block: add example ioctl

block: add example ioctl

Grab op == 1, BLOCK_URING_OP_IOCTL, and use it to implement basic
ioctl functionality.

Example code, to issue BLKBSZGET through IORING_OP_URING_CMD:

struct block_uring_cmd {
	__u16 	op;
	__u16	pad;
	union {
		__u32	size;
		__u32	ioctl_cmd;
	};
	__u64	addr;
	__u64	unused[4];
};

static int get_bs(struct io_uring *ring, const char *dev)
{
	struct io_uring_cqe *cqe;
	struct io_uring_sqe *sqe;
	struct block_uring_cmd *cmd;
	int ret, fd;

	fd = open(dev, O_RDONLY);
	if (fd < 0) {
		perror("open");
		return 1;
	}

	sqe = io_uring_get_sqe(ring);
	if (!sqe) {
		fprintf(stderr, "get sqe failed\n");
		goto err;
	}

	memset(sqe, 0, sizeof(*sqe));
	sqe->opcode = IORING_OP_URING_CMD;
	sqe->fd = fd;
	cmd = (void *) &sqe->cmd_pdu_start;
	cmd->op = BLOCK_URING_OP_IOCTL;
	cmd->ioctl_cmd = BLKBSZGET;
	sqe->cmd_user_data = 0x1234;

	ret = io_uring_submit(ring);
	if (ret <= 0) {
		fprintf(stderr, "sqe submit failed: %d\n", ret);
		goto err;
	}

	ret = io_uring_wait_cqe(ring, &cqe);
	if (ret < 0) {
		fprintf(stderr, "wait completion %d\n", ret);
		goto err;
	}
	printf("bs=%d\n", cqe->res);
	io_uring_cqe_seen(ring, cqe);
	return 0;
err:
	return 1;
}

Signed-off-by: Jens Axboe <axboe@kernel.dk>