Re: [PATCH v2 2/5] virtio-scsi: use functions for piecewise composition of buffers

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

On Tue, Dec 18, 2012 at 01:32:49PM +0100, Paolo Bonzini wrote:
> Using the new virtio_scsi_add_sg function lets us simplify the queueing
> path.  In particular, all data protected by the tgt_lock is just gone
> (multiqueue will find a new use for the lock).

vq access still needs some protection: virtio is not reentrant
by itself. with tgt_lock gone what protects vq against
concurrent add_buf calls?

> The speedup is relatively small (2-4%) but it is worthwhile because of
> the code simplification---both in this patches and in the next ones.
> 
> Signed-off-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
> 	v1->v2: new
> 
>  drivers/scsi/virtio_scsi.c |   94 +++++++++++++++++++------------------------
>  1 files changed, 42 insertions(+), 52 deletions(-)
> 
> diff --git a/drivers/scsi/virtio_scsi.c b/drivers/scsi/virtio_scsi.c
> index 74ab67a..2b93b6e 100644
> --- a/drivers/scsi/virtio_scsi.c
> +++ b/drivers/scsi/virtio_scsi.c
> @@ -59,11 +59,8 @@ struct virtio_scsi_vq {
>  
>  /* Per-target queue state */
>  struct virtio_scsi_target_state {
> -	/* Protects sg.  Lock hierarchy is tgt_lock -> vq_lock.  */
> +	/* Never held at the same time as vq_lock.  */
>  	spinlock_t tgt_lock;
> -
> -	/* For sglist construction when adding commands to the virtqueue.  */
> -	struct scatterlist sg[];
>  };
>  
>  /* Driver instance state */
> @@ -351,57 +348,58 @@ static void virtscsi_event_done(struct virtqueue *vq)
>  	spin_unlock_irqrestore(&vscsi->event_vq.vq_lock, flags);
>  };
>  
> -static void virtscsi_map_sgl(struct scatterlist *sg, unsigned int *p_idx,
> -			     struct scsi_data_buffer *sdb)
> -{
> -	struct sg_table *table = &sdb->table;
> -	struct scatterlist *sg_elem;
> -	unsigned int idx = *p_idx;
> -	int i;
> -
> -	for_each_sg(table->sgl, sg_elem, table->nents, i)
> -		sg[idx++] = *sg_elem;
> -
> -	*p_idx = idx;
> -}
> -
>  /**
> - * virtscsi_map_cmd - map a scsi_cmd to a virtqueue scatterlist
> + * virtscsi_add_cmd - add a virtio_scsi_cmd to a virtqueue
>   * @vscsi	: virtio_scsi state
>   * @cmd		: command structure
> - * @out_num	: number of read-only elements
> - * @in_num	: number of write-only elements
>   * @req_size	: size of the request buffer
>   * @resp_size	: size of the response buffer
> - *
> - * Called with tgt_lock held.
> + * @gfp	: flags to use for memory allocations
>   */
> -static void virtscsi_map_cmd(struct virtio_scsi_target_state *tgt,
> -			     struct virtio_scsi_cmd *cmd,
> -			     unsigned *out_num, unsigned *in_num,
> -			     size_t req_size, size_t resp_size)
> +static int virtscsi_add_cmd(struct virtqueue *vq,
> +			    struct virtio_scsi_cmd *cmd,
> +			    size_t req_size, size_t resp_size, gfp_t gfp)
>  {
>  	struct scsi_cmnd *sc = cmd->sc;
> -	struct scatterlist *sg = tgt->sg;
> -	unsigned int idx = 0;
> +	struct scatterlist sg;
> +	unsigned int count, count_sg;
> +	struct sg_table *out, *in;
> +	struct virtqueue_buf buf;
> +	int ret;
> +
> +	out = in = NULL;
> +
> +	if (sc && sc->sc_data_direction != DMA_NONE) {
> +		if (sc->sc_data_direction != DMA_FROM_DEVICE)
> +			out = &scsi_out(sc)->table;
> +		if (sc->sc_data_direction != DMA_TO_DEVICE)
> +			in = &scsi_in(sc)->table;
> +	}
> +
> +	count_sg = 2 + (out ? 1 : 0)          + (in ? 1 : 0);
> +	count    = 2 + (out ? out->nents : 0) + (in ? in->nents : 0);
> +	ret = virtqueue_start_buf(vq, &buf, cmd, count, count_sg, gfp);
> +	if (ret < 0)
> +		return ret;
>  
>  	/* Request header.  */
> -	sg_set_buf(&sg[idx++], &cmd->req, req_size);
> +	sg_init_one(&sg, &cmd->req, req_size);
> +	virtqueue_add_sg(&buf, &sg, 1, DMA_TO_DEVICE);
>  
>  	/* Data-out buffer.  */
> -	if (sc && sc->sc_data_direction != DMA_FROM_DEVICE)
> -		virtscsi_map_sgl(sg, &idx, scsi_out(sc));
> -
> -	*out_num = idx;
> +	if (out)
> +		virtqueue_add_sg(&buf, out->sgl, out->nents, DMA_TO_DEVICE);
>  
>  	/* Response header.  */
> -	sg_set_buf(&sg[idx++], &cmd->resp, resp_size);
> +	sg_init_one(&sg, &cmd->resp, resp_size);
> +	virtqueue_add_sg(&buf, &sg, 1, DMA_FROM_DEVICE);
>  
>  	/* Data-in buffer */
> -	if (sc && sc->sc_data_direction != DMA_TO_DEVICE)
> -		virtscsi_map_sgl(sg, &idx, scsi_in(sc));
> +	if (in)
> +		virtqueue_add_sg(&buf, in->sgl, in->nents, DMA_FROM_DEVICE);
>  
> -	*in_num = idx - *out_num;
> +	virtqueue_end_buf(&buf);
> +	return 0;
>  }
>  
>  static int virtscsi_kick_cmd(struct virtio_scsi_target_state *tgt,
> @@ -409,25 +407,20 @@ static int virtscsi_kick_cmd(struct virtio_scsi_target_state *tgt,
>  			     struct virtio_scsi_cmd *cmd,
>  			     size_t req_size, size_t resp_size, gfp_t gfp)
>  {
> -	unsigned int out_num, in_num;
>  	unsigned long flags;
> -	int err;
> +	int ret;
>  	bool needs_kick = false;
>  
> -	spin_lock_irqsave(&tgt->tgt_lock, flags);
> -	virtscsi_map_cmd(tgt, cmd, &out_num, &in_num, req_size, resp_size);
> -
> -	spin_lock(&vq->vq_lock);
> -	err = virtqueue_add_buf(vq->vq, tgt->sg, out_num, in_num, cmd, gfp);
> -	spin_unlock(&tgt->tgt_lock);
> -	if (!err)
> +	spin_lock_irqsave(&vq->vq_lock, flags);
> +	ret = virtscsi_add_cmd(vq->vq, cmd, req_size, resp_size, gfp);
> +	if (!ret)
>  		needs_kick = virtqueue_kick_prepare(vq->vq);
>  
>  	spin_unlock_irqrestore(&vq->vq_lock, flags);
>  
>  	if (needs_kick)
>  		virtqueue_notify(vq->vq);
> -	return err;
> +	return ret;
>  }
>  
>  static int virtscsi_queuecommand(struct Scsi_Host *sh, struct scsi_cmnd *sc)
> @@ -592,14 +585,11 @@ static struct virtio_scsi_target_state *virtscsi_alloc_tgt(
>  	gfp_t gfp_mask = GFP_KERNEL;
>  
>  	/* We need extra sg elements at head and tail.  */
> -	tgt = kmalloc(sizeof(*tgt) + sizeof(tgt->sg[0]) * (sg_elems + 2),
> -		      gfp_mask);
> -
> +	tgt = kmalloc(sizeof(*tgt), gfp_mask);
>  	if (!tgt)
>  		return NULL;
>  
>  	spin_lock_init(&tgt->tgt_lock);
> -	sg_init_table(tgt->sg, sg_elems + 2);
>  	return tgt;
>  }
>  
> -- 
> 1.7.1
>