1 // SPDX-License-Identifier: GPL-2.0
3 * NVMe over Fabrics loopback device.
4 * Copyright (c) 2015-2016 HGST, a Western Digital Company.
6 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
7 #include <linux/scatterlist.h>
8 #include <linux/blk-mq.h>
9 #include <linux/nvme.h>
10 #include <linux/module.h>
11 #include <linux/parser.h>
13 #include "../host/nvme.h"
14 #include "../host/fabrics.h"
16 #define NVME_LOOP_MAX_SEGMENTS 256
18 struct nvme_loop_iod {
19 struct nvme_request nvme_req;
20 struct nvme_command cmd;
21 struct nvme_completion cqe;
23 struct nvme_loop_queue *queue;
24 struct work_struct work;
25 struct sg_table sg_table;
26 struct scatterlist first_sgl[];
29 struct nvme_loop_ctrl {
30 struct nvme_loop_queue *queues;
32 struct blk_mq_tag_set admin_tag_set;
34 struct list_head list;
35 struct blk_mq_tag_set tag_set;
36 struct nvme_loop_iod async_event_iod;
37 struct nvme_ctrl ctrl;
39 struct nvmet_ctrl *target_ctrl;
40 struct nvmet_port *port;
43 static inline struct nvme_loop_ctrl *to_loop_ctrl(struct nvme_ctrl *ctrl)
45 return container_of(ctrl, struct nvme_loop_ctrl, ctrl);
48 enum nvme_loop_queue_flags {
52 struct nvme_loop_queue {
53 struct nvmet_cq nvme_cq;
54 struct nvmet_sq nvme_sq;
55 struct nvme_loop_ctrl *ctrl;
59 static LIST_HEAD(nvme_loop_ports);
60 static DEFINE_MUTEX(nvme_loop_ports_mutex);
62 static LIST_HEAD(nvme_loop_ctrl_list);
63 static DEFINE_MUTEX(nvme_loop_ctrl_mutex);
65 static void nvme_loop_queue_response(struct nvmet_req *nvme_req);
66 static void nvme_loop_delete_ctrl(struct nvmet_ctrl *ctrl);
68 static const struct nvmet_fabrics_ops nvme_loop_ops;
70 static inline int nvme_loop_queue_idx(struct nvme_loop_queue *queue)
72 return queue - queue->ctrl->queues;
75 static void nvme_loop_complete_rq(struct request *req)
77 struct nvme_loop_iod *iod = blk_mq_rq_to_pdu(req);
79 nvme_cleanup_cmd(req);
80 sg_free_table_chained(&iod->sg_table, SG_CHUNK_SIZE);
81 nvme_complete_rq(req);
84 static struct blk_mq_tags *nvme_loop_tagset(struct nvme_loop_queue *queue)
86 u32 queue_idx = nvme_loop_queue_idx(queue);
89 return queue->ctrl->admin_tag_set.tags[queue_idx];
90 return queue->ctrl->tag_set.tags[queue_idx - 1];
93 static void nvme_loop_queue_response(struct nvmet_req *req)
95 struct nvme_loop_queue *queue =
96 container_of(req->sq, struct nvme_loop_queue, nvme_sq);
97 struct nvme_completion *cqe = req->cqe;
100 * AEN requests are special as they don't time out and can
101 * survive any kind of queue freeze and often don't respond to
102 * aborts. We don't even bother to allocate a struct request
103 * for them but rather special case them here.
105 if (unlikely(nvme_loop_queue_idx(queue) == 0 &&
106 cqe->command_id >= NVME_AQ_BLK_MQ_DEPTH)) {
107 nvme_complete_async_event(&queue->ctrl->ctrl, cqe->status,
112 rq = blk_mq_tag_to_rq(nvme_loop_tagset(queue), cqe->command_id);
114 dev_err(queue->ctrl->ctrl.device,
115 "tag 0x%x on queue %d not found\n",
116 cqe->command_id, nvme_loop_queue_idx(queue));
120 nvme_end_request(rq, cqe->status, cqe->result);
124 static void nvme_loop_execute_work(struct work_struct *work)
126 struct nvme_loop_iod *iod =
127 container_of(work, struct nvme_loop_iod, work);
129 nvmet_req_execute(&iod->req);
132 static blk_status_t nvme_loop_queue_rq(struct blk_mq_hw_ctx *hctx,
133 const struct blk_mq_queue_data *bd)
135 struct nvme_ns *ns = hctx->queue->queuedata;
136 struct nvme_loop_queue *queue = hctx->driver_data;
137 struct request *req = bd->rq;
138 struct nvme_loop_iod *iod = blk_mq_rq_to_pdu(req);
139 bool queue_ready = test_bit(NVME_LOOP_Q_LIVE, &queue->flags);
142 if (!nvmf_check_ready(&queue->ctrl->ctrl, req, queue_ready))
143 return nvmf_fail_nonready_command(&queue->ctrl->ctrl, req);
145 ret = nvme_setup_cmd(ns, req, &iod->cmd);
149 blk_mq_start_request(req);
150 iod->cmd.common.flags |= NVME_CMD_SGL_METABUF;
151 iod->req.port = queue->ctrl->port;
152 if (!nvmet_req_init(&iod->req, &queue->nvme_cq,
153 &queue->nvme_sq, &nvme_loop_ops))
156 if (blk_rq_nr_phys_segments(req)) {
157 iod->sg_table.sgl = iod->first_sgl;
158 if (sg_alloc_table_chained(&iod->sg_table,
159 blk_rq_nr_phys_segments(req),
160 iod->sg_table.sgl, SG_CHUNK_SIZE))
161 return BLK_STS_RESOURCE;
163 iod->req.sg = iod->sg_table.sgl;
164 iod->req.sg_cnt = blk_rq_map_sg(req->q, req, iod->sg_table.sgl);
165 iod->req.transfer_len = blk_rq_payload_bytes(req);
168 schedule_work(&iod->work);
172 static void nvme_loop_submit_async_event(struct nvme_ctrl *arg)
174 struct nvme_loop_ctrl *ctrl = to_loop_ctrl(arg);
175 struct nvme_loop_queue *queue = &ctrl->queues[0];
176 struct nvme_loop_iod *iod = &ctrl->async_event_iod;
178 memset(&iod->cmd, 0, sizeof(iod->cmd));
179 iod->cmd.common.opcode = nvme_admin_async_event;
180 iod->cmd.common.command_id = NVME_AQ_BLK_MQ_DEPTH;
181 iod->cmd.common.flags |= NVME_CMD_SGL_METABUF;
183 if (!nvmet_req_init(&iod->req, &queue->nvme_cq, &queue->nvme_sq,
185 dev_err(ctrl->ctrl.device, "failed async event work\n");
189 schedule_work(&iod->work);
192 static int nvme_loop_init_iod(struct nvme_loop_ctrl *ctrl,
193 struct nvme_loop_iod *iod, unsigned int queue_idx)
195 iod->req.cmd = &iod->cmd;
196 iod->req.cqe = &iod->cqe;
197 iod->queue = &ctrl->queues[queue_idx];
198 INIT_WORK(&iod->work, nvme_loop_execute_work);
202 static int nvme_loop_init_request(struct blk_mq_tag_set *set,
203 struct request *req, unsigned int hctx_idx,
204 unsigned int numa_node)
206 struct nvme_loop_ctrl *ctrl = set->driver_data;
208 nvme_req(req)->ctrl = &ctrl->ctrl;
209 return nvme_loop_init_iod(ctrl, blk_mq_rq_to_pdu(req),
210 (set == &ctrl->tag_set) ? hctx_idx + 1 : 0);
213 static int nvme_loop_init_hctx(struct blk_mq_hw_ctx *hctx, void *data,
214 unsigned int hctx_idx)
216 struct nvme_loop_ctrl *ctrl = data;
217 struct nvme_loop_queue *queue = &ctrl->queues[hctx_idx + 1];
219 BUG_ON(hctx_idx >= ctrl->ctrl.queue_count);
221 hctx->driver_data = queue;
225 static int nvme_loop_init_admin_hctx(struct blk_mq_hw_ctx *hctx, void *data,
226 unsigned int hctx_idx)
228 struct nvme_loop_ctrl *ctrl = data;
229 struct nvme_loop_queue *queue = &ctrl->queues[0];
231 BUG_ON(hctx_idx != 0);
233 hctx->driver_data = queue;
237 static const struct blk_mq_ops nvme_loop_mq_ops = {
238 .queue_rq = nvme_loop_queue_rq,
239 .complete = nvme_loop_complete_rq,
240 .init_request = nvme_loop_init_request,
241 .init_hctx = nvme_loop_init_hctx,
244 static const struct blk_mq_ops nvme_loop_admin_mq_ops = {
245 .queue_rq = nvme_loop_queue_rq,
246 .complete = nvme_loop_complete_rq,
247 .init_request = nvme_loop_init_request,
248 .init_hctx = nvme_loop_init_admin_hctx,
251 static void nvme_loop_destroy_admin_queue(struct nvme_loop_ctrl *ctrl)
253 clear_bit(NVME_LOOP_Q_LIVE, &ctrl->queues[0].flags);
254 nvmet_sq_destroy(&ctrl->queues[0].nvme_sq);
255 blk_cleanup_queue(ctrl->ctrl.admin_q);
256 blk_mq_free_tag_set(&ctrl->admin_tag_set);
259 static void nvme_loop_free_ctrl(struct nvme_ctrl *nctrl)
261 struct nvme_loop_ctrl *ctrl = to_loop_ctrl(nctrl);
263 if (list_empty(&ctrl->list))
266 mutex_lock(&nvme_loop_ctrl_mutex);
267 list_del(&ctrl->list);
268 mutex_unlock(&nvme_loop_ctrl_mutex);
271 blk_cleanup_queue(ctrl->ctrl.connect_q);
272 blk_mq_free_tag_set(&ctrl->tag_set);
275 nvmf_free_options(nctrl->opts);
280 static void nvme_loop_destroy_io_queues(struct nvme_loop_ctrl *ctrl)
284 for (i = 1; i < ctrl->ctrl.queue_count; i++) {
285 clear_bit(NVME_LOOP_Q_LIVE, &ctrl->queues[i].flags);
286 nvmet_sq_destroy(&ctrl->queues[i].nvme_sq);
290 static int nvme_loop_init_io_queues(struct nvme_loop_ctrl *ctrl)
292 struct nvmf_ctrl_options *opts = ctrl->ctrl.opts;
293 unsigned int nr_io_queues;
296 nr_io_queues = min(opts->nr_io_queues, num_online_cpus());
297 ret = nvme_set_queue_count(&ctrl->ctrl, &nr_io_queues);
298 if (ret || !nr_io_queues)
301 dev_info(ctrl->ctrl.device, "creating %d I/O queues.\n", nr_io_queues);
303 for (i = 1; i <= nr_io_queues; i++) {
304 ctrl->queues[i].ctrl = ctrl;
305 ret = nvmet_sq_init(&ctrl->queues[i].nvme_sq);
307 goto out_destroy_queues;
309 ctrl->ctrl.queue_count++;
315 nvme_loop_destroy_io_queues(ctrl);
319 static int nvme_loop_connect_io_queues(struct nvme_loop_ctrl *ctrl)
323 for (i = 1; i < ctrl->ctrl.queue_count; i++) {
324 ret = nvmf_connect_io_queue(&ctrl->ctrl, i, false);
327 set_bit(NVME_LOOP_Q_LIVE, &ctrl->queues[i].flags);
333 static int nvme_loop_configure_admin_queue(struct nvme_loop_ctrl *ctrl)
337 memset(&ctrl->admin_tag_set, 0, sizeof(ctrl->admin_tag_set));
338 ctrl->admin_tag_set.ops = &nvme_loop_admin_mq_ops;
339 ctrl->admin_tag_set.queue_depth = NVME_AQ_MQ_TAG_DEPTH;
340 ctrl->admin_tag_set.reserved_tags = 2; /* connect + keep-alive */
341 ctrl->admin_tag_set.numa_node = NUMA_NO_NODE;
342 ctrl->admin_tag_set.cmd_size = sizeof(struct nvme_loop_iod) +
343 SG_CHUNK_SIZE * sizeof(struct scatterlist);
344 ctrl->admin_tag_set.driver_data = ctrl;
345 ctrl->admin_tag_set.nr_hw_queues = 1;
346 ctrl->admin_tag_set.timeout = ADMIN_TIMEOUT;
347 ctrl->admin_tag_set.flags = BLK_MQ_F_NO_SCHED;
349 ctrl->queues[0].ctrl = ctrl;
350 error = nvmet_sq_init(&ctrl->queues[0].nvme_sq);
353 ctrl->ctrl.queue_count = 1;
355 error = blk_mq_alloc_tag_set(&ctrl->admin_tag_set);
358 ctrl->ctrl.admin_tagset = &ctrl->admin_tag_set;
360 ctrl->ctrl.admin_q = blk_mq_init_queue(&ctrl->admin_tag_set);
361 if (IS_ERR(ctrl->ctrl.admin_q)) {
362 error = PTR_ERR(ctrl->ctrl.admin_q);
363 goto out_free_tagset;
366 error = nvmf_connect_admin_queue(&ctrl->ctrl);
368 goto out_cleanup_queue;
370 set_bit(NVME_LOOP_Q_LIVE, &ctrl->queues[0].flags);
372 error = nvmf_reg_read64(&ctrl->ctrl, NVME_REG_CAP, &ctrl->ctrl.cap);
374 dev_err(ctrl->ctrl.device,
375 "prop_get NVME_REG_CAP failed\n");
376 goto out_cleanup_queue;
380 min_t(int, NVME_CAP_MQES(ctrl->ctrl.cap), ctrl->ctrl.sqsize);
382 error = nvme_enable_ctrl(&ctrl->ctrl, ctrl->ctrl.cap);
384 goto out_cleanup_queue;
386 ctrl->ctrl.max_hw_sectors =
387 (NVME_LOOP_MAX_SEGMENTS - 1) << (PAGE_SHIFT - 9);
389 error = nvme_init_identify(&ctrl->ctrl);
391 goto out_cleanup_queue;
396 blk_cleanup_queue(ctrl->ctrl.admin_q);
398 blk_mq_free_tag_set(&ctrl->admin_tag_set);
400 nvmet_sq_destroy(&ctrl->queues[0].nvme_sq);
404 static void nvme_loop_shutdown_ctrl(struct nvme_loop_ctrl *ctrl)
406 if (ctrl->ctrl.queue_count > 1) {
407 nvme_stop_queues(&ctrl->ctrl);
408 blk_mq_tagset_busy_iter(&ctrl->tag_set,
409 nvme_cancel_request, &ctrl->ctrl);
410 nvme_loop_destroy_io_queues(ctrl);
413 if (ctrl->ctrl.state == NVME_CTRL_LIVE)
414 nvme_shutdown_ctrl(&ctrl->ctrl);
416 blk_mq_quiesce_queue(ctrl->ctrl.admin_q);
417 blk_mq_tagset_busy_iter(&ctrl->admin_tag_set,
418 nvme_cancel_request, &ctrl->ctrl);
419 blk_mq_unquiesce_queue(ctrl->ctrl.admin_q);
420 nvme_loop_destroy_admin_queue(ctrl);
423 static void nvme_loop_delete_ctrl_host(struct nvme_ctrl *ctrl)
425 nvme_loop_shutdown_ctrl(to_loop_ctrl(ctrl));
428 static void nvme_loop_delete_ctrl(struct nvmet_ctrl *nctrl)
430 struct nvme_loop_ctrl *ctrl;
432 mutex_lock(&nvme_loop_ctrl_mutex);
433 list_for_each_entry(ctrl, &nvme_loop_ctrl_list, list) {
434 if (ctrl->ctrl.cntlid == nctrl->cntlid)
435 nvme_delete_ctrl(&ctrl->ctrl);
437 mutex_unlock(&nvme_loop_ctrl_mutex);
440 static void nvme_loop_reset_ctrl_work(struct work_struct *work)
442 struct nvme_loop_ctrl *ctrl =
443 container_of(work, struct nvme_loop_ctrl, ctrl.reset_work);
447 nvme_stop_ctrl(&ctrl->ctrl);
448 nvme_loop_shutdown_ctrl(ctrl);
450 if (!nvme_change_ctrl_state(&ctrl->ctrl, NVME_CTRL_CONNECTING)) {
451 /* state change failure should never happen */
456 ret = nvme_loop_configure_admin_queue(ctrl);
460 ret = nvme_loop_init_io_queues(ctrl);
462 goto out_destroy_admin;
464 ret = nvme_loop_connect_io_queues(ctrl);
468 blk_mq_update_nr_hw_queues(&ctrl->tag_set,
469 ctrl->ctrl.queue_count - 1);
471 changed = nvme_change_ctrl_state(&ctrl->ctrl, NVME_CTRL_LIVE);
472 WARN_ON_ONCE(!changed);
474 nvme_start_ctrl(&ctrl->ctrl);
479 nvme_loop_destroy_io_queues(ctrl);
481 nvme_loop_destroy_admin_queue(ctrl);
483 dev_warn(ctrl->ctrl.device, "Removing after reset failure\n");
484 nvme_uninit_ctrl(&ctrl->ctrl);
485 nvme_put_ctrl(&ctrl->ctrl);
488 static const struct nvme_ctrl_ops nvme_loop_ctrl_ops = {
490 .module = THIS_MODULE,
491 .flags = NVME_F_FABRICS,
492 .reg_read32 = nvmf_reg_read32,
493 .reg_read64 = nvmf_reg_read64,
494 .reg_write32 = nvmf_reg_write32,
495 .free_ctrl = nvme_loop_free_ctrl,
496 .submit_async_event = nvme_loop_submit_async_event,
497 .delete_ctrl = nvme_loop_delete_ctrl_host,
498 .get_address = nvmf_get_address,
501 static int nvme_loop_create_io_queues(struct nvme_loop_ctrl *ctrl)
505 ret = nvme_loop_init_io_queues(ctrl);
509 memset(&ctrl->tag_set, 0, sizeof(ctrl->tag_set));
510 ctrl->tag_set.ops = &nvme_loop_mq_ops;
511 ctrl->tag_set.queue_depth = ctrl->ctrl.opts->queue_size;
512 ctrl->tag_set.reserved_tags = 1; /* fabric connect */
513 ctrl->tag_set.numa_node = NUMA_NO_NODE;
514 ctrl->tag_set.flags = BLK_MQ_F_SHOULD_MERGE;
515 ctrl->tag_set.cmd_size = sizeof(struct nvme_loop_iod) +
516 SG_CHUNK_SIZE * sizeof(struct scatterlist);
517 ctrl->tag_set.driver_data = ctrl;
518 ctrl->tag_set.nr_hw_queues = ctrl->ctrl.queue_count - 1;
519 ctrl->tag_set.timeout = NVME_IO_TIMEOUT;
520 ctrl->ctrl.tagset = &ctrl->tag_set;
522 ret = blk_mq_alloc_tag_set(&ctrl->tag_set);
524 goto out_destroy_queues;
526 ctrl->ctrl.connect_q = blk_mq_init_queue(&ctrl->tag_set);
527 if (IS_ERR(ctrl->ctrl.connect_q)) {
528 ret = PTR_ERR(ctrl->ctrl.connect_q);
529 goto out_free_tagset;
532 ret = nvme_loop_connect_io_queues(ctrl);
534 goto out_cleanup_connect_q;
538 out_cleanup_connect_q:
539 blk_cleanup_queue(ctrl->ctrl.connect_q);
541 blk_mq_free_tag_set(&ctrl->tag_set);
543 nvme_loop_destroy_io_queues(ctrl);
547 static struct nvmet_port *nvme_loop_find_port(struct nvme_ctrl *ctrl)
549 struct nvmet_port *p, *found = NULL;
551 mutex_lock(&nvme_loop_ports_mutex);
552 list_for_each_entry(p, &nvme_loop_ports, entry) {
553 /* if no transport address is specified use the first port */
554 if ((ctrl->opts->mask & NVMF_OPT_TRADDR) &&
555 strcmp(ctrl->opts->traddr, p->disc_addr.traddr))
560 mutex_unlock(&nvme_loop_ports_mutex);
564 static struct nvme_ctrl *nvme_loop_create_ctrl(struct device *dev,
565 struct nvmf_ctrl_options *opts)
567 struct nvme_loop_ctrl *ctrl;
571 ctrl = kzalloc(sizeof(*ctrl), GFP_KERNEL);
573 return ERR_PTR(-ENOMEM);
574 ctrl->ctrl.opts = opts;
575 INIT_LIST_HEAD(&ctrl->list);
577 INIT_WORK(&ctrl->ctrl.reset_work, nvme_loop_reset_ctrl_work);
579 ret = nvme_init_ctrl(&ctrl->ctrl, dev, &nvme_loop_ctrl_ops,
580 0 /* no quirks, we're perfect! */);
586 ctrl->ctrl.sqsize = opts->queue_size - 1;
587 ctrl->ctrl.kato = opts->kato;
588 ctrl->port = nvme_loop_find_port(&ctrl->ctrl);
590 ctrl->queues = kcalloc(opts->nr_io_queues + 1, sizeof(*ctrl->queues),
593 goto out_uninit_ctrl;
595 ret = nvme_loop_configure_admin_queue(ctrl);
597 goto out_free_queues;
599 if (opts->queue_size > ctrl->ctrl.maxcmd) {
600 /* warn if maxcmd is lower than queue_size */
601 dev_warn(ctrl->ctrl.device,
602 "queue_size %zu > ctrl maxcmd %u, clamping down\n",
603 opts->queue_size, ctrl->ctrl.maxcmd);
604 opts->queue_size = ctrl->ctrl.maxcmd;
607 if (opts->nr_io_queues) {
608 ret = nvme_loop_create_io_queues(ctrl);
610 goto out_remove_admin_queue;
613 nvme_loop_init_iod(ctrl, &ctrl->async_event_iod, 0);
615 dev_info(ctrl->ctrl.device,
616 "new ctrl: \"%s\"\n", ctrl->ctrl.opts->subsysnqn);
618 nvme_get_ctrl(&ctrl->ctrl);
620 changed = nvme_change_ctrl_state(&ctrl->ctrl, NVME_CTRL_LIVE);
621 WARN_ON_ONCE(!changed);
623 mutex_lock(&nvme_loop_ctrl_mutex);
624 list_add_tail(&ctrl->list, &nvme_loop_ctrl_list);
625 mutex_unlock(&nvme_loop_ctrl_mutex);
627 nvme_start_ctrl(&ctrl->ctrl);
631 out_remove_admin_queue:
632 nvme_loop_destroy_admin_queue(ctrl);
636 nvme_uninit_ctrl(&ctrl->ctrl);
638 nvme_put_ctrl(&ctrl->ctrl);
644 static int nvme_loop_add_port(struct nvmet_port *port)
646 mutex_lock(&nvme_loop_ports_mutex);
647 list_add_tail(&port->entry, &nvme_loop_ports);
648 mutex_unlock(&nvme_loop_ports_mutex);
652 static void nvme_loop_remove_port(struct nvmet_port *port)
654 mutex_lock(&nvme_loop_ports_mutex);
655 list_del_init(&port->entry);
656 mutex_unlock(&nvme_loop_ports_mutex);
659 static const struct nvmet_fabrics_ops nvme_loop_ops = {
660 .owner = THIS_MODULE,
661 .type = NVMF_TRTYPE_LOOP,
662 .add_port = nvme_loop_add_port,
663 .remove_port = nvme_loop_remove_port,
664 .queue_response = nvme_loop_queue_response,
665 .delete_ctrl = nvme_loop_delete_ctrl,
668 static struct nvmf_transport_ops nvme_loop_transport = {
670 .module = THIS_MODULE,
671 .create_ctrl = nvme_loop_create_ctrl,
672 .allowed_opts = NVMF_OPT_TRADDR,
675 static int __init nvme_loop_init_module(void)
679 ret = nvmet_register_transport(&nvme_loop_ops);
683 ret = nvmf_register_transport(&nvme_loop_transport);
685 nvmet_unregister_transport(&nvme_loop_ops);
690 static void __exit nvme_loop_cleanup_module(void)
692 struct nvme_loop_ctrl *ctrl, *next;
694 nvmf_unregister_transport(&nvme_loop_transport);
695 nvmet_unregister_transport(&nvme_loop_ops);
697 mutex_lock(&nvme_loop_ctrl_mutex);
698 list_for_each_entry_safe(ctrl, next, &nvme_loop_ctrl_list, list)
699 nvme_delete_ctrl(&ctrl->ctrl);
700 mutex_unlock(&nvme_loop_ctrl_mutex);
702 flush_workqueue(nvme_delete_wq);
705 module_init(nvme_loop_init_module);
706 module_exit(nvme_loop_cleanup_module);
708 MODULE_LICENSE("GPL v2");
709 MODULE_ALIAS("nvmet-transport-254"); /* 254 == NVMF_TRTYPE_LOOP */