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

[Avi Kivity <avi@xxxxxxxxxxxx>]

Rusty Russell wrote:
> 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).
>
>   
So you map the first sg entry for output, and the rest for input.

Less pretty, but more efficient.

--
error compiling committee.c: too many arguments to function


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