linux-kernel - Re: [PATCH v3 3/3] KVM: x86/pmu: Limit the maximum number of supported AMD GP counters

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Date:   Mon, 7 Nov 2022 19:34:49 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Like Xu <like.xu.linux@...il.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     Jim Mattson <jmattson@...gle.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/3] KVM: x86/pmu: Limit the maximum number of
 supported AMD GP counters

On 9/19/22 11:10, Like Xu wrote:
> @@ -506,6 +506,7 @@ struct kvm_pmc {
>   #define MSR_ARCH_PERFMON_PERFCTR_MAX	(MSR_ARCH_PERFMON_PERFCTR0 + KVM_INTEL_PMC_MAX_GENERIC - 1)
>   #define MSR_ARCH_PERFMON_EVENTSEL_MAX	(MSR_ARCH_PERFMON_EVENTSEL0 + KVM_INTEL_PMC_MAX_GENERIC - 1)
>   #define KVM_PMC_MAX_FIXED	3
> +#define KVM_AMD_PMC_MAX_GENERIC	AMD64_NUM_COUNTERS_CORE
>   struct kvm_pmu {

Even though the BUILD_BUG_ON prevents out-of-bounds accesses, this 
should be hardcoded to 6 to avoid mismatches with msrs_to_save_all[].

> +	BUILD_BUG_ON(AMD64_NUM_COUNTERS_CORE > KVM_AMD_PMC_MAX_GENERIC);

This should be KVM_AMD_PMC_MAX_GENERIC > AMD64_NUM_COUNTERS_CORE, not 
the opposite.

Fixed up and changed the commit message to follow:

     The AMD PerfMonV2 specification allows for a maximum of 16 GP
     counters, but currently only 6 pairs of MSRs are accepted by KVM.

     While AMD64_NUM_COUNTERS_CORE is already equal to 6, increasing
     without adjusting msrs_to_save_all[] could result in out-of-bounds
     accesses.  Therefore introduce a macro (named
     KVM_AMD_PMC_MAX_GENERIC) to refer to the number of counters
     supported by KVM.

Paolo