On 2018年05月10日 15:32, Jason Wang wrote:

On 2018年04月25日 13:15, Tiwei Bie wrote:

quoted

+    /* We're using some buffers from the free list. */
+    vq->vq.num_free -= descs_used;
+
+    /* Update free pointer */
+    if (indirect) {
+        n = head + 1;
+        if (n >= vq->vring_packed.num) {
+            n = 0;
+            vq->wrap_counter ^= 1;
+        }
+        vq->next_avail_idx = n;
+    } else
+        vq->next_avail_idx = i;

During testing zerocopy (out of order completion), I found driver may 
submit two identical buffer id to vhost. So the above code may not 
work well.

Consider the case that driver adds 3 buffer and virtqueue size is 8.

a) id = 0,count = 2,next_avail = 2

b) id = 2,count = 4,next_avail = 2

next_avail should be 6 here.

c) id = 4,count = 2,next_avail = 0

id should be 6 here.

Thanks

if packet b is done before packet a, driver may think buffer id 0 is 
available and try to use it if even if the real buffer 0 was not done.

Thanks
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