linux-kernel - Re: [PATCH v4 30/38] KVM: x86/pmu: Handle emulated instruction for mediated vPMU

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Message-ID: <aCaQedEhZwj9BsVK@google.com>
Date: Thu, 15 May 2025 18:10:17 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Mingwei Zhang <mizhang@...gle.com>
Cc: 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>, Mark Rutland <mark.rutland@....com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, 
	Ian Rogers <irogers@...gle.com>, Adrian Hunter <adrian.hunter@...el.com>, Liang@...gle.com, 
	Kan <kan.liang@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>, 
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org, 
	kvm@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	Yongwei Ma <yongwei.ma@...el.com>, Xiong Zhang <xiong.y.zhang@...ux.intel.com>, 
	Dapeng Mi <dapeng1.mi@...ux.intel.com>, Jim Mattson <jmattson@...gle.com>, 
	Sandipan Das <sandipan.das@....com>, Zide Chen <zide.chen@...el.com>, 
	Eranian Stephane <eranian@...gle.com>, Shukla Manali <Manali.Shukla@....com>, 
	Nikunj Dadhania <nikunj.dadhania@....com>
Subject: Re: [PATCH v4 30/38] KVM: x86/pmu: Handle emulated instruction for
 mediated vPMU

On Mon, Mar 24, 2025, Mingwei Zhang wrote:
>  static void kvm_pmu_incr_counter(struct kvm_pmc *pmc)
>  {
> -	pmc->emulated_counter++;
> -	kvm_pmu_request_counter_reprogram(pmc);
> +	struct kvm_vcpu *vcpu = pmc->vcpu;
> +
> +	/*
> +	 * For perf-based PMUs, accumulate software-emulated events separately
> +	 * from pmc->counter, as pmc->counter is offset by the count of the
> +	 * associated perf event. Request reprogramming, which will consult
> +	 * both emulated and hardware-generated events to detect overflow.
> +	 */
> +	if (!kvm_mediated_pmu_enabled(vcpu)) {
> +		pmc->emulated_counter++;
> +		kvm_pmu_request_counter_reprogram(pmc);
> +		return;
> +	}
> +
> +	/*
> +	 * For mediated PMUs, pmc->counter is updated when the vCPU's PMU is
> +	 * put, and will be loaded into hardware when the PMU is loaded. Simply
> +	 * increment the counter and signal overflow if it wraps to zero.
> +	 */
> +	pmc->counter = (pmc->counter + 1) & pmc_bitmask(pmc);
> +	if (!pmc->counter) {

Ugh, this is broken for the fastpath.  If kvm_skip_emulated_instruction() is
invoked by handle_fastpath_set_msr_irqoff() or handle_fastpath_hlt(), KVM may
consume stale information (GLOBAL_CTRL, GLOBAL_STATUS and PMCs), and even if KVM
gets lucky and those are all fresh, the PMC and GLOBAL_STATUS changes won't be
propagated back to hardware.

The best idea I have is to track whether or not the guest may be counting branches
and/or instruction based on eventsels, and then bail from fastpaths that need to
skip instructions.  That flag would also be useful to further optimize
kvm_pmu_trigger_event().

> +		pmc_to_pmu(pmc)->global_status |= BIT_ULL(pmc->idx);
> +		if (pmc_pmi_enabled(pmc))
> +			kvm_make_request(KVM_REQ_PMI, vcpu);
> +	}
>  }
>  
>  static inline bool cpl_is_matched(struct kvm_pmc *pmc)
> -- 
> 2.49.0.395.g12beb8f557-goog
>