virtio: Add ability to detach unused buffers from vrings

There's currently no way for a virtio driver to ask for unused
buffers, so it has to keep a list itself to reclaim them at shutdown.
This is redundant, since virtio_ring stores that information.  So
add a new hook to do this.

Signed-off-by: Shirley Ma <xma@us.ibm.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

1 files changed, 4 insertions, 0 deletions

diff --git a/include/linux/virtio.h b/include/linux/virtio.h
index 057a2e010758..f508c651e53d 100644
--- a/include/linux/virtio.h
+++ b/include/linux/virtio.h
@@ -51,6 +51,9 @@ struct virtqueue {
  *      This re-enables callbacks; it returns "false" if there are pending
  *      buffers in the queue, to detect a possible race between the driver
  *      checking for more work, and enabling callbacks.
+ * @detach_unused_buf: detach first unused buffer
+ *      vq: the struct virtqueue we're talking about.
+ *      Returns NULL or the "data" token handed to add_buf
  *
  * Locking rules are straightforward: the driver is responsible for
  * locking.  No two operations may be invoked simultaneously, with the exception
@@ -71,6 +74,7 @@ struct virtqueue_ops {
 
         void (*disable_cb)(struct virtqueue *vq);
         bool (*enable_cb)(struct virtqueue *vq);
+        void *(*detach_unused_buf)(struct virtqueue *vq);
 };
 
 /**