vhost: relax used address alignment

virtio 1.0 only requires used address to be 4 byte aligned,
vhost required 8 bytes (size of vring_used_elem).
Fix up vhost to match that.

Additionally, while vhost correctly requires 8 byte
alignment for log, it's unconnected to used ring:
it's a consequence that log has u64 entries.
Tweak code to make that clearer.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

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;
 		}