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>

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

@@ -56,6 +56,7 @@ void kvm_vgic_early_init(struct kvm *kvm)
 	INIT_LIST_HEAD(&dist->lpi_list_head);
 	INIT_LIST_HEAD(&dist->lpi_translation_cache);
 	raw_spin_lock_init(&dist->lpi_list_lock);
+	xa_init_flags(&dist->lpi_xa, XA_FLAGS_LOCK_IRQ);
 }
 
 /* CREATION */
@@ -366,6 +367,8 @@ static void kvm_vgic_dist_destroy(struct kvm *kvm)
 
 	if (vgic_supports_direct_msis(kvm))
 		vgic_v4_teardown(kvm);
+
+	xa_destroy(&dist->lpi_xa);
 }
 
 static void __kvm_vgic_vcpu_destroy(struct kvm_vcpu *vcpu)

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

@@ -52,6 +52,12 @@ static struct vgic_irq *vgic_add_lpi(struct kvm *kvm, u32 intid,
 	if (!irq)
 		return ERR_PTR(-ENOMEM);
 
+	ret = xa_reserve_irq(&dist->lpi_xa, intid, GFP_KERNEL_ACCOUNT);
+	if (ret) {
+		kfree(irq);
+		return ERR_PTR(ret);
+	}
+
 	INIT_LIST_HEAD(&irq->lpi_list);
 	INIT_LIST_HEAD(&irq->ap_list);
 	raw_spin_lock_init(&irq->irq_lock);
@@ -86,12 +92,22 @@ static struct vgic_irq *vgic_add_lpi(struct kvm *kvm, u32 intid,
 		goto out_unlock;
 	}
 
+	ret = xa_err(xa_store(&dist->lpi_xa, intid, irq, 0));
+	if (ret) {
+		xa_release(&dist->lpi_xa, intid);
+		kfree(irq);
+		goto out_unlock;
+	}
+
 	list_add_tail(&irq->lpi_list, &dist->lpi_list_head);
 	dist->lpi_list_count++;
 
 out_unlock:
 	raw_spin_unlock_irqrestore(&dist->lpi_list_lock, flags);
 
+	if (ret)
+		return ERR_PTR(ret);
+
 	/*
 	 * We "cache" the configuration table entries in our struct vgic_irq's.
 	 * However we only have those structs for mapped IRQs, so we read in

arch/arm64/kvm/vgic/vgic.c

@@ -30,7 +30,8 @@ struct vgic_global kvm_vgic_global_state __ro_after_init = {
  *         its->its_lock (mutex)
  *           vgic_cpu->ap_list_lock		must be taken with IRQs disabled
  *             kvm->lpi_list_lock		must be taken with IRQs disabled
- *               vgic_irq->irq_lock		must be taken with IRQs disabled
+ *               vgic_dist->lpi_xa.xa_lock	must be taken with IRQs disabled
+ *                 vgic_irq->irq_lock		must be taken with IRQs disabled
  *
  * As the ap_list_lock might be taken from the timer interrupt handler,
  * we have to disable IRQs before taking this lock and everything lower
@@ -131,6 +132,7 @@ void __vgic_put_lpi_locked(struct kvm *kvm, struct vgic_irq *irq)
 		return;
 
 	list_del(&irq->lpi_list);
+	xa_erase(&dist->lpi_xa, irq->intid);
 	dist->lpi_list_count--;
 
 	kfree(irq);

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;