Re: [PATCH v7 13/20] KVM: arm64: Apply dynamic guest counter reservations

[James Clark <james.clark@linaro.org>]

On 04/05/2026 10:18 pm, Colton Lewis wrote:
> Apply dynamic guest counter reservations by checking if the requested
> guest mask collides with any events the host has scheduled and calling
> pmu_perf_resched_update() with a hook that updates the mask of
> available counters in between schedule out and schedule in.
> 
> Signed-off-by: Colton Lewis <coltonlewis@google.com>
> ---
>   arch/arm64/kvm/pmu-direct.c  | 69 ++++++++++++++++++++++++++++++++++++
>   include/linux/perf/arm_pmu.h |  1 +
>   2 files changed, 70 insertions(+)
> 
> diff --git a/arch/arm64/kvm/pmu-direct.c b/arch/arm64/kvm/pmu-direct.c
> index 2252d3b905db9..14cc419dbafad 100644
> --- a/arch/arm64/kvm/pmu-direct.c
> +++ b/arch/arm64/kvm/pmu-direct.c
> @@ -100,6 +100,73 @@ u8 kvm_pmu_hpmn(struct kvm_vcpu *vcpu)
>   	return *host_data_ptr(nr_event_counters);
>   }
>   
> +/* Callback to update counter mask between perf scheduling */
> +static void kvm_pmu_update_mask(struct pmu *pmu, void *data)
> +{
> +	struct arm_pmu *arm_pmu = to_arm_pmu(pmu);
> +	unsigned long *new_mask = data;
> +
> +	bitmap_copy(arm_pmu->cntr_mask, new_mask, ARMPMU_MAX_HWEVENTS);
> +}
> +
> +/**
> + * kvm_pmu_set_guest_counters() - Handle dynamic counter reservations
> + * @cpu_pmu: struct arm_pmu to potentially modify
> + * @guest_mask: new guest mask for the pmu
> + *
> + * Check if guest counters will interfere with current host events and
> + * call into perf_pmu_resched_update if a reschedule is required.
> + */
> +static void kvm_pmu_set_guest_counters(struct arm_pmu *cpu_pmu, u64 guest_mask)
> +{
> +	struct pmu_hw_events *cpuc = this_cpu_ptr(cpu_pmu->hw_events);
> +	DECLARE_BITMAP(guest_bitmap, ARMPMU_MAX_HWEVENTS);
> +	DECLARE_BITMAP(new_mask, ARMPMU_MAX_HWEVENTS);
> +	bool need_resched = false;
> +
> +	bitmap_from_arr64(guest_bitmap, &guest_mask, ARMPMU_MAX_HWEVENTS);
> +	bitmap_copy(new_mask, cpu_pmu->hw_cntr_mask, ARMPMU_MAX_HWEVENTS);
> +
> +	if (guest_mask) {
> +		/* Subtract guest counters from available host mask */
> +		bitmap_andnot(new_mask, new_mask, guest_bitmap, ARMPMU_MAX_HWEVENTS);
> +
> +		/* Did we collide with an active host event? */
> +		if (bitmap_intersects(cpuc->used_mask, guest_bitmap, ARMPMU_MAX_HWEVENTS)) {
> +			int idx;
> +
> +			need_resched = true;
> +			cpuc->host_squeezed = true;
> +
> +			/* Look for pinned events that are about to be preempted */
> +			for_each_set_bit(idx, guest_bitmap, ARMPMU_MAX_HWEVENTS) {
> +				if (test_bit(idx, cpuc->used_mask) && cpuc->events[idx] &&
> +				    cpuc->events[idx]->attr.pinned) {
> +					pr_warn_ratelimited("perf: Pinned host event squeezed out by KVM guest PMU partition\n");

Hi Colton,

I get "perf: Pinned host event squeezed out by KVM guest PMU partition" 
even with arm_pmuv3.reserved_host_counters=3 for example. I would have 
expected any non zero value to stop the warning.

I think armv8pmu_get_single_idx() needs to be changed to allocate from 
the high end host counters first. A more complicated option would be 
checking to see if there are any non-pinned counters in the host 
reserved half when a new pinned counter is opened, then swapping the 
places of the new pinned and existing non-pinned counters so pinned 
always prefer being put into the host half. But it's probably not worth 
doing that.

James

> +					break;
> +				}
> +			}
> +		}
> +	} else {
> +		/*
> +		 * Restoring to hw_cntr_mask.
> +		 * Only resched if we previously squeezed an event.
> +		 */
> +		if (cpuc->host_squeezed) {
> +			need_resched = true;
> +			cpuc->host_squeezed = false;
> +		}
> +	}
> +
> +	if (need_resched) {
> +		/* Collision: run full perf reschedule */
> +		perf_pmu_resched_update(&cpu_pmu->pmu, kvm_pmu_update_mask, new_mask);
> +	} else {
> +		/* Host was never using guest counters anyway */
> +		bitmap_copy(cpu_pmu->cntr_mask, new_mask, ARMPMU_MAX_HWEVENTS);
> +	}
> +}
> +
>   /**
>    * kvm_pmu_host_counter_mask() - Compute bitmask of host-reserved counters
>    * @pmu: Pointer to arm_pmu struct
> @@ -218,6 +285,7 @@ void kvm_pmu_load(struct kvm_vcpu *vcpu)
>   
>   	pmu = vcpu->kvm->arch.arm_pmu;
>   	guest_counters = kvm_pmu_guest_counter_mask(pmu);
> +	kvm_pmu_set_guest_counters(pmu, guest_counters);
>   	kvm_pmu_apply_event_filter(vcpu);
>   
>   	for_each_set_bit(i, &guest_counters, ARMPMU_MAX_HWEVENTS) {
> @@ -319,5 +387,6 @@ void kvm_pmu_put(struct kvm_vcpu *vcpu)
>   	val = read_sysreg(pmintenset_el1);
>   	__vcpu_assign_sys_reg(vcpu, PMINTENSET_EL1, val & mask);
>   
> +	kvm_pmu_set_guest_counters(pmu, 0);
>   	preempt_enable();
>   }
> diff --git a/include/linux/perf/arm_pmu.h b/include/linux/perf/arm_pmu.h
> index f7b000bb3eca8..63f88fec5e80f 100644
> --- a/include/linux/perf/arm_pmu.h
> +++ b/include/linux/perf/arm_pmu.h
> @@ -75,6 +75,7 @@ struct pmu_hw_events {
>   
>   	/* Active events requesting branch records */
>   	unsigned int		branch_users;
> +	bool host_squeezed;
>   };
>   
>   enum armpmu_attr_groups {