Re: [PATCH 14/21] KVM: arm64: Apply dynamic guest counter reservations

Hi Zide, thanks for the review.

"Chen, Zide" <[email protected]> writes:

> On 6/12/2026 2:29 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 <[email protected]>
>> ---
>>   arch/arm64/kvm/pmu-direct.c  | 69 +++++++++++++++++++++++++++++++++++-
>>   include/linux/perf/arm_pmu.h |  1 +
>>   2 files changed, 69 insertions(+), 1 deletion(-)

>> diff --git a/arch/arm64/kvm/pmu-direct.c b/arch/arm64/kvm/pmu-direct.c
>> index 49f1feb5d280c..044f011c9c84b 100644
>> --- a/arch/arm64/kvm/pmu-direct.c
>> +++ b/arch/arm64/kvm/pmu-direct.c
>> @@ -87,6 +87,73 @@ u64 kvm_pmu_direct_pmcr_read(struct kvm_vcpu *vcpu)
>>   		ARMV8_PMU_PMCR_N);
>>   }

>> +/* 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_impl, 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_once("perf: Pinned host event squeezed out by KVM guest  
>> PMU partition\n");
>> +					break;
>> +				}
>> +			}
>> +		}
>> +	} else {
>> +		/*
>> +		 * Restoring to hw_cntr_impl.
>> +		 * 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
>> @@ -209,6 +276,7 @@ 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) {
>> @@ -317,7 +385,6 @@ void kvm_pmu_put(struct kvm_vcpu *vcpu)
>>   	/* Stop guest counters and disable interrupts in hardware. */
>>   	write_sysreg(mask, pmcntenclr_el0);
>>   	write_sysreg(mask, pmintenclr_el1);
>> -
>>   	kvm_pmu_set_guest_counters(pmu, 0);

> I must have missed something.

> Here, vcpu->kvm->arch.arm_pmu->cntr_mask is restored to cntr_mask_impl.
> Is this same struct shared with the perf driver
> (to_arm_pmu(event->pmu)), and shared across other vCPUs as well?

> If so, if another pCPU still has a PMU-partition-enabled vcpu loaded
> with guest-owned counters (MDCR_EL2.HPMN is set), could a host event be
> scheduled onto one of those guest-owned counters, conflicting with the
> guest?

You're correct that is shared more widely than appropriate and could
cause such a conflict.

cntr_mask should be moved to the per-CPU struct pmu_hw_events inside
struct arm_pmu.

>>   	preempt_enable();
>>   }