On 2020/6/10 下午7:05, Michael S. Tsirkin wrote:

quoted

quoted

+EXPORT_SYMBOL_GPL(vhost_get_vq_desc);
   /* Reverse the effect of vhost_get_vq_desc. Useful for error handling. */
   void vhost_discard_vq_desc(struct vhost_virtqueue *vq, int n)
   {
+	unfetch_descs(vq);
   	vq->last_avail_idx -= n;

So unfetch_descs() has decreased last_avail_idx.
Can we fix this by letting unfetch_descs() return the number and then we can
do:

int d = unfetch_descs(vq);
vq->last_avail_idx -= (n > d) ? n - d: 0;

Thanks

That's intentional I think - we need both.

Yes, but:

Unfetch_descs drops the descriptors in the cache that were
*not returned to caller*  through get_vq_desc.

vhost_discard_vq_desc drops the ones that were returned through get_vq_desc.

Did I miss anything?

We could count some descriptors twice, consider the case e.g we only 
cache on descriptor:

fetch_descs()
     fetch_buf()
         last_avail_idx++;

Then we want do discard it:
vhost_discard_avail_buf(1)
     unfetch_descs()
         last_avail_idx--;
     last_avail_idx -= 1;

Thanks