is_valid_msr(struct kvm_vcpu *vcpu, u32 msr)
> case MSR_CORE_PERF_FIXED_CTR_CTRL:
> return kvm_pmu_has_perf_global_ctrl(pmu);
> case MSR_IA32_PEBS_ENABLE:
> - ret = vcpu_get_perf_capabilities(vcpu) & PERF_CAP_PEBS_FORMAT;
> + ret = vcpu->arch.perf_capabilities & PERF_CAP_PEBS_FORMAT;
> break;
> case MSR_IA32_DS_AREA:
> ret = guest_cpuid_has(vcpu, X86_FEATURE_DS);
> break;
> case MSR_PEBS_DATA_CFG:
> - perf_capabilities = vcpu_get_perf_capabilities(vcpu);
> + perf_capabilities = vcpu->arch.perf_capabilities;
> ret = (perf_capabilities & PERF_CAP_PEBS_BASELINE) &&
> ((perf_capabilities & PERF_CAP_PEBS_FORMAT) > 3);
> break;
> @@ -577,7 +569,7 @@ static void intel_pmu_refresh(struct kvm_vcpu *vcpu)
> bitmap_set(pmu->all_valid_pmc_idx,
> INTEL_PMC_MAX_GENERIC, pmu->nr_arch_fixed_counters);
>
> - perf_capabilities = vcpu_get_perf_capabilities(vcpu);
> + perf_capabilities = vcpu->arch.perf_capabilities;
> if (cpuid_model_is_consistent(vcpu) &&
> (perf_capabilities & PMU_CAP_LBR_FMT))
> x86_perf_get_lbr(&lbr_desc->records);
> --
> 2.43.0.429.g432eaa2c6b-goog
>