virtio_ring: document alignment requirements

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 <rusty@rustcorp.com.au> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

virtio_ring: document alignment requirements
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 <rusty@rustcorp.com.au> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Michael S. Tsirkin
1 parent b7392d2247
Showing 1 changed file with 7 additions and 0 deletions Side-by-side Diff
include/uapi/linux/virtio_ring.h
@@ -101,6 +101,13 @@
 	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.
  *
...	...	@@ -101,6 +101,13 @@
101	101	struct vring_used *used;
102	102	};
103	103
	104	+/* Alignment requirements for vring elements.
	105	+ * When using pre-virtio 1.0 layout, these fall out naturally.
	106	+ */
	107	+#define VRING_AVAIL_ALIGN_SIZE 2
	108	+#define VRING_USED_ALIGN_SIZE 4
	109	+#define VRING_DESC_ALIGN_SIZE 16
	110	+
104	111	/* The standard layout for the ring is a continuous chunk of memory which looks
105	112	* like this. We assume num is a power of 2.
106	113	*