KVM: arm64: vgic: Use atomics to count LPIs

Switch to using atomics for LPI accounting, allowing vgic_irq references
to be dropped in parallel.
Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20240221054253.3848076-7-oliver.upton@linux.devSigned-off-by: Oliver Upton <oliver.upton@linux.dev>

arch/arm64/kvm/vgic/vgic-debug.c

@@ -149,7 +149,7 @@ static void print_dist_state(struct seq_file *s, struct vgic_dist *dist)
 	seq_printf(s, "vgic_model:\t%s\n", v3 ? "GICv3" : "GICv2");
 	seq_printf(s, "nr_spis:\t%d\n", dist->nr_spis);
 	if (v3)
-		seq_printf(s, "nr_lpis:\t%d\n", dist->lpi_list_count);
+		seq_printf(s, "nr_lpis:\t%d\n", atomic_read(&dist->lpi_count));
 	seq_printf(s, "enabled:\t%d\n", dist->enabled);
 	seq_printf(s, "\n");
 

arch/arm64/kvm/vgic/vgic-its.c

@@ -96,7 +96,7 @@ static struct vgic_irq *vgic_add_lpi(struct kvm *kvm, u32 intid,
 		goto out_unlock;
 	}
 
-	dist->lpi_list_count++;
+	atomic_inc(&dist->lpi_count);
 
 out_unlock:
 	raw_spin_unlock_irqrestore(&dist->lpi_list_lock, flags);
@@ -346,7 +346,7 @@ int vgic_copy_lpi_list(struct kvm *kvm, struct kvm_vcpu *vcpu, u32 **intid_ptr)
 	 * command). If coming from another path (such as enabling LPIs),
 	 * we must be careful not to overrun the array.
 	 */
-	irq_count = READ_ONCE(dist->lpi_list_count);
+	irq_count = atomic_read(&dist->lpi_count);
 	intids = kmalloc_array(irq_count, sizeof(intids[0]), GFP_KERNEL_ACCOUNT);
 	if (!intids)
 		return -ENOMEM;

arch/arm64/kvm/vgic/vgic.c

@@ -123,7 +123,7 @@ void __vgic_put_lpi_locked(struct kvm *kvm, struct vgic_irq *irq)
 		return;
 
 	xa_erase(&dist->lpi_xa, irq->intid);
-	dist->lpi_list_count--;
+	atomic_dec(&dist->lpi_count);
 
 	kfree(irq);
 }

include/kvm/arm_vgic.h

@@ -273,10 +273,10 @@ struct vgic_dist {
 	 */
 	u64			propbaser;
 
-	/* Protects the lpi_list and the count value below. */
+	/* Protects the lpi_list. */
 	raw_spinlock_t		lpi_list_lock;
 	struct xarray		lpi_xa;
-	int			lpi_list_count;
+	atomic_t		lpi_count;
 
 	/* LPI translation cache */
 	struct list_head	lpi_translation_cache;