virtio: make add_buf return capacity remaining

This API change means that virtio_net can tell how much capacity
remains for buffers.  It's necessarily fuzzy, since
VIRTIO_RING_F_INDIRECT_DESC means we can fit any number of descriptors
in one, *if* we can kmalloc.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Dinesh Subhraveti <dineshs@us.ibm.com>

1 files changed, 5 insertions, 1 deletions

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index a882f2606515..f53600580726 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -208,7 +208,11 @@ add_head:
 
         pr_debug("Added buffer head %i to %p\n", head, vq);
         END_USE(vq);
-        return 0;
+
+        /* If we're indirect, we can fit many (assuming not OOM). */
+        if (vq->indirect)
+                return vq->num_free ? vq->vring.num : 0;
+        return vq->num_free;
 }
 
 static void vring_kick(struct virtqueue *_vq)