Subject: Re: [PATCH v2 1/2] KVM: arm64: Add PMU event filtering infrastructure

From: Marc Zyngier <[email-masked]>
Date: Tue, 10 Mar 2020 11:03:14 +0000

Hi Eric,

On 2020-03-09 18:05, Auger Eric wrote:
> Hi Marc,
> 
> On 3/9/20 1:48 PM, Marc Zyngier wrote:
>> It can be desirable to expose a PMU to a guest, and yet not want the
>> guest to be able to count some of the implemented events (because this
>> would give information on shared resources, for example.
>> 
>> For this, let's extend the PMUv3 device API, and offer a way to setup 
>> a
>> bitmap of the allowed events (the default being no bitmap, and thus no
>> filtering).
>> 
>> Userspace can thus allow/deny ranges of event. The default policy
>> depends on the "polarity" of the first filter setup (default deny if 
>> the
>> filter allows events, and default allow if the filter denies events).
>> This allows to setup exactly what is allowed for a given guest.
>> 
>> Note that although the ioctl is per-vcpu, the map of allowed events is
>> global to the VM (it can be setup from any vcpu until the vcpu PMU is
>> initialized).
>> 
>> Signed-off-by: Marc Zyngier <[email-masked]>
>> ---
>>  arch/arm64/include/asm/kvm_host.h |  6 +++
>>  arch/arm64/include/uapi/asm/kvm.h | 16 ++++++
>>  virt/kvm/arm/arm.c                |  2 +
>>  virt/kvm/arm/pmu.c                | 84 
>> +++++++++++++++++++++++++------
>>  4 files changed, 92 insertions(+), 16 deletions(-)
>> 
>> diff --git a/arch/arm64/include/asm/kvm_host.h 
>> b/arch/arm64/include/asm/kvm_host.h
>> index 57fd46acd058..8e63c618688d 100644
>> --- a/arch/arm64/include/asm/kvm_host.h
>> +++ b/arch/arm64/include/asm/kvm_host.h
>> @@ -91,6 +91,12 @@ struct kvm_arch {
>>  	 * supported.
>>  	 */
>>  	bool return_nisv_io_abort_to_user;
>> +
>> +	/*
>> +	 * VM-wide PMU filter, implemented as a bitmap and big enough
>> +	 * for up to 65536 events
>> +	 */
>> +	unsigned long *pmu_filter;
>>  };
>> 
>>  #define KVM_NR_MEM_OBJS     40
>> diff --git a/arch/arm64/include/uapi/asm/kvm.h 
>> b/arch/arm64/include/uapi/asm/kvm.h
>> index ba85bb23f060..7b1511d6ce44 100644
>> --- a/arch/arm64/include/uapi/asm/kvm.h
>> +++ b/arch/arm64/include/uapi/asm/kvm.h
>> @@ -159,6 +159,21 @@ struct kvm_sync_regs {
>>  struct kvm_arch_memory_slot {
>>  };
>> 
>> +/*
>> + * PMU filter structure. Describe a range of events with a particular
>> + * action. To be used with KVM_ARM_VCPU_PMU_V3_FILTER.
>> + */
>> +struct kvm_pmu_event_filter {
>> +	__u16	base_event;
>> +	__u16	nevents;
>> +
>> +#define KVM_PMU_EVENT_ALLOW	0
>> +#define KVM_PMU_EVENT_DENY	1
>> +
>> +	__u8	action;
>> +	__u8	pad[3];
>> +};
>> +
>>  /* for KVM_GET/SET_VCPU_EVENTS */
>>  struct kvm_vcpu_events {
>>  	struct {
>> @@ -329,6 +344,7 @@ struct kvm_vcpu_events {
>>  #define KVM_ARM_VCPU_PMU_V3_CTRL	0
>>  #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_TIMER_CTRL		1
>>  #define   KVM_ARM_VCPU_TIMER_IRQ_VTIMER		0
>>  #define   KVM_ARM_VCPU_TIMER_IRQ_PTIMER		1
>> diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
>> index eda7b624eab8..8d849ac88a44 100644
>> --- a/virt/kvm/arm/arm.c
>> +++ b/virt/kvm/arm/arm.c
>> @@ -164,6 +164,8 @@ void kvm_arch_destroy_vm(struct kvm *kvm)
>>  	free_percpu(kvm->arch.last_vcpu_ran);
>>  	kvm->arch.last_vcpu_ran = NULL;
>> 
>> +	bitmap_free(kvm->arch.pmu_filter);
>> +
>>  	for (i = 0; i < KVM_MAX_VCPUS; ++i) {
>>  		if (kvm->vcpus[i]) {
>>  			kvm_vcpu_destroy(kvm->vcpus[i]);
>> diff --git a/virt/kvm/arm/pmu.c b/virt/kvm/arm/pmu.c
>> index f0d0312c0a55..9f0fd0224d5b 100644
>> --- a/virt/kvm/arm/pmu.c
>> +++ b/virt/kvm/arm/pmu.c
>> @@ -579,10 +579,19 @@ static void kvm_pmu_create_perf_event(struct 
>> kvm_vcpu *vcpu, u64 select_idx)
>> 
>>  	kvm_pmu_stop_counter(vcpu, pmc);
>>  	eventsel = data & ARMV8_PMU_EVTYPE_EVENT;
>> +	if (pmc->idx == ARMV8_PMU_CYCLE_IDX)
>> +		eventsel = ARMV8_PMUV3_PERFCTR_CPU_CYCLES;
> nit:
> 	if (pmc->idx == ARMV8_PMU_CYCLE_IDX)
> 		eventsel = ARMV8_PMUV3_PERFCTR_CPU_CYCLES;
> 	else
> 		eventsel = data & ARMV8_PMU_EVTYPE_EVENT;

You don't like it? ;-)

>> 
>>  	/* Software increment event does't need to be backed by a perf event 
>> */
> nit: while wer are at it fix the does't typo
>> -	if (eventsel == ARMV8_PMUV3_PERFCTR_SW_INCR &&
>> -	    pmc->idx != ARMV8_PMU_CYCLE_IDX)
>> +	if (eventsel == ARMV8_PMUV3_PERFCTR_SW_INCR)
>> +		return;
>> +
>> +	/*
>> +	 * If we have a filter in place and that the event isn't allowed, do
>> +	 * not install a perf event either.
>> +	 */
>> +	if (vcpu->kvm->arch.pmu_filter &&
>> +	    !test_bit(eventsel, vcpu->kvm->arch.pmu_filter))
>>  		return;
>> 
>>  	memset(&attr, 0, sizeof(struct perf_event_attr));
>> @@ -594,8 +603,7 @@ static void kvm_pmu_create_perf_event(struct 
>> kvm_vcpu *vcpu, u64 select_idx)
>>  	attr.exclude_kernel = data & ARMV8_PMU_EXCLUDE_EL1 ? 1 : 0;
>>  	attr.exclude_hv = 1; /* Don't count EL2 events */
>>  	attr.exclude_host = 1; /* Don't count host events */
>> -	attr.config = (pmc->idx == ARMV8_PMU_CYCLE_IDX) ?
>> -		ARMV8_PMUV3_PERFCTR_CPU_CYCLES : eventsel;
>> +	attr.config = eventsel;
> So in that case the guest counter will not increment but the guest does
> not know the counter is not implemented. Can't this lead to bad user
> experience. Shouldn't this disablement be reflected in PMCEID0/1 regs?

The whole point is that we want to keep things hidden from the guest.
Also, PMCEID{0,1} only describe a small set of events (the architected
common events), and not the whole range of microarchitectural events
that the CPU implements.

>> 
>>  	counter = kvm_pmu_get_pair_counter_value(vcpu, pmc);
>> 
>> @@ -735,15 +743,6 @@ int kvm_arm_pmu_v3_enable(struct kvm_vcpu *vcpu)
>> 
>>  static int kvm_arm_pmu_v3_init(struct kvm_vcpu *vcpu)
>>  {
>> -	if (!kvm_arm_support_pmu_v3())
>> -		return -ENODEV;
>> -
>> -	if (!test_bit(KVM_ARM_VCPU_PMU_V3, vcpu->arch.features))
>> -		return -ENXIO;
>> -
>> -	if (vcpu->arch.pmu.created)
>> -		return -EBUSY;
>> -
>>  	if (irqchip_in_kernel(vcpu->kvm)) {
>>  		int ret;
>> 
>> @@ -794,8 +793,19 @@ static bool pmu_irq_is_valid(struct kvm *kvm, int 
>> irq)
>>  	return true;
>>  }
>> 
>> +#define NR_EVENTS	(ARMV8_PMU_EVTYPE_EVENT + 1)
>> +
>>  int kvm_arm_pmu_v3_set_attr(struct kvm_vcpu *vcpu, struct 
>> kvm_device_attr *attr)
>>  {
>> +	if (!kvm_arm_support_pmu_v3())
>> +		return -ENODEV;
>> +
>> +	if (!test_bit(KVM_ARM_VCPU_PMU_V3, vcpu->arch.features))
>> +		return -ENODEV;
> I see you changed -ENXIO into -ENODEV. wanted?

Probably not... but see below.

>> +
>> +	if (vcpu->arch.pmu.created)
>> +		return -EBUSY;
>> +
>>  	switch (attr->attr) {
>>  	case KVM_ARM_VCPU_PMU_V3_IRQ: {
>>  		int __user *uaddr = (int __user *)(long)attr->addr;
>> @@ -804,9 +814,6 @@ int kvm_arm_pmu_v3_set_attr(struct kvm_vcpu *vcpu, 
>> struct kvm_device_attr *attr)
>>  		if (!irqchip_in_kernel(vcpu->kvm))
>>  			return -EINVAL;
>> 
>> -		if (!test_bit(KVM_ARM_VCPU_PMU_V3, vcpu->arch.features))
>> -			return -ENODEV;
>> -

Here's why. I wonder if we already have a problem with the consistency 
of the
error codes returned to userspace.

>>  		if (get_user(irq, uaddr))
>>  			return -EFAULT;
>> 
>> @@ -824,6 +831,50 @@ int kvm_arm_pmu_v3_set_attr(struct kvm_vcpu 
>> *vcpu, struct kvm_device_attr *attr)
>>  		vcpu->arch.pmu.irq_num = irq;
>>  		return 0;
>>  	}
>> +	case KVM_ARM_VCPU_PMU_V3_FILTER: {
>> +		struct kvm_pmu_event_filter __user *uaddr;
>> +		struct kvm_pmu_event_filter filter;
>> +
>> +		uaddr = (struct kvm_pmu_event_filter __user *)(long)attr->addr;
>> +
>> +		if (copy_from_user(&filter, uaddr, sizeof(filter)))
>> +			return -EFAULT;
>> +
>> +		if (((u32)filter.base_event + filter.nevents) > NR_EVENTS ||
> isnt't it >= ?

No, I think this is correct. I want to be able to filter event 0xFFFF, 
for example,
so I have base_event=0xFFFF and nevents=1.

>> +		    (filter.action != KVM_PMU_EVENT_ALLOW &&
>> +		     filter.action != KVM_PMU_EVENT_DENY))
>> +			return -EINVAL;
> -EINVAL is not documented in the API doc.

Good point.

>> +
>> +		mutex_lock(&vcpu->kvm->lock);
>> +
>> +		if (!vcpu->kvm->arch.pmu_filter) {
>> +			vcpu->kvm->arch.pmu_filter = bitmap_alloc(NR_EVENTS, GFP_KERNEL);
>> +			if (!vcpu->kvm->arch.pmu_filter) {
>> +				mutex_unlock(&vcpu->kvm->lock);
>> +				return -ENOMEM;
>> +			}
>> +
>> +			/*
>> +			 * The default depends on the first applied filter.
>> +			 * If it allows events, the default is to deny.
>> +			 * Conversely, if the first filter denies a set of
>> +			 * events, the default is to allow.
>> +			 */
>> +			if (filter.action == KVM_PMU_EVENT_ALLOW)
>> +				bitmap_zero(vcpu->kvm->arch.pmu_filter, NR_EVENTS);
>> +			else
>> +				bitmap_fill(vcpu->kvm->arch.pmu_filter, NR_EVENTS);
>> +		}
>> +
>> +		if (filter.action == KVM_PMU_EVENT_ALLOW)
>> +			bitmap_set(vcpu->kvm->arch.pmu_filter, filter.base_event, 
>> filter.nevents);
>> +		else
>> +			bitmap_clear(vcpu->kvm->arch.pmu_filter, filter.base_event, 
>> filter.nevents);
>> +
>> +		mutex_unlock(&vcpu->kvm->lock);
>> +
>> +		return 0;
>> +	}
>>  	case KVM_ARM_VCPU_PMU_V3_INIT:
>>  		return kvm_arm_pmu_v3_init(vcpu);
>>  	}
>> @@ -860,6 +911,7 @@ int kvm_arm_pmu_v3_has_attr(struct kvm_vcpu *vcpu, 
>> struct kvm_device_attr *attr)
>>  	switch (attr->attr) {
>>  	case KVM_ARM_VCPU_PMU_V3_IRQ:
> not related to this patch but shouldn't we advertise this only with
> in-kernel irqchip?

We do support the PMU without the in-kernel chip, unfortunately... Yes,
supporting this feature was a big mistake.

>>  	case KVM_ARM_VCPU_PMU_V3_INIT:
>> +	case KVM_ARM_VCPU_PMU_V3_FILTER:
>>  		if (kvm_arm_support_pmu_v3() &&
>>  		    test_bit(KVM_ARM_VCPU_PMU_V3, vcpu->arch.features))
>>  			return 0;

Thanks for the review,

         M.
-- 
Jazz is not dead. It just smells funny...

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