[PULL 4/7] hw/virtio: Fix packed virtqueue flush used_idx

["Michael S. Tsirkin" <mst@redhat.com>]

From: Wafer <wafer@jaguarmicro.com>

In the event of writing many chains of descriptors, the device must
write just the id of the last buffer in the descriptor chain, skip
forward the number of descriptors in the chain, and then repeat the
operations for the rest of chains.

Current QEMU code writes all the buffer ids consecutively, and then
skips all the buffers altogether. This is a bug, and can be reproduced
with a VirtIONet device with _F_MRG_RXBUB and without
_F_INDIRECT_DESC:

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'.

Fixes: 86044b24e8 ("virtio: basic packed virtqueue support")
Acked-by: Eugenio Pérez <eperezma@redhat.com>
Signed-off-by: Wafer <wafer@jaguarmicro.com>
Message-Id: <20240407015451.5228-2-wafer@jaguarmicro.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 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 d229755eae..c5bedca848 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;
-- 
MST