Re: [PATCH 2/6] KVM: x86/pmu: Disable HG_ONLY events as appropriate for current vCPU state

[Sean Christopherson <seanjc@google.com>]

On Wed, Jan 21, 2026, Jim Mattson wrote:
> Introduce amd_pmu_dormant_hg_event(), which determines whether an AMD PMC
> should be dormant (i.e. not count) based on the guest's Host-Only and
> Guest-Only event selector bits and the current vCPU state.
> 
> Update amd_pmu_set_eventsel_hw() to clear the event selector's enable bit
> when the event is dormant.
> 
> Signed-off-by: Jim Mattson <jmattson@google.com>
> ---
>  arch/x86/include/asm/perf_event.h |  2 ++
>  arch/x86/kvm/svm/pmu.c            | 23 +++++++++++++++++++++++
>  2 files changed, 25 insertions(+)
> 
> diff --git a/arch/x86/include/asm/perf_event.h b/arch/x86/include/asm/perf_event.h
> index 0d9af4135e0a..7649d79d91a6 100644
> --- a/arch/x86/include/asm/perf_event.h
> +++ b/arch/x86/include/asm/perf_event.h
> @@ -58,6 +58,8 @@
>  #define AMD64_EVENTSEL_INT_CORE_ENABLE			(1ULL << 36)
>  #define AMD64_EVENTSEL_GUESTONLY			(1ULL << 40)
>  #define AMD64_EVENTSEL_HOSTONLY				(1ULL << 41)
> +#define AMD64_EVENTSEL_HG_ONLY				\

I would strongly prefer to avoid the HG acronym, as it's not immediately obvious
that it's HOST_GUEST, and avoiding long lines even with the full HOST_GUEST is
pretty easy.

The name should also have "MASK" at the end to make it more obvious this is a
multi-flag macro, i.e. not a single-flag value.  Otherwise the intent and thus
correctness of code like this isn't obvious:

	if (eventsel & AMD64_EVENTSEL_HG_ONLY)

How about AMD64_EVENTSEL_HOST_GUEST_MASK?

> +	(AMD64_EVENTSEL_HOSTONLY | AMD64_EVENTSEL_GUESTONLY)
>  
>  #define AMD64_EVENTSEL_INT_CORE_SEL_SHIFT		37
>  #define AMD64_EVENTSEL_INT_CORE_SEL_MASK		\
> diff --git a/arch/x86/kvm/svm/pmu.c b/arch/x86/kvm/svm/pmu.c
> index 33c139b23a9e..f619417557f9 100644
> --- a/arch/x86/kvm/svm/pmu.c
> +++ b/arch/x86/kvm/svm/pmu.c
> @@ -147,10 +147,33 @@ static int amd_pmu_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  	return 1;
>  }
>  
> +static bool amd_pmu_dormant_hg_event(struct kvm_pmc *pmc)

I think I would prefer to flip the polarity, even though the only caller would
then need to invert the return value.  Partly because I think we can come up with
a more intuitive name, partly because it'll make the last check in particular
more intutive, i.e. IMO, checking "guest == guest"

	return !!(hg_only & AMD64_EVENTSEL_GUESTONLY) == is_guest_mode(vcpu);

is more obvious than checking "host == guest":

	return !!(hg_only & AMD64_EVENTSEL_GUESTONLY) == is_guest_mode(vcpu);

Maybe amd_pmc_is_active() or amd_pmc_counts_in_current_mode()?

> +{
> +	u64 hg_only = pmc->eventsel & AMD64_EVENTSEL_HG_ONLY;
> +	struct kvm_vcpu *vcpu = pmc->vcpu;
> +
> +	if (hg_only == 0)

!hg_only

In the spirit of avoiding the "hg" acronym, what if we do something like this?

	const u64 HOST_GUEST_MASK = AMD64_EVENTSEL_HOST_GUEST_MASK;
	struct kvm_vcpu *vcpu = pmc->vcpu;
	u64 eventsel = pmc->eventsel;

	/*
	 * PMCs count in both host and guest if neither {HOST,GUEST}_ONLY flags
	 * are set, or if both flags are set.
	 */
	if (!(eventsel & HOST_GUEST_MASK) ||
	    ((eventsel & HOST_GUEST_MASK) == HOST_GUEST_MASK))
		return true;

	/* {HOST,GUEST}_ONLY bits are ignored when SVME is clear. */
	if (!(vcpu->arch.efer & EFER_SVME))
		return true;

	return !!(eventsel & AMD64_EVENTSEL_GUESTONLY) == is_guest_mode(vcpu);

> +		/* Not an HG_ONLY event */

Please don't put comments inside single-line if-statements.  99% of the time
it's easy to put the comment outside of the if-statement, and doing so encourages
a more verbose comment and avoids a "does this if-statement need curly-braces"
debate.

> +		return false;
> +
> +	if (!(vcpu->arch.efer & EFER_SVME))
> +		/* HG_ONLY bits are ignored when SVME is clear */
> +		return false;
> +
> +	/* Always active if both HG_ONLY bits are set */
> +	if (hg_only == AMD64_EVENTSEL_HG_ONLY)

I vote to check this condition at the same time !hg_only is checked.  From a
*very* pedantic perspective, one could argue it's "wrong" to check the bits when
SVME=0, but the purpose of the helper is to detect if the PMC is active or not.
Precisely following the architectural behavior is unnecessary.

> +		return false;
> +
> +	return !!(hg_only & AMD64_EVENTSEL_HOSTONLY) == is_guest_mode(vcpu);
> +}
> +
>  static void amd_pmu_set_eventsel_hw(struct kvm_pmc *pmc)
>  {
>  	pmc->eventsel_hw = (pmc->eventsel & ~AMD64_EVENTSEL_HOSTONLY) |
>  		AMD64_EVENTSEL_GUESTONLY;
> +
> +	if (amd_pmu_dormant_hg_event(pmc))
> +		pmc->eventsel_hw &= ~ARCH_PERFMON_EVENTSEL_ENABLE;
>  }
>  
>  static int amd_pmu_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> -- 
> 2.52.0.457.g6b5491de43-goog
>