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Date: Wed, 24 Jan 2024 07:52:00 -0800
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References: <20240124003858.3954822-1-mizhang@google.com> <20240124003858.3954822-3-mizhang@google.com>
Message-ID: <ZbEyIM41MGYTXcK_@google.com>
Subject: Re: [PATCH 2/2] KVM: x86/pmu: Remove vcpu_get_perf_capabilities()
From: Sean Christopherson <seanjc@google.com>
To: Mingwei Zhang <mizhang@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>, kvm@vger.kernel.org, 
	linux-kernel@vger.kernel.org, Aaron Lewis <aaronlewis@google.com>
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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
>