virtio_ring: document alignment requirements

mstsirkin · mstsirkin · commit b28e0506fafd · 2014-12-29T10:54:54.000+02:00
Host needs to know vring element alignment requirements:
simply doing alignof on structures doesn't work reliably: on some
platforms gcc has alignof(uint32_t) == 2.

Add macros for alignment as specified in virtio 1.0 cs01,
export them to userspace as well.

Acked-by: Rusty Russell &lt;rusty@rustcorp.com.au&gt;
Signed-off-by: Michael S. Tsirkin &lt;mst@redhat.com&gt;
diff --git a/include/uapi/linux/virtio_ring.h b/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.
  *
