linux-kernel - Re: [PATCH v4 27/38] KVM: x86/pmu: Handle PMU MSRs interception and event filtering

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Message-ID: <aCemClX6rKnVFqLt@google.com>
Date: Fri, 16 May 2025 13:54:34 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Dapeng Mi <dapeng1.mi@...ux.intel.com>
Cc: Mingwei Zhang <mizhang@...gle.com>, 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>, 
	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 27/38] KVM: x86/pmu: Handle PMU MSRs interception and
 event filtering

On Fri, May 16, 2025, Dapeng Mi wrote:
> On 3/25/2025 1:31 AM, Mingwei Zhang wrote:
> > +	if (kvm_mediated_pmu_enabled(pmu_to_vcpu(pmu))) {
> > +		bool allowed = check_pmu_event_filter(pmc);
> > +
> > +		if (pmc_is_gp(pmc)) {
> > +			if (allowed)
> > +				pmc->eventsel_hw |= pmc->eventsel &
> > +						    ARCH_PERFMON_EVENTSEL_ENABLE;
> > +			else
> > +				pmc->eventsel_hw &= ~ARCH_PERFMON_EVENTSEL_ENABLE;
> > +		} else {
> > +			int idx = pmc->idx - KVM_FIXED_PMC_BASE_IDX;
> > +
> > +			if (allowed)
> > +				pmu->fixed_ctr_ctrl_hw = pmu->fixed_ctr_ctrl;
> 
> Sean, just found there is a potential bug here.  The
> "pmu->fixed_ctr_ctrl_hw" should not be assigned to "pmu->fixed_ctr_ctrl"
> here, otherwise the other filtered fixed counter (not this allowed fixed
> counter) could be enabled accidentally.
> 
> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> index ba9d336f1d1d..f32e5f66f73b 100644
> --- a/arch/x86/kvm/pmu.c
> +++ b/arch/x86/kvm/pmu.c
> @@ -473,7 +473,8 @@ static int reprogram_counter(struct kvm_pmc *pmc)
>                         int idx = pmc->idx - KVM_FIXED_PMC_BASE_IDX;
> 
>                         if (allowed)
> -                               pmu->fixed_ctr_ctrl_hw = pmu->fixed_ctr_ctrl;
> +                               pmu->fixed_ctr_ctrl_hw |= pmu->fixed_ctr_ctrl &
> +                                              
> intel_fixed_bits_by_idx(idx, 0xf);

Hmm, I think that's fine, since pmu->fixed_ctr_ctrl should have changed?  But I'd
rather play it safe and do (completely untested):

	if (pmc_is_gp(pmc)) {
		pmc->eventsel_hw &= ~ARCH_PERFMON_EVENTSEL_ENABLE;
		if (allowed)
			pmc->eventsel_hw |= pmc->eventsel &
					    ARCH_PERFMON_EVENTSEL_ENABLE;
	} else {
		u64 mask = intel_fixed_bits_by_idx(pmc->idx - KVM_FIXED_PMC_BASE_IDX, 0xf);

		pmu->fixed_ctr_ctrl_hw &= ~mask;
		if (allowed)
			pmu->fixed_ctr_ctrl_hw |= pmu->fixed_ctr_ctrl & mask;
	}