Merge tag 'for_linus' of git://git.kernel.org/pub/scm/linux/kernel/gi…

…t/mst/vhost

Pull vhost cleanup and virtio bugfix
 "There's a single change here, fixing a vhost bug where vhost
  initialization fails due to used ring alignment check being too
  strict"

* tag 'for_linus' of git://git.kernel.org/pub/scm/linux/kernel/git/mst/vhost:
  vhost: relax used address alignment
  virtio_ring: document alignment requirements

drivers/vhost/vhost.c

@@ -713,9 +713,13 @@ long vhost_vring_ioctl(struct vhost_dev *d, int ioctl, void __user *argp)
 			r = -EFAULT;
 			break;
 		}
-		if ((a.avail_user_addr & (sizeof *vq->avail->ring - 1)) ||
-		    (a.used_user_addr & (sizeof *vq->used->ring - 1)) ||
-		    (a.log_guest_addr & (sizeof *vq->used->ring - 1))) {
+
+		/* Make sure it's safe to cast pointers to vring types. */
+		BUILD_BUG_ON(__alignof__ *vq->avail > VRING_AVAIL_ALIGN_SIZE);
+		BUILD_BUG_ON(__alignof__ *vq->used > VRING_USED_ALIGN_SIZE);
+		if ((a.avail_user_addr & (VRING_AVAIL_ALIGN_SIZE - 1)) ||
+		    (a.used_user_addr & (VRING_USED_ALIGN_SIZE - 1)) ||
+		    (a.log_guest_addr & (sizeof(u64) - 1))) {
 			r = -EINVAL;
 			break;
 		}

include/uapi/linux/virtio_ring.h

@@ -101,6 +101,13 @@ struct vring {
 	struct vring_used *used;
 };
 
+/* Alignment requirements for vring elements.
+ * When using pre-virtio 1.0 layout, these fall out naturally.
+ */
+#define VRING_AVAIL_ALIGN_SIZE 2
+#define VRING_USED_ALIGN_SIZE 4
+#define VRING_DESC_ALIGN_SIZE 16
+
 /* The standard layout for the ring is a continuous chunk of memory which looks
  * like this.  We assume num is a power of 2.
  *