linux-kernel - Re: [PATCH v4 12/12] KVM: x86/cpuid: Add AMD CPUID ExtPerfMonAndDbg leaf 0x80000022

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Message-ID: <ZC92/WkJf9/AnABP@google.com>
Date:   Thu, 6 Apr 2023 18:50:53 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Sandipan Das <sandipan.das@....com>
Subject: Re: [PATCH v4 12/12] KVM: x86/cpuid: Add AMD CPUID ExtPerfMonAndDbg
 leaf 0x80000022

On Tue, Feb 14, 2023, Like Xu wrote:
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index f4a4691b4f4e..2472fa8746c2 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -4916,6 +4916,12 @@ static __init void svm_set_cpu_caps(void)
>  		} else {
>  			/* AMD PMU PERFCTR_CORE CPUID */
>  			kvm_cpu_cap_check_and_set(X86_FEATURE_PERFCTR_CORE);
> +			/*
> +			 * KVM only supports AMD PerfMon V2, even if it supports V3+.

Ha!  A perfect example of why I strongly prefer that changelogs and comments avoid
pronouns.  The above "it" reads as:

			 * KVM only supports AMD PerfMon V2, even if KVM supports V3+.

which is clearly nonsensical.


> +			 * For PerfMon V3+, it's unsafe to expect V2 bit is set or cleared.

If it's unsafe to assume anything v3+ implying v2 support, then it's definitely
not safe to assume that KVM can blindly set v2 without future changes.  I don't
see any reason not to do

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index bd324962bb7e..1192f605ad47 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -756,6 +756,10 @@ void kvm_set_cpu_caps(void)
                F(NULL_SEL_CLR_BASE) | F(AUTOIBRS) | 0 /* PrefetchCtlMsr */
        );
 
+       kvm_cpu_cap_mask(CPUID_8000_0022_EAX,
+               F(PERFMON_V2)
+       );
+
        /*
         * Synthesize "LFENCE is serializing" into the AMD-defined entry in
         * KVM's supported CPUID if the feature is reported as supported by the


and then this code can be:

			if (kvm_pmu_cap.version != 2)
				kvm_cpu_cap_clear(X86_FEATURE_PERFMON_V2);

Ah, but presumably the

		if (kvm_pmu_cap.num_counters_gp < AMD64_NUM_COUNTERS_CORE)

path also needs to clear PERFMON_V2.  I think I'd still vote to grab host CPUID
and clear here (instead of setting).

What is the relationship between PERFCTR_CORE and PERFMON_V2?  E.g. if v2 depends
on having PERFCTR_CORE, then we can do:

	if (enable_pmu) {
		if (kvm_pmu_cap.num_counters_gp < AMD64_NUM_COUNTERS_CORE)
			kvm_pmu_cap.num_counters_gp = AMD64_NUM_COUNTERS;
		else
			kvm_cpu_cap_check_and_set(X86_FEATURE_PERFCTR_CORE);

		if (kvm_pmu_cap.version != 2 ||
		    !kvm_cpu_cap_has(X86_FEATURE_PERFCTR_CORE))
			kvm_cpu_cap_clear(X86_FEATURE_PERFMON_V2);