linux-kernel - Re: [PATCH v5 38/44] KVM: x86/pmu: Disallow emulation in the fastpath if mediated PMCs are active

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Message-ID: <91412178-04c6-4319-a5bc-a957217fea14@amd.com>
Date: Wed, 13 Aug 2025 15:23:17 +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 38/44] KVM: x86/pmu: Disallow emulation in the fastpath
 if mediated PMCs are active

On 07-08-2025 01:27, Sean Christopherson wrote:
> Don't handle exits in the fastpath if emulation is required, i.e. if an
> instruction needs to be skipped, the mediated PMU is enabled, and one or
> more PMCs is counting instructions.  With the mediated PMU, KVM's cache of
> PMU state is inconsistent with respect to hardware until KVM exits the
> inner run loop (when the mediated PMU is "put").
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/pmu.h | 10 ++++++++++
>  arch/x86/kvm/x86.c |  9 +++++++++
>  2 files changed, 19 insertions(+)
> 
> diff --git a/arch/x86/kvm/pmu.h b/arch/x86/kvm/pmu.h
> index e2e2d8476a3f..a0cd42cbea9d 100644
> --- a/arch/x86/kvm/pmu.h
> +++ b/arch/x86/kvm/pmu.h
> @@ -234,6 +234,16 @@ static inline bool pmc_is_globally_enabled(struct kvm_pmc *pmc)
>  	return test_bit(pmc->idx, (unsigned long *)&pmu->global_ctrl);
>  }
>  
> +static inline bool kvm_pmu_is_fastpath_emulation_allowed(struct kvm_vcpu *vcpu)
> +{
> +	struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
> +
> +	return !kvm_vcpu_has_mediated_pmu(vcpu) ||
> +	       !bitmap_intersects(pmu->pmc_counting_instructions,
> +				  (unsigned long *)&pmu->global_ctrl,
> +				  X86_PMC_IDX_MAX);
> +}
> +
>  void kvm_pmu_deliver_pmi(struct kvm_vcpu *vcpu);
>  void kvm_pmu_handle_event(struct kvm_vcpu *vcpu);
>  int kvm_pmu_rdpmc(struct kvm_vcpu *vcpu, unsigned pmc, u64 *data);
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 7fb94ef64e18..6bdf7ef0b535 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2092,6 +2092,9 @@ EXPORT_SYMBOL_GPL(kvm_emulate_invd);
>  
>  fastpath_t handle_fastpath_invd(struct kvm_vcpu *vcpu)
>  {
> +	if (!kvm_pmu_is_fastpath_emulation_allowed(vcpu))
> +		return EXIT_FASTPATH_NONE;
> +
>  	if (!kvm_emulate_invd(vcpu))
>  		return EXIT_FASTPATH_EXIT_USERSPACE;
>  
> @@ -2151,6 +2154,9 @@ fastpath_t handle_fastpath_set_msr_irqoff(struct kvm_vcpu *vcpu)
>  	u64 data = kvm_read_edx_eax(vcpu);
>  	u32 msr = kvm_rcx_read(vcpu);
>  
> +	if (!kvm_pmu_is_fastpath_emulation_allowed(vcpu))
> +		return EXIT_FASTPATH_NONE;
> +
>  	switch (msr) {
>  	case APIC_BASE_MSR + (APIC_ICR >> 4):
>  		if (!lapic_in_kernel(vcpu) || !apic_x2apic_mode(vcpu->arch.apic) ||
> @@ -11267,6 +11273,9 @@ EXPORT_SYMBOL_GPL(kvm_emulate_halt);
>  
>  fastpath_t handle_fastpath_hlt(struct kvm_vcpu *vcpu)
>  {
> +	if (!kvm_pmu_is_fastpath_emulation_allowed(vcpu))
> +		return EXIT_FASTPATH_NONE;
> +
>  	if (!kvm_emulate_halt(vcpu))
>  		return EXIT_FASTPATH_EXIT_USERSPACE;
>  

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