linux-kernel - Re: [PATCH v9 10/28] KVM: x86/pmu: Explicitly check for RDPMC of unsupported Intel PMC types

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Date:   Mon, 11 Dec 2023 14:26:11 +0800
From:   "Mi, Dapeng" <dapeng1.mi@...ux.intel.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Kan Liang <kan.liang@...ux.intel.com>,
        Jim Mattson <jmattson@...gle.com>,
        Jinrong Liang <cloudliang@...cent.com>,
        Aaron Lewis <aaronlewis@...gle.com>,
        Like Xu <likexu@...cent.com>
Subject: Re: [PATCH v9 10/28] KVM: x86/pmu: Explicitly check for RDPMC of
 unsupported Intel PMC types


On 12/2/2023 8:03 AM, Sean Christopherson wrote:
> Explicitly check for attempts to read unsupported PMC types instead of
> letting the bounds check fail.  Functionally, letting the check fail is
> ok, but it's unnecessarily subtle and does a poor job of documenting the
> architectural behavior that KVM is emulating.
>
> Opportunistically add macros for the type vs. index to further document
> what is going on.
>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   arch/x86/kvm/vmx/pmu_intel.c | 11 +++++++++--
>   1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/pmu_intel.c b/arch/x86/kvm/vmx/pmu_intel.c
> index 644de27bd48a..bd4f4bdf5419 100644
> --- a/arch/x86/kvm/vmx/pmu_intel.c
> +++ b/arch/x86/kvm/vmx/pmu_intel.c
> @@ -23,6 +23,9 @@
>   /* Perf's "BASE" is wildly misleading, this is a single-bit flag, not a base. */
>   #define INTEL_RDPMC_FIXED	INTEL_PMC_FIXED_RDPMC_BASE
>   
> +#define INTEL_RDPMC_TYPE_MASK	GENMASK(31, 16)
> +#define INTEL_RDPMC_INDEX_MASK	GENMASK(15, 0)
> +
>   #define MSR_PMC_FULL_WIDTH_BIT      (MSR_IA32_PMC0 - MSR_IA32_PERFCTR0)
>   
>   static void reprogram_fixed_counters(struct kvm_pmu *pmu, u64 data)
> @@ -82,9 +85,13 @@ static struct kvm_pmc *intel_rdpmc_ecx_to_pmc(struct kvm_vcpu *vcpu,
>   	/*
>   	 * Fixed PMCs are supported on all architectural PMUs.  Note, KVM only
>   	 * emulates fixed PMCs for PMU v2+, but the flag itself is still valid,
> -	 * i.e. let RDPMC fail due to accessing a non-existent counter.
> +	 * i.e. let RDPMC fail due to accessing a non-existent counter.  Reject
> +	 * attempts to read all other types, which are unknown/unsupported.
>   	 */
> -	idx &= ~INTEL_RDPMC_FIXED;
> +	if (idx & INTEL_RDPMC_TYPE_MASK & ~INTEL_RDPMC_FIXED)
> +		return NULL;
> +
> +	idx &= INTEL_RDPMC_INDEX_MASK;
>   	if (fixed) {
>   		counters = pmu->fixed_counters;
>   		num_counters = pmu->nr_arch_fixed_counters;
Reviewed-by: Dapeng Mi <dapeng1.mi@...ux.intel.com>