[PATCH 06/18] virtio_ring: avail event index interface

["Michael S. Tsirkin" <mst@redhat.com>]

Define a new feature bit for the host to
declare that it uses an avail_event index
(like Xen) instead of a feature bit
to enable/disable interrupts.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 include/linux/virtio_ring.h |   11 ++++++++---
 1 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
index f5c1b75..f791772 100644
--- a/include/linux/virtio_ring.h
+++ b/include/linux/virtio_ring.h
@@ -32,6 +32,9 @@
 /* The Guest publishes the used index for which it expects an interrupt
  * at the end of the avail ring. Host should ignore the avail->flags field. */
 #define VIRTIO_RING_F_USED_EVENT_IDX	29
+/* The Host publishes the avail index for which it expects a kick
+ * at the end of the used ring. Guest should ignore the used->flags field. */
+#define VIRTIO_RING_F_AVAIL_EVENT_IDX	32
 
 /* Virtio ring descriptors: 16 bytes.  These can chain together via "next". */
 struct vring_desc {
@@ -96,11 +99,13 @@ struct vring {
  *	__u16 used_flags;
  *	__u16 used_idx;
  *	struct vring_used_elem used[num];
+ *	__u16 avail_event_idx;
  * };
  */
-/* We publish the used event index at the end of the available ring.
- * It is at the end for backwards compatibility. */
+/* We publish the used event index at the end of the available ring, and vice
+ * versa. They are at the end for backwards compatibility. */
 #define vring_used_event(vr) ((vr)->avail->ring[(vr)->num])
+#define vring_avail_event(vr) (*(__u16 *)&(vr)->used->ring[(vr)->num])
 
 static inline void vring_init(struct vring *vr, unsigned int num, void *p,
 			      unsigned long align)
@@ -116,7 +121,7 @@ static inline unsigned vring_size(unsigned int num, unsigned long align)
 {
 	return ((sizeof(struct vring_desc) * num + sizeof(__u16) * (2 + num)
 		 + align - 1) & ~(align - 1))
-		+ sizeof(__u16) * 2 + sizeof(struct vring_used_elem) * num;
+		+ sizeof(__u16) * 3 + sizeof(struct vring_used_elem) * num;
 }
 
 #ifdef __KERNEL__
-- 
1.7.5.53.gc233e