KVM: arm/arm64: timer: Add active state caching

Programming the active state in the (re)distributor can be an
expensive operation so it makes some sense to try and reduce
the number of accesses as much as possible. So far, we
program the active state on each VM entry, but there is some
opportunity to do less.

An obvious solution is to cache the active state in memory,
and only program it in the HW when conditions change. But
because the HW can also change things under our feet (the active
state can transition from 1 to 0 when the guest does an EOI),
some precautions have to be taken, which amount to only caching
an "inactive" state, and always programing it otherwise.

With this in place, we observe a reduction of around 700 cycles
on a 2GHz GICv2 platform for a NULL hypercall.

Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

include/kvm/arm_arch_timer.h

@@ -55,6 +55,9 @@ struct arch_timer_cpu {
 
 	/* VGIC mapping */
 	struct irq_phys_map		*map;
+
+	/* Active IRQ state caching */
+	bool				active_cleared_last;
 };
 
 int kvm_timer_hyp_init(void);
@@ -74,4 +77,6 @@ bool kvm_timer_should_fire(struct kvm_vcpu *vcpu);
 void kvm_timer_schedule(struct kvm_vcpu *vcpu);
 void kvm_timer_unschedule(struct kvm_vcpu *vcpu);
 
+void kvm_timer_vcpu_put(struct kvm_vcpu *vcpu);
+
 #endif