KVM: arm64: Document PMU filtering API

virtual GIC implementation, this must be done after initializing the in-kernel

irqchip.

1.3 ATTRIBUTE: KVM_ARM_VCPU_PMU_V3_FILTER

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:Parameters: in kvm_device_attr.addr the address for a PMU event filter is a

             pointer to a struct kvm_pmu_event_filter

:Returns:

	 =======  ======================================================

	 -ENODEV: PMUv3 not supported or GIC not initialized

	 -ENXIO:  PMUv3 not properly configured or in-kernel irqchip not

	 	  configured as required prior to calling this attribute

	 -EBUSY:  PMUv3 already initialized

	 -EINVAL: Invalid filter range

	 =======  ======================================================

Request the installation of a PMU event filter described as follows:

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];

};

A filter range is defined as the range [@base_event, @base_event + @nevents),

together with an @action (KVM_PMU_EVENT_ALLOW or KVM_PMU_EVENT_DENY). The

first registered range defines the global policy (global ALLOW if the first

@action is DENY, global DENY if the first @action is ALLOW). Multiple ranges

can be programmed, and must fit within the event space defined by the PMU

architecture (10 bits on ARMv8.0, 16 bits from ARMv8.1 onwards).

Note: "Cancelling" a filter by registering the opposite action for the same

range doesn't change the default action. For example, installing an ALLOW

filter for event range [0:10) as the first filter and then applying a DENY

action for the same range will leave the whole range as disabled.

Restrictions: Event 0 (SW_INCR) is never filtered, as it doesn't count a

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).

2. GROUP: KVM_ARM_VCPU_TIMER_CTRL

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