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authorMichael S. Tsirkin <mst@redhat.com>2014-12-25 16:03:27 +0200
committerMichael S. Tsirkin <mst@redhat.com>2014-12-29 10:54:54 +0200
commitb28e0506fafd9c987bba7a6a71ea02a37fcabdea (patch)
treed106164f315a92e8894454fdf93226bff5f53d3e
parentb7392d2247cfe6771f95d256374f1a8e6a6f48d6 (diff)
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>
-rw-r--r--include/uapi/linux/virtio_ring.h7
1 files changed, 7 insertions, 0 deletions
diff --git a/include/uapi/linux/virtio_ring.h b/include/uapi/linux/virtio_ring.h
index 61c818a7fe70..a3318f31e8e7 100644
--- 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.
*