On Wed, Jun 10, 2020 at 02:37:50PM +0200, Eugenio Perez Martin wrote:

quoted

+/* This function returns a value > 0 if a descriptor was found, or 0 if none were found.
+ * A negative code is returned on error. */
+static int fetch_descs(struct vhost_virtqueue *vq)
+{
+       int ret;
+
+       if (unlikely(vq->first_desc >= vq->ndescs)) {
+               vq->first_desc = 0;
+               vq->ndescs = 0;
+       }
+
+       if (vq->ndescs)
+               return 1;
+
+       for (ret = 1;
+            ret > 0 && vq->ndescs <= vhost_vq_num_batch_descs(vq);
+            ret = fetch_buf(vq))
+               ;

(Expanding comment in V6):

We get an infinite loop this way:
* vq->ndescs == 0, so we call fetch_buf() here
* fetch_buf gets less than vhost_vq_num_batch_descs(vq); descriptors. ret = 1
* This loop calls again fetch_buf, but vq->ndescs > 0 (and avail_vq ==
last_avail_vq), so it just return 1

That's what
	 [PATCH RFC v7 08/14] fixup! vhost: use batched get_vq_desc version
is supposed to fix.

-- 
MST