[RFC PATCH 6/6] nvme: enable passthrough with fixed-buffer

[Christoph Hellwig]

> +/*
> + * Unlike blk_rq_map_user () this is only for fixed-buffer async passthrough.
> + * And hopefully faster as well.
> + */

This belongs into io_uring.c.  And that hopeful comment needs to be
validated and removed.

> +int nvme_rq_map_user_fixedb(struct request_queue *q, struct request *rq,
> +		     void __user *ubuf, unsigned long len, gfp_t gfp_mask,
> +		     struct io_uring_cmd *ioucmd)
> +{
> +	struct iov_iter iter;
> +	size_t iter_count, nr_segs;
> +	struct bio *bio;
> +	int ret;
> +
> +	/*
> +	 * Talk to io_uring to obtain BVEC iterator for the buffer.
> +	 * And use that iterator to form bio/request.
> +	 */
> +	ret = io_uring_cmd_import_fixed(ubuf, len, rq_data_dir(rq), &iter,
> +			ioucmd);

io_uring_cmd_import_fixed takes a non-__user pointer, so this will cause
a sparse warning.