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

[Colton Lewis <coltonlewis@google.com>]

James Clark <james.clark@linaro.org> writes:

> 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


I agree it makes the most sense to allocate from the top, but I'm happy
the basic idea works.

>> +					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 {