[PATCH v2 3/4] KVM: arm64: Add KVM_ARM_VCPU_PMU_V3_SET_PMU attribute
Alexandru Elisei
alexandru.elisei at arm.com
Wed Dec 8 04:23:44 PST 2021
Hi Reiji,
On Tue, Dec 07, 2021 at 07:13:17PM -0800, Reiji Watanabe wrote:
> Hi Alex,
>
> On Mon, Dec 6, 2021 at 9:02 AM Alexandru Elisei
> <alexandru.elisei at arm.com> wrote:
> >
> > When KVM creates an event and there are more than one PMUs present on the
> > system, perf_init_event() will go through the list of available PMUs and
> > will choose the first one that can create the event. The order of the PMUs
> > in the PMU list depends on the probe order, which can change under various
> > circumstances, for example if the order of the PMU nodes change in the DTB
> > or if asynchronous driver probing is enabled on the kernel command line
> > (with the driver_async_probe=armv8-pmu option).
> >
> > Another consequence of this approach is that, on heteregeneous systems,
> > all virtual machines that KVM creates will use the same PMU. This might
> > cause unexpected behaviour for userspace: when a VCPU is executing on
> > the physical CPU that uses this PMU, PMU events in the guest work
> > correctly; but when the same VCPU executes on another CPU, PMU events in
> > the guest will suddenly stop counting.
> >
> > Fortunately, perf core allows user to specify on which PMU to create an
> > event by using the perf_event_attr->type field, which is used by
> > perf_init_event() as an index in the radix tree of available PMUs.
> >
> > Add the KVM_ARM_VCPU_PMU_V3_CTRL(KVM_ARM_VCPU_PMU_V3_SET_PMU) VCPU
> > attribute to allow userspace to specify the arm_pmu that KVM will use when
> > creating events for that VCPU. KVM will make no attempt to run the VCPU on
> > the physical CPUs that share this PMU, leaving it up to userspace to
> > manage the VCPU threads' affinity accordingly.
> >
> > Signed-off-by: Alexandru Elisei <alexandru.elisei at arm.com>
> > ---
> > Documentation/virt/kvm/devices/vcpu.rst | 25 +++++++++++++++++
> > arch/arm64/include/uapi/asm/kvm.h | 1 +
> > arch/arm64/kvm/pmu-emul.c | 37 +++++++++++++++++++++++--
> > include/kvm/arm_pmu.h | 1 +
> > tools/arch/arm64/include/uapi/asm/kvm.h | 1 +
> > 5 files changed, 63 insertions(+), 2 deletions(-)
> >
> > diff --git a/Documentation/virt/kvm/devices/vcpu.rst b/Documentation/virt/kvm/devices/vcpu.rst
> > index 60a29972d3f1..c82be5cbc268 100644
> > --- a/Documentation/virt/kvm/devices/vcpu.rst
> > +++ b/Documentation/virt/kvm/devices/vcpu.rst
> > @@ -104,6 +104,31 @@ hardware event. Filtering event 0x1E (CHAIN) has no effect either, as it
> > isn't strictly speaking an event. Filtering the cycle counter is possible
> > using event 0x11 (CPU_CYCLES).
> >
> > +1.4 ATTRIBUTE: KVM_ARM_VCPU_PMU_V3_SET_PMU
> > +------------------------------------------
> > +
> > +:Parameters: in kvm_device_attr.addr the address to an int representing the PMU
> > + identifier.
> > +
> > +:Returns:
> > +
> > + ======= ===============================================
> > + -EBUSY PMUv3 already initialized
> > + -EFAULT Error accessing the PMU identifier
> > + -ENXIO PMU not found
> > + -ENODEV PMUv3 not supported or GIC not initialized
> > + -ENOMEM Could not allocate memory
> > + ======= ===============================================
> > +
> > +Request that the VCPU uses the specified hardware PMU when creating guest events
> > +for the purpose of PMU emulation. The PMU identifier can be read from the "type"
> > +file for the desired PMU instance under /sys/devices (or, equivalent,
> > +/sys/bus/even_source). This attribute is particularly useful on heterogeneous
> > +systems where there are at least two CPU PMUs on the system.
> > +
> > +Note that KVM will not make any attempts to run the VCPU on the physical CPUs
> > +associated with the PMU specified by this attribute. This is entirely left to
> > +userspace.
> >
> > 2. GROUP: KVM_ARM_VCPU_TIMER_CTRL
> > =================================
> > diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h
> > index b3edde68bc3e..1d0a0a2a9711 100644
> > --- a/arch/arm64/include/uapi/asm/kvm.h
> > +++ b/arch/arm64/include/uapi/asm/kvm.h
> > @@ -362,6 +362,7 @@ struct kvm_arm_copy_mte_tags {
> > #define KVM_ARM_VCPU_PMU_V3_IRQ 0
> > #define KVM_ARM_VCPU_PMU_V3_INIT 1
> > #define KVM_ARM_VCPU_PMU_V3_FILTER 2
> > +#define KVM_ARM_VCPU_PMU_V3_SET_PMU 3
> > #define KVM_ARM_VCPU_TIMER_CTRL 1
> > #define KVM_ARM_VCPU_TIMER_IRQ_VTIMER 0
> > #define KVM_ARM_VCPU_TIMER_IRQ_PTIMER 1
> > diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c
> > index eaaad4c06561..618138c5f792 100644
> > --- a/arch/arm64/kvm/pmu-emul.c
> > +++ b/arch/arm64/kvm/pmu-emul.c
> > @@ -603,6 +603,7 @@ static bool kvm_pmu_counter_is_enabled(struct kvm_vcpu *vcpu, u64 select_idx)
> > static void kvm_pmu_create_perf_event(struct kvm_vcpu *vcpu, u64 select_idx)
> > {
> > struct kvm_pmu *pmu = &vcpu->arch.pmu;
> > + struct arm_pmu *arm_pmu = pmu->arm_pmu;
> > struct kvm_pmc *pmc;
> > struct perf_event *event;
> > struct perf_event_attr attr;
> > @@ -638,8 +639,7 @@ static void kvm_pmu_create_perf_event(struct kvm_vcpu *vcpu, u64 select_idx)
> > return;
> >
> > memset(&attr, 0, sizeof(struct perf_event_attr));
> > - attr.type = PERF_TYPE_RAW;
> > - attr.size = sizeof(attr);
> > + attr.type = arm_pmu ? arm_pmu->pmu.type : PERF_TYPE_RAW;
> > attr.pinned = 1;
> > attr.disabled = !kvm_pmu_counter_is_enabled(vcpu, pmc->idx);
> > attr.exclude_user = data & ARMV8_PMU_EXCLUDE_EL0 ? 1 : 0;
> > @@ -941,6 +941,29 @@ static bool pmu_irq_is_valid(struct kvm *kvm, int irq)
> > return true;
> > }
> >
> > +static int kvm_arm_pmu_v3_set_pmu(struct kvm_vcpu *vcpu, int pmu_id)
> > +{
> > + struct kvm_pmu *kvm_pmu = &vcpu->arch.pmu;
> > + struct arm_pmu_entry *entry;
> > + struct arm_pmu *arm_pmu;
> > + int ret = -ENXIO;
> > +
> > + mutex_lock(&arm_pmus_lock);
> > +
> > + list_for_each_entry(entry, &arm_pmus, entry) {
> > + arm_pmu = entry->arm_pmu;
> > + if (arm_pmu->pmu.type == pmu_id) {
> > + kvm_pmu->arm_pmu = arm_pmu;
>
> Shouldn't kvm->arch.pmuver be updated based on the pmu that
> is used for the guest ?
As far as I can tell, kvm->arch.pmuver is used in kvm_pmu_event_mask() to
get the number of available perf events, which is then used for configuring
events (via the PMEVTYPER_EL0) register or for masking out events when the
guest reads PMCEID{0,1}_EL0; the events that are masked out are the events
that are unsupported by the PMU that perf will choose for creating events.
This series doesn't forbid userspace from setting the PMU for only a subset
of VCPUs, leaving the other VCPUs with the default PMU, so setting
kvm->arch.pmuver to a particular VCPU's PMU is not correct.
I think the correct fix here would be to have kvm_pmu_event_mask() use the
VCPU's PMU PMUVer, and fallback to kvm->arch.pmuver if that isn't set.
This makes me wonder. Should KVM enforce having userspace either not
setting the PMU for any VCPU, either setting it for all VCPUs? I think this
would be a good idea and will reduce complexity in the long run. I also
don't see a use case for userspace choosing to set the PMU for a subset of
VCPUs, leaving the other VCPUs with the default behaviour.
Thanks,
Alex
>
> Thanks,
> Reiji
>
>
> > + ret = 0;
> > + goto out_unlock;
> > + }
> > + }
> > +
> > +out_unlock:
> > + mutex_unlock(&arm_pmus_lock);
> > + return ret;
> > +}
> > +
> > int kvm_arm_pmu_v3_set_attr(struct kvm_vcpu *vcpu, struct kvm_device_attr *attr)
> > {
> > if (!kvm_vcpu_has_pmu(vcpu))
> > @@ -1027,6 +1050,15 @@ int kvm_arm_pmu_v3_set_attr(struct kvm_vcpu *vcpu, struct kvm_device_attr *attr)
> >
> > return 0;
> > }
> > + case KVM_ARM_VCPU_PMU_V3_SET_PMU: {
> > + int __user *uaddr = (int __user *)(long)attr->addr;
> > + int pmu_id;
> > +
> > + if (get_user(pmu_id, uaddr))
> > + return -EFAULT;
> > +
> > + return kvm_arm_pmu_v3_set_pmu(vcpu, pmu_id);
> > + }
> > case KVM_ARM_VCPU_PMU_V3_INIT:
> > return kvm_arm_pmu_v3_init(vcpu);
> > }
> > @@ -1064,6 +1096,7 @@ int kvm_arm_pmu_v3_has_attr(struct kvm_vcpu *vcpu, struct kvm_device_attr *attr)
> > case KVM_ARM_VCPU_PMU_V3_IRQ:
> > case KVM_ARM_VCPU_PMU_V3_INIT:
> > case KVM_ARM_VCPU_PMU_V3_FILTER:
> > + case KVM_ARM_VCPU_PMU_V3_SET_PMU:
> > if (kvm_vcpu_has_pmu(vcpu))
> > return 0;
> > }
> > diff --git a/include/kvm/arm_pmu.h b/include/kvm/arm_pmu.h
> > index e249c5f172aa..ab3046a8f9bb 100644
> > --- a/include/kvm/arm_pmu.h
> > +++ b/include/kvm/arm_pmu.h
> > @@ -34,6 +34,7 @@ struct kvm_pmu {
> > bool created;
> > bool irq_level;
> > struct irq_work overflow_work;
> > + struct arm_pmu *arm_pmu;
> > };
> >
> > struct arm_pmu_entry {
> > diff --git a/tools/arch/arm64/include/uapi/asm/kvm.h b/tools/arch/arm64/include/uapi/asm/kvm.h
> > index b3edde68bc3e..1d0a0a2a9711 100644
> > --- a/tools/arch/arm64/include/uapi/asm/kvm.h
> > +++ b/tools/arch/arm64/include/uapi/asm/kvm.h
> > @@ -362,6 +362,7 @@ struct kvm_arm_copy_mte_tags {
> > #define KVM_ARM_VCPU_PMU_V3_IRQ 0
> > #define KVM_ARM_VCPU_PMU_V3_INIT 1
> > #define KVM_ARM_VCPU_PMU_V3_FILTER 2
> > +#define KVM_ARM_VCPU_PMU_V3_SET_PMU 3
> > #define KVM_ARM_VCPU_TIMER_CTRL 1
> > #define KVM_ARM_VCPU_TIMER_IRQ_VTIMER 0
> > #define KVM_ARM_VCPU_TIMER_IRQ_PTIMER 1
> > --
> > 2.34.1
> >
> > _______________________________________________
> > kvmarm mailing list
> > kvmarm at lists.cs.columbia.edu
> > https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
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