Re: [PATCH] virtio-blk: add DISCARD support to virtio-blk driver

[Stefan Hajnoczi]

On Tue, Mar 28, 2017 at 04:39:25PM +0800, Changpeng Liu wrote:
> Currently virtio-blk driver does not provide discard feature flag, so the
> filesystems which built on top of the block device will not send discard
> command. This is okay for HDD backend, but it will impact the performance
> for SSD backend.
> 
> Add a feature flag VIRTIO_BLK_F_DISCARD and command VIRTIO_BLK_T_DISCARD
> to extend exist virtio-blk protocol. virtio-blk protocol uses a single
> 8 bytes descriptor containing type,reserved and sector, currently Linux
> uses the reserved field as IO priority, here we also re-use the reserved
> field as number of discard sectors.
> 
> Signed-off-by: Changpeng Liu <changpeng.liu@xxxxxxxxx>
> ---
>  drivers/block/virtio_blk.c      | 38 +++++++++++++++++++++++++++++---------
>  include/uapi/linux/virtio_blk.h | 12 ++++++++++--
>  2 files changed, 39 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 1d4c9f8..550cfe7 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -241,6 +241,9 @@ static int virtio_queue_rq(struct blk_mq_hw_ctx *hctx,
>  	case REQ_OP_FLUSH:
>  		type = VIRTIO_BLK_T_FLUSH;
>  		break;
> +	case REQ_OP_DISCARD:
> +		type = VIRTIO_BLK_T_DISCARD;
> +		break;
>  	case REQ_OP_SCSI_IN:
>  	case REQ_OP_SCSI_OUT:
>  		type = VIRTIO_BLK_T_SCSI_CMD;
> @@ -256,16 +259,24 @@ static int virtio_queue_rq(struct blk_mq_hw_ctx *hctx,
>  	vbr->out_hdr.type = cpu_to_virtio32(vblk->vdev, type);
>  	vbr->out_hdr.sector = type ?
>  		0 : cpu_to_virtio64(vblk->vdev, blk_rq_pos(req));
> -	vbr->out_hdr.ioprio = cpu_to_virtio32(vblk->vdev, req_get_ioprio(req));
> +	vbr->out_hdr.u.ioprio = cpu_to_virtio32(vblk->vdev, req_get_ioprio(req));
>  
>  	blk_mq_start_request(req);
>  
> -	num = blk_rq_map_sg(hctx->queue, req, vbr->sg);
> -	if (num) {
> -		if (rq_data_dir(req) == WRITE)
> -			vbr->out_hdr.type |= cpu_to_virtio32(vblk->vdev, VIRTIO_BLK_T_OUT);
> -		else
> -			vbr->out_hdr.type |= cpu_to_virtio32(vblk->vdev, VIRTIO_BLK_T_IN);
> +	if (type == VIRTIO_BLK_T_DISCARD) {
> +		vbr->out_hdr.u.discard_nr_sectors = cpu_to_virtio32(vblk->vdev,
> +								    blk_rq_sectors(req));
> +		num = 0;
> +	} else {
> +		num = blk_rq_map_sg(hctx->queue, req, vbr->sg);
> +		if (num) {
> +			if (rq_data_dir(req) == WRITE)
> +				vbr->out_hdr.type |= cpu_to_virtio32(vblk->vdev,
> +								     VIRTIO_BLK_T_OUT);
> +			else
> +				vbr->out_hdr.type |= cpu_to_virtio32(vblk->vdev,
> +								     VIRTIO_BLK_T_IN);
> +		}
>  	}
>  
>  	spin_lock_irqsave(&vblk->vqs[qid].lock, flags);
> @@ -775,6 +786,15 @@ static int virtblk_probe(struct virtio_device *vdev)
>  	if (!err && opt_io_size)
>  		blk_queue_io_opt(q, blk_size * opt_io_size);
>  
> +	if (virtio_has_feature(vdev, VIRTIO_BLK_F_DISCARD)) {
> +		q->limits.discard_zeroes_data = 0;
> +		q->limits.discard_alignment = blk_size;
> +		q->limits.discard_granularity = blk_size;
> +		blk_queue_max_discard_sectors(q, UINT_MAX);
> +		blk_queue_max_discard_segments(q, 1);
> +		queue_flag_set_unlocked(QUEUE_FLAG_DISCARD, q);
> +	}

Please add configuration space fields for these limits.  Looking at the
virtio-scsi block limits code in QEMU's scsi_disk_emulate_inquiry() I
can see that the hypervisor has useful values that it wants to
communicate.  They shouldn't be hardcoded to blk_size.

> +
>  	virtio_device_ready(vdev);
>  
>  	device_add_disk(&vdev->dev, vblk->disk);
> @@ -882,14 +902,14 @@ static int virtblk_restore(struct virtio_device *vdev)
>  	VIRTIO_BLK_F_SCSI,
>  #endif
>  	VIRTIO_BLK_F_FLUSH, VIRTIO_BLK_F_TOPOLOGY, VIRTIO_BLK_F_CONFIG_WCE,
> -	VIRTIO_BLK_F_MQ,
> +	VIRTIO_BLK_F_MQ, VIRTIO_BLK_F_DISCARD,
>  }
>  ;
>  static unsigned int features[] = {
>  	VIRTIO_BLK_F_SEG_MAX, VIRTIO_BLK_F_SIZE_MAX, VIRTIO_BLK_F_GEOMETRY,
>  	VIRTIO_BLK_F_RO, VIRTIO_BLK_F_BLK_SIZE,
>  	VIRTIO_BLK_F_FLUSH, VIRTIO_BLK_F_TOPOLOGY, VIRTIO_BLK_F_CONFIG_WCE,
> -	VIRTIO_BLK_F_MQ,
> +	VIRTIO_BLK_F_MQ, VIRTIO_BLK_F_DISCARD,
>  };
>  
>  static struct virtio_driver virtio_blk = {
> diff --git a/include/uapi/linux/virtio_blk.h b/include/uapi/linux/virtio_blk.h
> index 9ebe4d9..d608649 100644
> --- a/include/uapi/linux/virtio_blk.h
> +++ b/include/uapi/linux/virtio_blk.h
> @@ -38,6 +38,7 @@
>  #define VIRTIO_BLK_F_BLK_SIZE	6	/* Block size of disk is available*/
>  #define VIRTIO_BLK_F_TOPOLOGY	10	/* Topology information is available */
>  #define VIRTIO_BLK_F_MQ		12	/* support more than one vq */
> +#define VIRTIO_BLK_F_DISCARD	13	/* DISCARD command is supported */
>  
>  /* Legacy feature bits */
>  #ifndef VIRTIO_BLK_NO_LEGACY
> @@ -114,6 +115,9 @@ struct virtio_blk_config {
>  /* Get device ID command */
>  #define VIRTIO_BLK_T_GET_ID    8
>  
> +/* Discard command */
> +#define VIRTIO_BLK_T_DISCARD	16
> +
>  #ifndef VIRTIO_BLK_NO_LEGACY
>  /* Barrier before this op. */
>  #define VIRTIO_BLK_T_BARRIER	0x80000000
> @@ -127,8 +131,12 @@ struct virtio_blk_config {
>  struct virtio_blk_outhdr {
>  	/* VIRTIO_BLK_T* */
>  	__virtio32 type;
> -	/* io priority. */
> -	__virtio32 ioprio;
> +	union {
> +		/* io priority. */
> +		__virtio32 ioprio;
> +		/* discard number of sectors */
> +		__virtio32 discard_nr_sectors;
> +	} u;

DISCARD commands have no io priority?  Perhaps it's better to add an
extended header.

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