On Tue, May 29, 2018 at 09:42:24AM +0800, Changpeng Liu wrote:
>  	num = blk_rq_map_sg(hctx->queue, req, vbr->sg);
>  	if (num) {
> -		if (rq_data_dir(req) == WRITE)
> +		if (rq_data_dir(req) == WRITE || type == VIRTIO_BLK_T_DISCARD ||
> +		    type == VIRTIO_BLK_T_WRITE_ZEROES)
>  			vbr->out_hdr.type |= cpu_to_virtio32(vblk->vdev, VIRTIO_BLK_T_OUT);

The VIRTIO specification says:

  The type of the request is either a read (VIRTIO_BLK_T_IN), a write
  (VIRTIO_BLK_T_OUT), a discard (VIRTIO_BLK_T_DISCARD), a write zeroes
  (VIRTIO_BLK_T_WRITE_ZEROES) or a flush (VIRTIO_BLK_T_FLUSH).

But this patch actually uses VIRTIO_BLK_T_DISCARD | VIRTIO_BLK_T_OUT or
VIRTIO_BLK_T_WRITE_ZEROES | VIRTIO_BLK_T_OUT.  "either A or B" is
exclusive, it does not mean "A and B".

Can you clarify whether the spec needs to be changed or what the purpose
of ORing in the VIRTIO_BLK_T_OUT bit is?