From mboxrd@z Thu Jan  1 00:00:00 1970
From: Rusty Russell <rusty-8n+1lVoiYb80n/F98K4Iww@public.gmane.org>
Subject: Re: [PATCH RFC 3/3] Virtio draft III: example block
	driver
Date: Mon, 18 Jun 2007 18:08:15 +1000
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Cc: Stephen Rothwell <sfr-3FnU+UHB4dNDw9hX6IcOSA@public.gmane.org>,
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To: Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>
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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.


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