vhost/vsock: fix incorrect used length reported to the guest

The "used length" reported by calling vhost_add_used() must be the
number of bytes written by the device (using "in" buffers).

In vhost_vsock_handle_tx_kick() the device only reads the guest
buffers (they are all "out" buffers), without writing anything,
so we must pass 0 as "used length" to comply virtio spec.

Fixes: 433fc58 ("VSOCK: Introduce vhost_vsock.ko")
Cc: [email protected]
Reported-by: Halil Pasic <[email protected]>
Suggested-by: Jason Wang <[email protected]>
Signed-off-by: Stefano Garzarella <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Michael S. Tsirkin <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Reviewed-by: Halil Pasic <[email protected]>

drivers/vhost/vsock.c

@@ -554,7 +554,7 @@ static void vhost_vsock_handle_tx_kick(struct vhost_work *work)
 			virtio_transport_free_pkt(pkt);
 
 		len += sizeof(pkt->hdr);
-		vhost_add_used(vq, head, len);
+		vhost_add_used(vq, head, 0);
 		total_len += len;
 		added = true;
 	} while(likely(!vhost_exceeds_weight(vq, ++pkts, total_len)));