Re: [PATCH v2 3/4] KVM: arm64: Add KVM_ARM_VCPU_PMU_V3_SET_PMU attribute

[Alexandru Elisei <alexandru.elisei@arm.com>]

Hi,

On Wed, Dec 08, 2021 at 12:23:44PM +0000, Alexandru Elisei wrote:
> 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@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@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.

I had a look and I don't think there's a way to enforce this, as there are
no restrictions on when a VCPU can be created. KVM must support only a
subset of VCPUs having a PMU set.

Thanks,
Alex

> 
> 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
> > >
> > > _______________________________________________
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> > > kvmarm@lists.cs.columbia.edu
> > > https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
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