[PATCH v2] hw/virtio: Fix packed virtqueue flush used_idx

[Wafer]

If a virtio-net device has the VIRTIO_NET_F_MRG_RXBUF feature
but not the VIRTIO_RING_F_INDIRECT_DESC feature,
'VirtIONetQueue->rx_vq' will use the merge feature
to store data in multiple 'elems'.
The 'num_buffers' in the virtio header indicates how many elements are merged.
If the value of 'num_buffers' is greater than 1,
all the merged elements will be filled into the descriptor ring.
The 'idx' of the elements should be the value of 'vq->used_idx' plus 'ndescs'.

Signed-off-by: Wafer <wa...@jaguarmicro.com>

---
Changes in v2:
  - Clarify more in commit message;
---
 hw/virtio/virtio.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index fb6b4ccd83..cab5832cac 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -957,12 +957,20 @@ static void virtqueue_packed_flush(VirtQueue *vq, 
unsigned int count)
         return;
     }
 
+    /*
+     * For indirect element's 'ndescs' is 1.
+     * For all other elemment's 'ndescs' is the
+     * number of descriptors chained by NEXT (as set in virtqueue_packed_pop).
+     * So When the 'elem' be filled into the descriptor ring,
+     * The 'idx' of this 'elem' shall be
+     * the value of 'vq->used_idx' plus the 'ndescs'.
+     */
+    ndescs += vq->used_elems[0].ndescs;
     for (i = 1; i < count; i++) {
-        virtqueue_packed_fill_desc(vq, &vq->used_elems[i], i, false);
+        virtqueue_packed_fill_desc(vq, &vq->used_elems[i], ndescs, false);
         ndescs += vq->used_elems[i].ndescs;
     }
     virtqueue_packed_fill_desc(vq, &vq->used_elems[0], 0, true);
-    ndescs += vq->used_elems[0].ndescs;
 
     vq->inuse -= ndescs;
     vq->used_idx += ndescs;
-- 
2.27.0