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

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

On 14/05/2026 8:05 pm, Colton Lewis wrote:
> 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.
> 

A warn_once might save someone a few hours of debugging, but we probably 
don't need more than that.

>>>>> +                    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 {