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

[Sean Christopherson]

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;
	}