linux-kernel - Re: [PATCH] virtio_blk: Add support for lifetime feature

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Date:   Mon, 12 Apr 2021 10:17:35 +0100
From:   Stefan Hajnoczi <stefanha@...hat.com>
To:     Enrico Granata <egranata@...gle.com>
Cc:     mst@...hat.com, jasowang@...hat.com, pbonzini@...hat.com,
        axboe@...nel.dk, virtualization@...ts.linux-foundation.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] virtio_blk: Add support for lifetime feature

On Tue, Mar 30, 2021 at 11:16:02PM +0000, Enrico Granata wrote:
> The VirtIO TC has adopted a new feature in virtio-blk enabling
> discovery of lifetime information.
> 
> This commit adds support for the VIRTIO_BLK_T_LIFETIME command
> to the virtio_blk driver, and adds two new attributes to the
> sysfs entry for virtio_blk:
> * pre_eol_info
> * life_time
> 
> which are defined in the same manner as the files of the same name
> for the eMMC driver, in line with the VirtIO specification.
> 
> Signed-off-by: Enrico Granata <egranata@...gle.com>
> ---
>  drivers/block/virtio_blk.c      | 76 ++++++++++++++++++++++++++++++++-
>  include/uapi/linux/virtio_blk.h | 11 +++++
>  2 files changed, 86 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index b9fa3ef5b57c..1fc0ec000b4f 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -246,7 +246,7 @@ static blk_status_t virtio_queue_rq(struct blk_mq_hw_ctx *hctx,
>  		unmap = !(req->cmd_flags & REQ_NOUNMAP);
>  		break;
>  	case REQ_OP_DRV_IN:
> -		type = VIRTIO_BLK_T_GET_ID;
> +		type = vbr->out_hdr.type;

This patch changes the endianness of vbr->out_hdr.type from
virtio-endian to cpu endian before virtio_queue_rq. That is error-prone
because someone skimming through the code will see some accesses with
cpu_to_virtio32() and others without it. They would have to audit the
code carefully to understand what is going on.

The following is cleaner:

  case REQ_OP_DRV_IN:
      break; /* type already set for custom requests */
  ...
  if (req_op(req) != REQ_OP_DRV_IN)
      vbr->out_hdr.type = cpu_to_virtio32(vblk->vdev, type);

Now vbr->out_hdr.type is virtio-endian everywhere. If we need to support
REQ_OP_DRV_OUT in the future it can use the same approach.

virtblk_get_id() and virtblk_get_lifetime() would be updated like this:

  vbreq->out_hdr.type = cpu_to_virtio32(VIRTIO_BLK_T_GET_*);

>  		break;
>  	default:
>  		WARN_ON_ONCE(1);
> @@ -310,11 +310,14 @@ static int virtblk_get_id(struct gendisk *disk, char *id_str)
>  	struct virtio_blk *vblk = disk->private_data;
>  	struct request_queue *q = vblk->disk->queue;
>  	struct request *req;
> +	struct virtblk_req *vbreq;

It's called vbr elsewhere in the driver. It would be nice to keep naming
consistent.

>  	int err;
>  
>  	req = blk_get_request(q, REQ_OP_DRV_IN, 0);
>  	if (IS_ERR(req))
>  		return PTR_ERR(req);
> +	vbreq = blk_mq_rq_to_pdu(req);
> +	vbreq->out_hdr.type = VIRTIO_BLK_T_GET_ID;
>  
>  	err = blk_rq_map_kern(q, req, id_str, VIRTIO_BLK_ID_BYTES, GFP_KERNEL);
>  	if (err)
> @@ -327,6 +330,34 @@ static int virtblk_get_id(struct gendisk *disk, char *id_str)
>  	return err;
>  }
>  
> +static int virtblk_get_lifetime(struct gendisk *disk, struct virtio_blk_lifetime *lifetime)
> +{
> +	struct virtio_blk *vblk = disk->private_data;
> +	struct request_queue *q = vblk->disk->queue;
> +	struct request *req;
> +	struct virtblk_req *vbreq;
> +	int err;
> +
> +	if (!virtio_has_feature(vblk->vdev, VIRTIO_BLK_F_LIFETIME))
> +		return -EOPNOTSUPP;
> +
> +	req = blk_get_request(q, REQ_OP_DRV_IN, 0);
> +	if (IS_ERR(req))
> +		return PTR_ERR(req);
> +	vbreq = blk_mq_rq_to_pdu(req);
> +	vbreq->out_hdr.type = VIRTIO_BLK_T_GET_LIFETIME;
> +
> +	err = blk_rq_map_kern(q, req, lifetime, sizeof(*lifetime), GFP_KERNEL);
> +	if (err)
> +		goto out;
> +
> +	blk_execute_rq(vblk->disk, req, false);
> +	err = blk_status_to_errno(virtblk_result(blk_mq_rq_to_pdu(req)));
> +out:
> +	blk_put_request(req);
> +	return err;
> +}
> +
>  static void virtblk_get(struct virtio_blk *vblk)
>  {
>  	refcount_inc(&vblk->refs);
> @@ -435,6 +466,46 @@ static ssize_t serial_show(struct device *dev,
>  
>  static DEVICE_ATTR_RO(serial);
>  
> +static ssize_t pre_eol_info_show(struct device *dev,
> +			   struct device_attribute *attr, char *buf)
> +{
> +	struct gendisk *disk = dev_to_disk(dev);
> +	struct virtio_blk_lifetime lft;
> +	int err;
> +
> +	/* sysfs gives us a PAGE_SIZE buffer */
> +	BUILD_BUG_ON(sizeof(lft) >= PAGE_SIZE);

Why is this necessary? In serial_show() it protects against a buffer
overflow. That's not the case here since sprintf() is used to write to
buf and the size of lft doesn't really matter.

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