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

[Avi Kivity <avi@xxxxxxxxxxxx>]

Rusty Russell wrote:
> +static bool do_read(request_queue_t *q, struct virtio_blk *vblk,
> +                   struct virtblk_req *vbr)
> +{
> +       unsigned long num;
> +
> +       vbr->out_hdr.type |= VIRTIO_BLK_T_READ;
> +
> +       /* 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?
--
error compiling committee.c: too many arguments to function


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