KVM: arm64: vgic-its: Spin off helper for finding ITS by doorbell addr

The fast path will soon need to find an ITS by doorbell address, as the
translation caches will become local to an ITS. Spin off a helper to do
just that.
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20240422200158.2606761-10-oliver.upton@linux.devSigned-off-by: Marc Zyngier <maz@kernel.org>

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

@@ -511,9 +511,29 @@ static unsigned long vgic_mmio_read_its_idregs(struct kvm *kvm,
 	return 0;
 }
 
+static struct vgic_its *__vgic_doorbell_to_its(struct kvm *kvm, gpa_t db)
+{
+	struct kvm_io_device *kvm_io_dev;
+	struct vgic_io_device *iodev;
+
+	kvm_io_dev = kvm_io_bus_get_dev(kvm, KVM_MMIO_BUS, db);
+	if (!kvm_io_dev)
+		return ERR_PTR(-EINVAL);
+
+	if (kvm_io_dev->ops != &kvm_io_gic_ops)
+		return ERR_PTR(-EINVAL);
+
+	iodev = container_of(kvm_io_dev, struct vgic_io_device, dev);
+	if (iodev->iodev_type != IODEV_ITS)
+		return ERR_PTR(-EINVAL);
+
+	return iodev->its;
+}
+
 static unsigned long vgic_its_cache_key(u32 devid, u32 eventid)
 {
 	return (((unsigned long)devid) << VITS_TYPER_IDBITS) | eventid;
+
 }
 
 static struct vgic_irq *__vgic_its_check_cache(struct vgic_dist *dist,
@@ -721,8 +741,6 @@ int vgic_its_resolve_lpi(struct kvm *kvm, struct vgic_its *its,
 struct vgic_its *vgic_msi_to_its(struct kvm *kvm, struct kvm_msi *msi)
 {
 	u64 address;
-	struct kvm_io_device *kvm_io_dev;
-	struct vgic_io_device *iodev;
 
 	if (!vgic_has_its(kvm))
 		return ERR_PTR(-ENODEV);
@@ -732,18 +750,7 @@ struct vgic_its *vgic_msi_to_its(struct kvm *kvm, struct kvm_msi *msi)
 
 	address = (u64)msi->address_hi << 32 | msi->address_lo;
 
-	kvm_io_dev = kvm_io_bus_get_dev(kvm, KVM_MMIO_BUS, address);
-	if (!kvm_io_dev)
-		return ERR_PTR(-EINVAL);
-
-	if (kvm_io_dev->ops != &kvm_io_gic_ops)
-		return ERR_PTR(-EINVAL);
-
-	iodev = container_of(kvm_io_dev, struct vgic_io_device, dev);
-	if (iodev->iodev_type != IODEV_ITS)
-		return ERR_PTR(-EINVAL);
-
-	return iodev->its;
+	return __vgic_doorbell_to_its(kvm, address);
 }
 
 /*