linux-kernel - Re: [PATCH v5 33/44] KVM: x86/pmu: Bypass perf checks when emulating mediated PMU counter accesses

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Message-ID: <affba07d-78e1-41f5-980b-d4df7fa66804@amd.com>
Date: Wed, 13 Aug 2025 15:31:39 +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 33/44] KVM: x86/pmu: Bypass perf checks when emulating
 mediated PMU counter accesses

On 07-08-2025 01:26, Sean Christopherson wrote:
> From: Dapeng Mi <dapeng1.mi@...ux.intel.com>
> 
> When emulating a PMC counter read or write for a mediated PMU, bypass the
> perf checks and emulated_counter logic as the counters aren't proxied
> through perf, i.e. pmc->counter always holds the guest's up-to-date value,
> and thus there's no need to defer emulated overflow checks.
> 
> Suggested-by: Sean Christopherson <seanjc@...gle.com>
> Signed-off-by: Dapeng Mi <dapeng1.mi@...ux.intel.com>
> Co-developed-by: Mingwei Zhang <mizhang@...gle.com>
> Signed-off-by: Mingwei Zhang <mizhang@...gle.com>
> [sean: split from event filtering change, write shortlog+changelog]
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/pmu.c | 5 +++++
>  arch/x86/kvm/pmu.h | 3 +++
>  2 files changed, 8 insertions(+)
> 
> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> index 817ef852bdf9..082d2905882b 100644
> --- a/arch/x86/kvm/pmu.c
> +++ b/arch/x86/kvm/pmu.c
> @@ -377,6 +377,11 @@ static void pmc_update_sample_period(struct kvm_pmc *pmc)
>  
>  void pmc_write_counter(struct kvm_pmc *pmc, u64 val)
>  {
> +	if (kvm_vcpu_has_mediated_pmu(pmc->vcpu)) {
> +		pmc->counter = val & pmc_bitmask(pmc);
> +		return;
> +	}
> +
>  	/*
>  	 * Drop any unconsumed accumulated counts, the WRMSR is a write, not a
>  	 * read-modify-write.  Adjust the counter value so that its value is
> diff --git a/arch/x86/kvm/pmu.h b/arch/x86/kvm/pmu.h
> index 51963a3a167a..1c9d26d60a60 100644
> --- a/arch/x86/kvm/pmu.h
> +++ b/arch/x86/kvm/pmu.h
> @@ -111,6 +111,9 @@ static inline u64 pmc_read_counter(struct kvm_pmc *pmc)
>  {
>  	u64 counter, enabled, running;
>  
> +	if (kvm_vcpu_has_mediated_pmu(pmc->vcpu))
> +		return pmc->counter & pmc_bitmask(pmc);
> +
>  	counter = pmc->counter + pmc->emulated_counter;
>  
>  	if (pmc->perf_event && !pmc->is_paused)

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