linux-kernel - Re: [PATCH v5 15/44] KVM: SVM: Check pmu->version, not enable

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Message-ID: <dac108d6-ad4a-4447-a67c-91600a5b8a9c@amd.com>
Date: Wed, 13 Aug 2025 15:28:50 +0530
From: Sandipan Das <sandipan.das@....com>
To: Sean Christopherson <seanjc@...gle.com>, Marc Zyngier <maz@...nel.org>,
 Oliver Upton <oliver.upton@...ux.dev>, Tianrui Zhao
 <zhaotianrui@...ngson.cn>, Bibo Mao <maobibo@...ngson.cn>,
 Huacai Chen <chenhuacai@...nel.org>, Anup Patel <anup@...infault.org>,
 Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt
 <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
 Xin Li <xin@...or.com>, "H. Peter Anvin" <hpa@...or.com>,
 Andy Lutomirski <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
 Ingo Molnar <mingo@...hat.com>, Arnaldo Carvalho de Melo <acme@...nel.org>,
 Namhyung Kim <namhyung@...nel.org>, Paolo Bonzini <pbonzini@...hat.com>
Cc: linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
 kvm@...r.kernel.org, loongarch@...ts.linux.dev,
 kvm-riscv@...ts.infradead.org, linux-riscv@...ts.infradead.org,
 linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
 Kan Liang <kan.liang@...ux.intel.com>, Yongwei Ma <yongwei.ma@...el.com>,
 Mingwei Zhang <mizhang@...gle.com>,
 Xiong Zhang <xiong.y.zhang@...ux.intel.com>,
 Dapeng Mi <dapeng1.mi@...ux.intel.com>
Subject: Re: [PATCH v5 15/44] KVM: SVM: Check pmu->version, not enable_pmu,
 when getting PMC MSRs

On 07-08-2025 01:26, Sean Christopherson wrote:
> Gate access to PMC MSRs based on pmu->version, not on kvm->arch.enable_pmu,
> to more accurately reflect KVM's behavior.  This is a glorified nop, as
> pmu->version and pmu->nr_arch_gp_counters can only be non-zero if
> amd_pmu_refresh() is reached, kvm_pmu_refresh() invokes amd_pmu_refresh()
> if and only if kvm->arch.enable_pmu is true, and amd_pmu_refresh() forces
> pmu->version to be 1 or 2.
> 
> I.e. the following holds true:
> 
>   !pmu->nr_arch_gp_counters || kvm->arch.enable_pmu == (pmu->version > 0)
> 
> and so the only way for amd_pmu_get_pmc() to return a non-NULL value is if
> both kvm->arch.enable_pmu and pmu->version evaluate to true.
> 
> No real functional change intended.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/svm/pmu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/svm/pmu.c b/arch/x86/kvm/svm/pmu.c
> index 288f7f2a46f2..7b8577f3c57a 100644
> --- a/arch/x86/kvm/svm/pmu.c
> +++ b/arch/x86/kvm/svm/pmu.c
> @@ -41,7 +41,7 @@ static inline struct kvm_pmc *get_gp_pmc_amd(struct kvm_pmu *pmu, u32 msr,
>  	struct kvm_vcpu *vcpu = pmu_to_vcpu(pmu);
>  	unsigned int idx;
>  
> -	if (!vcpu->kvm->arch.enable_pmu)
> +	if (!pmu->version)
>  		return NULL;
>  
>  	switch (msr) {

Reviewed-by: Sandipan Das <sandipan.das@....com>