Re: [PATCH v4] virtio_blk: add DISCARD and WRIET ZEROES commands support

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Thu, May 31, 2018 at 11:53:26PM +0000, Liu, Changpeng wrote:
> 
> 
> > -----Original Message-----
> > From: Stefan Hajnoczi [mailto:stefanha@redhat.com]
> > Sent: Thursday, May 31, 2018 6:31 PM
> > To: Liu, Changpeng <changpeng.liu@intel.com>
> > Cc: virtualization@lists.linux-foundation.org; cavery@redhat.com;
> > jasowang@redhat.com; pbonzini@redhat.com; Wang, Wei W
> > <wei.w.wang@intel.com>
> > Subject: Re: [PATCH v4] virtio_blk: add DISCARD and WRIET ZEROES commands
> > support
> > 
> > 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".
> Hi Stefan,
> 
> For the new add DISCARD and WRITE ZEROES commands, I defined the 
> type code to 11 and 13, so the last bit always is 1, there is no difference
> in practice.

Then the code is misleading.  This is clearer:

  if (type == VIRTIO_BLK_T_DISCARD || type == VIRTIO_BLK_T_WRITE_ZEROES)
      /* Do nothing, type already set */
  else if (rq_data_dir(req) == WRITE)
      vbr->out_hdr.type |= cpu_to_virtio32(vblk->vdev, VIRTIO_BLK_T_OUT);
  ...

> But I believe the specification says VIRTIO_BLK_T_OUT means direction, so
> OR the two bits together should compliance with the specification.

I cannot find that in the specification:

  http://docs.oasis-open.org/virtio/virtio/v1.0/cs04/virtio-v1.0-cs04.html#x1-2020002

and it would contradict the "The type of the request is either ... or
..." wording that I quoted from the spec above.

If you do find something in the spec, please let me know and we can
figure out how to make the spec consistent.

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