1 files changed, 49 insertions, 15 deletions

diff --git a/include/uapi/linux/virtio_ring.h b/include/uapi/linux/virtio_ring.h
index 559f42e73315..f8c20d3de8da 100644
--- a/include/uapi/linux/virtio_ring.h
+++ b/include/uapi/linux/virtio_ring.h
@@ -86,15 +86,28 @@
  * at the end of the used ring. Guest should ignore the used->flags field. */
 #define VIRTIO_RING_F_EVENT_IDX		29
 
-/* Virtio ring descriptors: 16 bytes.  These can chain together via "next". */
+/* 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
+
+/**
+ * struct vring_desc - Virtio ring descriptors,
+ * 16 bytes long. These can chain together via @next.
+ *
+ * @addr: buffer address (guest-physical)
+ * @len: buffer length
+ * @flags: descriptor flags
+ * @next: index of the next descriptor in the chain,
+ *        if the VRING_DESC_F_NEXT flag is set. We chain unused
+ *        descriptors via this, too.
+ */
 struct vring_desc {
-	/* Address (guest-physical). */
 	__virtio64 addr;
-	/* Length. */
 	__virtio32 len;
-	/* The flags as indicated above. */
 	__virtio16 flags;
-	/* We chain unused descriptors via this, too */
 	__virtio16 next;
 };
 
@@ -112,28 +125,47 @@ struct vring_used_elem {
 	__virtio32 len;
 };
 
+typedef struct vring_used_elem __attribute__((aligned(VRING_USED_ALIGN_SIZE)))
+	vring_used_elem_t;
+
 struct vring_used {
 	__virtio16 flags;
 	__virtio16 idx;
-	struct vring_used_elem ring[];
+	vring_used_elem_t ring[];
 };
 
+/*
+ * The ring element addresses are passed between components with different
+ * alignments assumptions. Thus, we might need to decrease the compiler-selected
+ * alignment, and so must use a typedef to make sure the aligned attribute
+ * actually takes hold:
+ *
+ * https://gcc.gnu.org/onlinedocs//gcc/Common-Type-Attributes.html#Common-Type-Attributes
+ *
+ * When used on a struct, or struct member, the aligned attribute can only
+ * increase the alignment; in order to decrease it, the packed attribute must
+ * be specified as well. When used as part of a typedef, the aligned attribute
+ * can both increase and decrease alignment, and specifying the packed
+ * attribute generates a warning.
+ */
+typedef struct vring_desc __attribute__((aligned(VRING_DESC_ALIGN_SIZE)))
+	vring_desc_t;
+typedef struct vring_avail __attribute__((aligned(VRING_AVAIL_ALIGN_SIZE)))
+	vring_avail_t;
+typedef struct vring_used __attribute__((aligned(VRING_USED_ALIGN_SIZE)))
+	vring_used_t;
+
 struct vring {
 	unsigned int num;
 
-	struct vring_desc *desc;
+	vring_desc_t *desc;
 
-	struct vring_avail *avail;
+	vring_avail_t *avail;
 
-	struct vring_used *used;
+	vring_used_t *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
+#ifndef VIRTIO_RING_NO_LEGACY
 
 /* The standard layout for the ring is a continuous chunk of memory which looks
  * like this.  We assume num is a power of 2.
@@ -181,6 +213,8 @@ static inline unsigned vring_size(unsigned int num, unsigned long align)
 		+ sizeof(__virtio16) * 3 + sizeof(struct vring_used_elem) * num;
 }
 
+#endif /* VIRTIO_RING_NO_LEGACY */
+
 /* The following is used with USED_EVENT_IDX and AVAIL_EVENT_IDX */
 /* Assuming a given event_idx value from the other side, if
  * we have just incremented index from old to new_idx,