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 13/05/2026 5:45 pm, Colton Lewis wrote:
>> 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.


> Another thing I forgot to mention is that even with the ratelimited
> warning, this spams the logs any time the host and guest are both using
> the PMU and I'm not sure how useful that is.

I'm sure it does. I'll delete it.

>>>> +                    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 {