Re: [PATCH 3/6] KVM: x86/pmu: Track enabled AMD PMCs with Host-Only xor Guest-Only bits set

[Sean Christopherson <seanjc@google.com>]

On Wed, Jan 21, 2026, Jim Mattson wrote:
> Add pmc_hostonly and pmc_guestonly bitmaps to struct kvm_pmu to track which
> guest-enabled performance counters have just one of the Host-Only and
> Guest-Only event selector bits set. PMCs that are disabled, have neither
> HG_ONLY bit set, or have both HG_ONLY bits set are not tracked, because
> they don't require special handling at vCPU state transitions.

Why bother with bitmaps?  The bitmaps are basically just eliding the checks in
amd_pmc_is_active() (my name), and those checks are super fast compared to
emulating transitions between L1 and L2.

Can't we simply do:

  void amd_pmu_refresh_host_guest_eventsels(struct kvm_vcpu *vcpu)
  {
	struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
	struct kvm_pmc *pmc;
	int i;

	kvm_for_each_pmc(pmu, pmc, i, pmu->all_valid_pmc_idx)
		amd_pmu_set_eventsel_hw(pmc);

  }

And then call that helper on all transitions?

> +static void amd_pmu_update_hg_bitmaps(struct kvm_pmc *pmc)
> +{
> +	struct kvm_pmu *pmu = pmc_to_pmu(pmc);
> +	u64 eventsel = pmc->eventsel;
> +
> +	if (!(eventsel & ARCH_PERFMON_EVENTSEL_ENABLE)) {
> +		bitmap_clear(pmu->pmc_hostonly, pmc->idx, 1);
> +		bitmap_clear(pmu->pmc_guestonly, pmc->idx, 1);
> +		return;
> +	}
> +
> +	switch (eventsel & AMD64_EVENTSEL_HG_ONLY) {
> +	case AMD64_EVENTSEL_HOSTONLY:
> +		bitmap_set(pmu->pmc_hostonly, pmc->idx, 1);
> +		bitmap_clear(pmu->pmc_guestonly, pmc->idx, 1);
> +		break;
> +	case AMD64_EVENTSEL_GUESTONLY:
> +		bitmap_clear(pmu->pmc_hostonly, pmc->idx, 1);
> +		bitmap_set(pmu->pmc_guestonly, pmc->idx, 1);
> +		break;
> +	default:
> +		bitmap_clear(pmu->pmc_hostonly, pmc->idx, 1);
> +		bitmap_clear(pmu->pmc_guestonly, pmc->idx, 1);
> +		break;
> +	}
> +}
> +
>  static bool amd_pmu_dormant_hg_event(struct kvm_pmc *pmc)
>  {
>  	u64 hg_only = pmc->eventsel & AMD64_EVENTSEL_HG_ONLY;
> @@ -196,6 +223,7 @@ static int amd_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  		if (data != pmc->eventsel) {
>  			pmc->eventsel = data;
>  			amd_pmu_set_eventsel_hw(pmc);
> +			amd_pmu_update_hg_bitmaps(pmc);

If we're going to bother adding amd_pmu_set_eventsel_hw(), and not reuse it as
suggested above, then it amd_pmu_set_eventsel_hw() should be renamed to just
amd_pmu_set_eventsel() and it should be the one configuring the bitmaps.  Because
KVM should never write to an eventsel without updating the bitmaps.  That would
also better capture the relationship between the bitmaps and eventsel_hw, e.g.

	pmc->eventsel_hw = (pmc->eventsel & ~AMD64_EVENTSEL_HOSTONLY) |
			   AMD64_EVENTSEL_GUESTONLY;

	if (!amd_pmc_is_active(pmc))
		pmc->eventsel_hw &= ~ARCH_PERFMON_EVENTSEL_ENABLE;

	/*
	 * Update the host/guest bitmaps used to reconfigure eventsel_hw on
	 * transitions to/from an L2 guest, so that KVM can quickly refresh
	 * the event selectors programmed into hardware, e.g. without having
	 * to 
	 */
	if (!(eventsel & ARCH_PERFMON_EVENTSEL_ENABLE)) {
		bitmap_clear(pmu->pmc_hostonly, pmc->idx, 1);
		bitmap_clear(pmu->pmc_guestonly, pmc->idx, 1);
		return;
	}

	switch (eventsel & AMD64_EVENTSEL_HG_ONLY) {
	case AMD64_EVENTSEL_HOSTONLY:
		bitmap_set(pmu->pmc_hostonly, pmc->idx, 1);
		bitmap_clear(pmu->pmc_guestonly, pmc->idx, 1);
		break;
	case AMD64_EVENTSEL_GUESTONLY:
		bitmap_clear(pmu->pmc_hostonly, pmc->idx, 1);
		bitmap_set(pmu->pmc_guestonly, pmc->idx, 1);
		break;
	default:
		bitmap_clear(pmu->pmc_hostonly, pmc->idx, 1);
		bitmap_clear(pmu->pmc_guestonly, pmc->idx, 1);
		break;
	}

But I still don't see any point in the bitmaps.

>  			kvm_pmu_request_counter_reprogram(pmc);
>  		}
>  		return 0;
> -- 
> 2.52.0.457.g6b5491de43-goog
>