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virtio: add virtqueue_rewind()

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virtqueue_discard() requires a VirtQueueElement but virtio-balloon does
not migrate its in-use element.  Introduce a new function that is
similar to virtqueue_discard() but doesn't require a VirtQueueElement.

This will allow virtio-balloon to access element again after migration
with the usual proviso that the guest may have modified the vring since
last time.

Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Roman Kagan <rkagan@virtuozzo.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Ladi Prosek <lprosek@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
This commit is contained in:
Stefan Hajnoczi 2016-09-07 17:20:48 +02:00 committed by Michael S. Tsirkin
parent 104e70cae7
commit 297a75e6c5
2 changed files with 23 additions and 0 deletions
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22
hw/virtio/virtio.c
View File

@ -272,6 +272,28 @@ void virtqueue_discard(VirtQueue *vq, const VirtQueueElement *elem,
virtqueue_unmap_sg(vq, elem, len);
}
/* virtqueue_rewind:
* @vq: The #VirtQueue
* @num: Number of elements to push back
*
* Pretend that elements weren't popped from the virtqueue. The next
* virtqueue_pop() will refetch the oldest element.
*
* Use virtqueue_discard() instead if you have a VirtQueueElement.
*
* Returns: true on success, false if @num is greater than the number of in use
* elements.
*/
bool virtqueue_rewind(VirtQueue *vq, unsigned int num)
{
if (num > vq->inuse) {
return false;
}
vq->last_avail_idx -= num;
vq->inuse -= num;
return true;
}
void virtqueue_fill(VirtQueue *vq, const VirtQueueElement *elem,
unsigned int len, unsigned int idx)
{

1
include/hw/virtio/virtio.h
View File

@ -154,6 +154,7 @@ void virtqueue_push(VirtQueue *vq, const VirtQueueElement *elem,
void virtqueue_flush(VirtQueue *vq, unsigned int count);
void virtqueue_discard(VirtQueue *vq, const VirtQueueElement *elem,
unsigned int len);
bool virtqueue_rewind(VirtQueue *vq, unsigned int num);
void virtqueue_fill(VirtQueue *vq, const VirtQueueElement *elem,
unsigned int len, unsigned int idx);