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[PATCHv2 RFC 1/4] virtio_ring: add capacity check API

Add API to check ring capacity. Because of the option
to use indirect buffers, this returns the worst
case, not the normal case capacity.

Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
 drivers/virtio/virtio_ring.c |    8 ++++++++
 include/linux/virtio.h       |    5 +++++
 2 files changed, 13 insertions(+), 0 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 68b9136..23422f1 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -344,6 +344,14 @@ void *virtqueue_get_buf(struct virtqueue *_vq, unsigned int *len)
+int virtqueue_min_capacity(struct virtqueue *_vq)
+	struct vring_virtqueue *vq = to_vvq(_vq);
+	return vq->num_free;
 void virtqueue_disable_cb(struct virtqueue *_vq)
 	struct vring_virtqueue *vq = to_vvq(_vq);
diff --git a/include/linux/virtio.h b/include/linux/virtio.h
index 7108857..209220d 100644
--- a/include/linux/virtio.h
+++ b/include/linux/virtio.h
@@ -42,6 +42,9 @@ struct virtqueue {
  *	vq: the struct virtqueue we're talking about.
  *	len: the length written into the buffer
  *	Returns NULL or the "data" token handed to add_buf.
+ * virtqueue_min_capacity: get the current capacity of the queue
+ *	vq: the struct virtqueue we're talking about.
+ *	Returns the current worst case capacity of the queue.
  * virtqueue_disable_cb: disable callbacks
  *	vq: the struct virtqueue we're talking about.
  *	Note that this is not necessarily synchronous, hence unreliable and only
@@ -89,6 +92,8 @@ void virtqueue_kick(struct virtqueue *vq);
 void *virtqueue_get_buf(struct virtqueue *vq, unsigned int *len);
+int virtqueue_min_capacity(struct virtqueue *vq);
 void virtqueue_disable_cb(struct virtqueue *vq);
 bool virtqueue_enable_cb(struct virtqueue *vq);

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