During device reset or similar situations a VirtQueueElement needs to be
freed without pushing it onto the used ring or rewinding the virtqueue.
Extract a new function to do this.

Later patches add virtio_detach_element() calls to existing device so
that scatter-gather lists are unmapped and vq->inuse goes back to zero
during device reset.  Currently some devices don't bother and simply
call g_free(elem) which is not a clean way to throw away a
VirtQueueElement.

Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com>
---
 hw/virtio/virtio.c         | 27 +++++++++++++++++++++++++--
 include/hw/virtio/virtio.h |  2 ++
 2 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index fcf3358..adcef45 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -264,12 +264,35 @@ static void virtqueue_unmap_sg(VirtQueue *vq, const 
VirtQueueElement *elem,
                                   0, elem->out_sg[i].iov_len);
 }
 
+/* virtqueue_detach_element:
+ * @vq: The #VirtQueue
+ * @elem: The #VirtQueueElement
+ * @len: number of bytes written
+ *
+ * Detach the element from the virtqueue.  This function is suitable for device
+ * reset or other situations where a #VirtQueueElement is simply freed and will
+ * not be pushed or discarded.
+ */
+void virtqueue_detach_element(VirtQueue *vq, const VirtQueueElement *elem,
+                              unsigned int len)
+{
+    vq->inuse--;
+    virtqueue_unmap_sg(vq, elem, len);
+}
+
+/* virtqueue_discard:
+ * @vq: The #VirtQueue
+ * @elem: The #VirtQueueElement
+ * @len: number of bytes written
+ *
+ * Pretend the most recent element wasn't popped from the virtqueue.  The next
+ * call to virtqueue_pop() will refetch the element.
+ */
 void virtqueue_discard(VirtQueue *vq, const VirtQueueElement *elem,
                        unsigned int len)
 {
     vq->last_avail_idx--;
-    vq->inuse--;
-    virtqueue_unmap_sg(vq, elem, len);
+    virtqueue_detach_element(vq, elem, len);
 }
 
 /* virtqueue_rewind:
diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
index f05559d..ad1e2d6 100644
--- a/include/hw/virtio/virtio.h
+++ b/include/hw/virtio/virtio.h
@@ -152,6 +152,8 @@ void *virtqueue_alloc_element(size_t sz, unsigned out_num, 
unsigned in_num);
 void virtqueue_push(VirtQueue *vq, const VirtQueueElement *elem,
                     unsigned int len);
 void virtqueue_flush(VirtQueue *vq, unsigned int count);
+void virtqueue_detach_element(VirtQueue *vq, const VirtQueueElement *elem,
+                              unsigned int len);
 void virtqueue_discard(VirtQueue *vq, const VirtQueueElement *elem,
                        unsigned int len);
 bool virtqueue_rewind(VirtQueue *vq, unsigned int num);
-- 
2.7.4


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