[PATCH 05/18] virtio: used event index interface

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

Define a new feature bit for the guest to utilize a used_event index
(like Xen) instead if a flag bit to enable/disable interrupts.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 include/linux/virtio_ring.h |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)

diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
index e4d144b..f5c1b75 100644
--- a/include/linux/virtio_ring.h
+++ b/include/linux/virtio_ring.h
@@ -29,6 +29,10 @@
 /* We support indirect buffer descriptors */
 #define VIRTIO_RING_F_INDIRECT_DESC	28
 
+/* 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
+
 /* Virtio ring descriptors: 16 bytes.  These can chain together via "next". */
 struct vring_desc {
 	/* Address (guest-physical). */
@@ -83,6 +87,7 @@ struct vring {
  *	__u16 avail_flags;
  *	__u16 avail_idx;
  *	__u16 available[num];
+ *	__u16 used_event_idx;
  *
  *	// Padding to the next align boundary.
  *	char pad[];
@@ -93,6 +98,10 @@ struct vring {
  *	struct vring_used_elem used[num];
  * };
  */
+/* We publish the used event index at the end of the available ring.
+ * It is at the end for backwards compatibility. */
+#define vring_used_event(vr) ((vr)->avail->ring[(vr)->num])
+
 static inline void vring_init(struct vring *vr, unsigned int num, void *p,
 			      unsigned long align)
 {
-- 
1.7.5.53.gc233e