KVM: arm64: vgic: Store LPIs in an xarray

Using a linked-list for LPIs is less than ideal as it of course requires
iterative searches to find a particular entry. An xarray is a better
data structure for this use case, as it provides faster searches and can
still handle a potentially sparse range of INTID allocations.

Start by storing LPIs in an xarray, punting usage of the xarray to a
subsequent change. The observant among you will notice that we added yet
another lock to the chain of locking order rules; document the ordering
of the xa_lock. Don't worry, we'll get rid of the lpi_list_lock one
day...

Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20240221054253.3848076-2-oliver.upton@linux.dev
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

include/kvm/arm_vgic.h

@@ -13,6 +13,7 @@
 #include <linux/spinlock.h>
 #include <linux/static_key.h>
 #include <linux/types.h>
+#include <linux/xarray.h>
 #include <kvm/iodev.h>
 #include <linux/list.h>
 #include <linux/jump_label.h>
@@ -275,6 +276,7 @@ struct vgic_dist {
 
 	/* Protects the lpi_list and the count value below. */
 	raw_spinlock_t		lpi_list_lock;
+	struct xarray		lpi_xa;
 	struct list_head	lpi_list_head;
 	int			lpi_list_count;