Re: [PATCH 3/3] virtio-blk: Add bio-based IO path for virtio-blk

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jun 18, 2012 at 02:53:10PM +0800, Asias He wrote:
> +static void virtblk_make_request(struct request_queue *q, struct bio *bio)
> +{
> +	struct virtio_blk *vblk = q->queuedata;
> +	unsigned int num, out = 0, in = 0;
> +	struct virtblk_req *vbr;
> +
> +	BUG_ON(bio->bi_phys_segments + 2 > vblk->sg_elems);
> +	BUG_ON(bio->bi_rw & (REQ_FLUSH | REQ_FUA));
> +
> +	vbr = virtblk_alloc_req(vblk, GFP_NOIO);
> +	if (!vbr) {
> +		bio_endio(bio, -ENOMEM);
> +		return;
> +	}
> +
> +	vbr->bio = bio;
> +	vbr->req = NULL;
> +	vbr->out_hdr.type = 0;
> +	vbr->out_hdr.sector = bio->bi_sector;
> +	vbr->out_hdr.ioprio = bio_prio(bio);
> +
> +	sg_set_buf(&vbr->sg[out++], &vbr->out_hdr, sizeof(vbr->out_hdr));
> +
> +	num = blk_bio_map_sg(q, bio, vbr->sg + out);
> +
> +	sg_set_buf(&vbr->sg[num + out + in++], &vbr->status,
> +		   sizeof(vbr->status));
> +
> +	if (num) {
> +		if (bio->bi_rw & REQ_WRITE) {
> +			vbr->out_hdr.type |= VIRTIO_BLK_T_OUT;
> +			out += num;
> +		} else {
> +			vbr->out_hdr.type |= VIRTIO_BLK_T_IN;
> +			in += num;
> +		}
> +	}
> +
> +	spin_lock_irq(vblk->disk->queue->queue_lock);
> +	if (virtqueue_add_buf(vblk->vq, vbr->sg, out, in, vbr,
> +			      GFP_ATOMIC) < 0) {
> +		spin_unlock_irq(vblk->disk->queue->queue_lock);

Any implications of dropping lock like that?
E.g. for suspend. like we are still discussing with
unlocked kick?

> +		virtblk_add_buf_wait(vblk, vbr, out, in);
> +	} else {
> +		virtqueue_kick(vblk->vq);

Why special case the first call? task state manipulation so expensive?

> +		spin_unlock_irq(vblk->disk->queue->queue_lock);
> +	}
> +}
> +