linux-kernel - Re: [PATCH v3 6/8] KVM: x86/svm/pmu: Add AMD PerfMonV2 support

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Message-ID: <Y9BzYzEjAwUA+wuy@google.com>
Date:   Wed, 25 Jan 2023 00:10:11 +0000
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 v3 6/8] KVM: x86/svm/pmu: Add AMD PerfMonV2 support

On Fri, Nov 11, 2022, Like Xu wrote:
On Fri, Nov 11, 2022, Like Xu wrote:
> @@ -162,20 +179,42 @@ static int amd_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  static void amd_pmu_refresh(struct kvm_vcpu *vcpu)
>  {
>       struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
> +     struct kvm_cpuid_entry2 *entry;
> +     union cpuid_0x80000022_ebx ebx;
>
> -     if (guest_cpuid_has(vcpu, X86_FEATURE_PERFCTR_CORE))
> -             pmu->nr_arch_gp_counters = AMD64_NUM_COUNTERS_CORE;
> +     pmu->version = 1;
> +     if (kvm_cpu_cap_has(X86_FEATURE_AMD_PMU_V2) &&

Why check kvm_cpu_cap support?  I.e. what will go wrong if userspace enumerates
PMU v2 to the guest without proper hardware/KVM support.

If this is _necessary_ to protect the host kernel, then we should probably have
a helper to query PMU features, e.g.

static __always_inline bool guest_pmu_has(struct kvm_vcpu *vcpu,
                                          unsigned int x86_feature)
{
        return kvm_cpu_cap_has(x86_feature) &&
               guest_cpuid_has(vcpu, x86_feature);
}

> +         guest_cpuid_has(vcpu, X86_FEATURE_AMD_PMU_V2)) {
> +             pmu->version = 2;
> +             entry = kvm_find_cpuid_entry_index(vcpu, 0x80000022, 0);
> +             ebx.full = entry->ebx;
> +             pmu->nr_arch_gp_counters = min3((unsigned int)ebx.split.num_core_pmc,
> +                                             (unsigned int)kvm_pmu_cap.num_counters_gp,
> +                                             (unsigned int)KVM_AMD_PMC_MAX_GENERIC);

Blech.  This really shouldn't be necessary, KVM should tweak kvm_pmu_cap.num_counters_gp
as needed during initialization to ensure num_counters_gp doesn't exceed KVM's
internal limits.

Posted a patch[*], please take a look.  As mentioned in that thread, I'll somewhat
speculatively apply that series sooner than later so that you can use it a base
for this series (assuming the patch isn't busted).

[*] https://lore.kernel.org/all/20230124234905.3774678-2-seanjc@google.com

> +     }
> +
> +     /* Commitment to minimal PMCs, regardless of CPUID.80000022 */

Please expand this comment.  I'm still not entirely sure I've interpreted it correctly,
and I'm not sure that I agree with the code.

> +     if (kvm_cpu_cap_has(X86_FEATURE_PERFCTR_CORE) &&

AFAICT, checking kvm_cpu_cap_has() is an unrelated change.  Either it's a bug fix
and belongs in a separate patch, or it's unnecessary and should be dropped.

> +         guest_cpuid_has(vcpu, X86_FEATURE_PERFCTR_CORE))
> +             pmu->nr_arch_gp_counters = max_t(unsigned int,
> +                                              pmu->nr_arch_gp_counters,
> +                                              AMD64_NUM_COUNTERS_CORE);
>       else
> -             pmu->nr_arch_gp_counters = AMD64_NUM_COUNTERS;
> +             pmu->nr_arch_gp_counters = max_t(unsigned int,
> +                                              pmu->nr_arch_gp_counters,
> +                                              AMD64_NUM_COUNTERS);

Using max() doesn't look right.  E.g. if KVM ends up running on some odd setup
where ebx.split.num_core_pmc/kvm_pmu_cap.num_counters_gp is less than
AMD64_NUM_COUNTERS_CORE or AMD64_NUM_COUNTERS.

Or more likely, if userspace says "only expose N counters to this guest".

Shouldn't this be something like?

	if (guest_cpuid_has(vcpu, X86_FEATURE_AMD_PMU_V2))
		pmu->nr_arch_gp_counters = min(ebx.split.num_core_pmc,
					       kvm_pmu_cap.num_counters_gp);
	else if (guest_cpuid_has(vcpu, X86_FEATURE_PERFCTR_CORE))
		pmu->nr_arch_gp_counters = AMD64_NUM_COUNTERS_CORE;
	else
		pmu->nr_arch_gp_counters = AMD64_NUM_COUNTERSE;