libvhost-user: add vu_queue_unpop()

vhost-user-input will make use of this function to undo some queue pop
in case the virtio queue does not have enough room.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20190308140454.32437-11-marcandre.lureau@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

contrib/libvhost-user/libvhost-user.c

@@ -1966,6 +1966,22 @@ vu_queue_pop(VuDev *dev, VuVirtq *vq, size_t sz)
     return elem;
 }
 
+static void
+vu_queue_detach_element(VuDev *dev, VuVirtq *vq, VuVirtqElement *elem,
+                        size_t len)
+{
+    vq->inuse--;
+    /* unmap, when DMA support is added */
+}
+
+void
+vu_queue_unpop(VuDev *dev, VuVirtq *vq, VuVirtqElement *elem,
+               size_t len)
+{
+    vq->last_avail_idx--;
+    vu_queue_detach_element(dev, vq, elem, len);
+}
+
 bool
 vu_queue_rewind(VuDev *dev, VuVirtq *vq, unsigned int num)
 {

contrib/libvhost-user/libvhost-user.h

@@ -458,6 +458,20 @@ void vu_queue_notify(VuDev *dev, VuVirtq *vq);
  */
 void *vu_queue_pop(VuDev *dev, VuVirtq *vq, size_t sz);
 
+
+/**
+ * vu_queue_unpop:
+ * @dev: a VuDev context
+ * @vq: a VuVirtq queue
+ * @elem: The #VuVirtqElement
+ * @len: number of bytes written
+ *
+ * Pretend the most recent element wasn't popped from the virtqueue.  The next
+ * call to vu_queue_pop() will refetch the element.
+ */
+void vu_queue_unpop(VuDev *dev, VuVirtq *vq, VuVirtqElement *elem,
+                    size_t len);
+
 /**
  * vu_queue_rewind:
  * @dev: a VuDev context