KVM: vgic-v4: Track the number of VLPIs per vcpu

In order to find out whether a vcpu is likely to be the target of
VLPIs (and to further optimize the way we deal with those), let's
track the number of VLPIs a vcpu can receive.

This gets implemented with an atomic variable that gets incremented
or decremented on map, unmap and move of a VLPI.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Zenghui Yu <yuzenghui@huawei.com>
Reviewed-by: Christoffer Dall <christoffer.dall@arm.com>
Link: https://lore.kernel.org/r/20191107160412.30301-2-maz@kernel.org

include/linux/irqchip/arm-gic-v4.h

@@ -32,6 +32,8 @@ struct its_vm {
 struct its_vpe {
 	struct page 		*vpt_page;
 	struct its_vm		*its_vm;
+	/* per-vPE VLPI tracking */
+	atomic_t		vlpi_count;
 	/* Doorbell interrupt */
 	int			irq;
 	irq_hw_number_t		vpe_db_lpi;