KVM: arm/arm64: vgic-v2: Avoid accessing GICH registers

GICv2 registers are *slow*. As in "terrifyingly slow". Which is bad.
But we're equaly bad, as we make a point in accessing them even if
we don't have any interrupt in flight.

A good solution is to first find out if we have anything useful to
write into the GIC, and if we don't, to simply not do it. This
involves tracking which LRs actually have something valid there.

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

include/kvm/arm_vgic.h

@@ -321,6 +321,8 @@ struct vgic_cpu {
 
 	/* Protected by the distributor's irq_phys_map_lock */
 	struct list_head	irq_phys_map_list;
+
+	u64		live_lrs;
 };
 
 #define LR_EMPTY	0xff