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Date: Mon, 27 Mar 2017 21:20:18 +0100
From: Stefan Hajnoczi <stefanha@...il.com>
To: Changpeng Liu <changpeng.liu@...el.com>
Cc: virtio-dev@...ts.oasis-open.org,
virtualization@...ts.linux-foundation.org,
linux-kernel@...r.kernel.org, hch@....de, qemu-devel@...gnu.org
Subject: Re: [PATCH] virtio-blk: add DISCARD support to virtio-blk driver
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@...el.com>
> ---
> 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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