Re: [PATCH RFC 3/3] Virtio draft III: example block driver

Rusty Russell
2007-06-18T08:08:15Z

On Sun, 2007-06-17 at 17:25 +0300, Avi Kivity wrote:
> Rusty Russell wrote:
> > + /* Set up for reply. */
> > + vblk->sg[0].page = virt_to_page(&vbr->in_hdr);
> > + vblk->sg[0].offset = offset_in_page(&vbr->in_hdr);
> > + vblk->sg[0].length = sizeof(vbr->in_hdr);
> > + num = blk_rq_map_sg(q, vbr->req, vblk->sg+1);
> > + vbr->out_hdr.id = vblk->vdev->ops->add_inbuf(vblk->vdev, vblk->sg,
> > + 1+num, vbr);
> > + if (IS_ERR_VALUE(vbr->out_hdr.id))
> > + goto full;
> > +
> > + vblk->sg[0].page = virt_to_page(&vbr->out_hdr);
> > + vblk->sg[0].offset = offset_in_page(&vbr->out_hdr);
> > + vblk->sg[0].length = sizeof(vbr->out_hdr);
> > +
> > + vbr->out_id = vblk->vdev->ops->add_outbuf(vblk->vdev, vblk->sg, 1,
> > + vbr);
>
> This strikes me as wasteful. Why not set up a single descriptor which
> contains both placement and the data itself?

We could actually do this for write, but not for read (where the length
& sector need to be sent to other end, and the data comes from other
end).

The strict separation of in and out in virtio is to support both
untrusted inter-guest I/O (admittedly not useful for block devices) and
socket-style hypercall interfaces.

Thanks,
Rusty.