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? > preempt_enable(); > }
