path: root/Documentation/target
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authorAndy Grover <>2014-10-01 16:07:04 -0700
committerNicholas Bellinger <>2014-10-02 21:35:54 -0700
commitce87685128f3e0fced2aca9f73fc8cc67704ae11 (patch)
treeeb190fafcbc45d7818b98c15bbe9cb3e8cf94090 /Documentation/target
parentf14bb039a4e8206439d3e9abd92bc76bd142f243 (diff)
target: Add documentation on the target userspace pass-through driver
Describes the driver and its interface to make it possible for user programs to back a LIO-exported LUN. Thanks to Richard W. M. Jones for review, and supplementing this doc with the first two paragraphs. Signed-off-by: Andy Grover <> Signed-off-by: Nicholas Bellinger <>
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+1) TCM Userspace Design
+ a) Background
+ b) Benefits
+ c) Design constraints
+ d) Implementation overview
+ i. Mailbox
+ ii. Command ring
+ iii. Data Area
+ e) Device discovery
+ f) Device events
+ g) Other contingencies
+2) Writing a user pass-through handler
+ a) Discovering and configuring TCMU uio devices
+ b) Waiting for events on the device(s)
+ c) Managing the command ring
+3) Command filtering and pass_level
+4) A final note
+TCM Userspace Design
+TCM is another name for LIO, an in-kernel iSCSI target (server).
+Existing TCM targets run in the kernel. TCMU (TCM in Userspace)
+allows userspace programs to be written which act as iSCSI targets.
+This document describes the design.
+The existing kernel provides modules for different SCSI transport
+protocols. TCM also modularizes the data storage. There are existing
+modules for file, block device, RAM or using another SCSI device as
+storage. These are called "backstores" or "storage engines". These
+built-in modules are implemented entirely as kernel code.
+In addition to modularizing the transport protocol used for carrying
+SCSI commands ("fabrics"), the Linux kernel target, LIO, also modularizes
+the actual data storage as well. These are referred to as "backstores"
+or "storage engines". The target comes with backstores that allow a
+file, a block device, RAM, or another SCSI device to be used for the
+local storage needed for the exported SCSI LUN. Like the rest of LIO,
+these are implemented entirely as kernel code.
+These backstores cover the most common use cases, but not all. One new
+use case that other non-kernel target solutions, such as tgt, are able
+to support is using Gluster's GLFS or Ceph's RBD as a backstore. The
+target then serves as a translator, allowing initiators to store data
+in these non-traditional networked storage systems, while still only
+using standard protocols themselves.
+If the target is a userspace process, supporting these is easy. tgt,
+for example, needs only a small adapter module for each, because the
+modules just use the available userspace libraries for RBD and GLFS.
+Adding support for these backstores in LIO is considerably more
+difficult, because LIO is entirely kernel code. Instead of undertaking
+the significant work to port the GLFS or RBD APIs and protocols to the
+kernel, another approach is to create a userspace pass-through
+backstore for LIO, "TCMU".
+In addition to allowing relatively easy support for RBD and GLFS, TCMU
+will also allow easier development of new backstores. TCMU combines
+with the LIO loopback fabric to become something similar to FUSE
+(Filesystem in Userspace), but at the SCSI layer instead of the
+filesystem layer. A SUSE, if you will.
+The disadvantage is there are more distinct components to configure, and
+potentially to malfunction. This is unavoidable, but hopefully not
+fatal if we're careful to keep things as simple as possible.
+Design constraints:
+- Good performance: high throughput, low latency
+- Cleanly handle if userspace:
+ 1) never attaches
+ 2) hangs
+ 3) dies
+ 4) misbehaves
+- Allow future flexibility in user & kernel implementations
+- Be reasonably memory-efficient
+- Simple to configure & run
+- Simple to write a userspace backend
+Implementation overview:
+The core of the TCMU interface is a memory region that is shared
+between kernel and userspace. Within this region is: a control area
+(mailbox); a lockless producer/consumer circular buffer for commands
+to be passed up, and status returned; and an in/out data buffer area.
+TCMU uses the pre-existing UIO subsystem. UIO allows device driver
+development in userspace, and this is conceptually very close to the
+TCMU use case, except instead of a physical device, TCMU implements a
+memory-mapped layout designed for SCSI commands. Using UIO also
+benefits TCMU by handling device introspection (e.g. a way for
+userspace to determine how large the shared region is) and signaling
+mechanisms in both directions.
+There are no embedded pointers in the memory region. Everything is
+expressed as an offset from the region's starting address. This allows
+the ring to still work if the user process dies and is restarted with
+the region mapped at a different virtual address.
+See target_core_user.h for the struct definitions.
+The Mailbox:
+The mailbox is always at the start of the shared memory region, and
+contains a version, details about the starting offset and size of the
+command ring, and head and tail pointers to be used by the kernel and
+userspace (respectively) to put commands on the ring, and indicate
+when the commands are completed.
+version - 1 (userspace should abort if otherwise)
+flags - none yet defined.
+cmdr_off - The offset of the start of the command ring from the start
+of the memory region, to account for the mailbox size.
+cmdr_size - The size of the command ring. This does *not* need to be a
+power of two.
+cmd_head - Modified by the kernel to indicate when a command has been
+placed on the ring.
+cmd_tail - Modified by userspace to indicate when it has completed
+processing of a command.
+The Command Ring:
+Commands are placed on the ring by the kernel incrementing
+mailbox.cmd_head by the size of the command, modulo cmdr_size, and
+then signaling userspace via uio_event_notify(). Once the command is
+completed, userspace updates mailbox.cmd_tail in the same way and
+signals the kernel via a 4-byte write(). When cmd_head equals
+cmd_tail, the ring is empty -- no commands are currently waiting to be
+processed by userspace.
+TCMU commands start with a common header containing "len_op", a 32-bit
+value that stores the length, as well as the opcode in the lowest
+unused bits. Currently only two opcodes are defined, TCMU_OP_PAD and
+TCMU_OP_CMD. When userspace encounters a command with PAD opcode, it
+should skip ahead by the bytes in "length". (The kernel inserts PAD
+entries to ensure each CMD entry fits contigously into the circular
+When userspace handles a CMD, it finds the SCSI CDB (Command Data
+Block) via tcmu_cmd_entry.req.cdb_off. This is an offset from the
+start of the overall shared memory region, not the entry. The data
+in/out buffers are accessible via tht req.iov[] array. Note that
+each iov.iov_base is also an offset from the start of the region.
+TCMU currently does not support BIDI operations.
+When completing a command, userspace sets rsp.scsi_status, and
+rsp.sense_buffer if necessary. Userspace then increments
+mailbox.cmd_tail by entry.hdr.length (mod cmdr_size) and signals the
+kernel via the UIO method, a 4-byte write to the file descriptor.
+The Data Area:
+This is shared-memory space after the command ring. The organization
+of this area is not defined in the TCMU interface, and userspace
+should access only the parts referenced by pending iovs.
+Device Discovery:
+Other devices may be using UIO besides TCMU. Unrelated user processes
+may also be handling different sets of TCMU devices. TCMU userspace
+processes must find their devices by scanning sysfs
+class/uio/uio*/name. For TCMU devices, these names will be of the
+where "tcm-user" is common for all TCMU-backed UIO devices. <hba_num>
+and <device_name> allow userspace to find the device's path in the
+kernel target's configfs tree. Assuming the usual mount point, it is
+found at:
+This location contains attributes such as "hw_block_size", that
+userspace needs to know for correct operation.
+<subtype> will be a userspace-process-unique string to identify the
+TCMU device as expecting to be backed by a certain handler, and <path>
+will be an additional handler-specific string for the user process to
+configure the device, if needed. The name cannot contain ':', due to
+LIO limitations.
+For all devices so discovered, the user handler opens /dev/uioX and
+calls mmap():
+where size must be equal to the value read from
+Device Events:
+If a new device is added or removed, a notification will be broadcast
+over netlink, using a generic netlink family name of "TCM-USER" and a
+multicast group named "config". This will include the UIO name as
+described in the previous section, as well as the UIO minor
+number. This should allow userspace to identify both the UIO device and
+the LIO device, so that after determining the device is supported
+(based on subtype) it can take the appropriate action.
+Other contingencies:
+Userspace handler process never attaches:
+- TCMU will post commands, and then abort them after a timeout period
+ (30 seconds.)
+Userspace handler process is killed:
+- It is still possible to restart and re-connect to TCMU
+ devices. Command ring is preserved. However, after the timeout period,
+ the kernel will abort pending tasks.
+Userspace handler process hangs:
+- The kernel will abort pending tasks after a timeout period.
+Userspace handler process is malicious:
+- The process can trivially break the handling of devices it controls,
+ but should not be able to access kernel memory outside its shared
+ memory areas.
+Writing a user pass-through handler (with example code)
+A user process handing a TCMU device must support the following:
+a) Discovering and configuring TCMU uio devices
+b) Waiting for events on the device(s)
+c) Managing the command ring: Parsing operations and commands,
+ performing work as needed, setting response fields (scsi_status and
+ possibly sense_buffer), updating cmd_tail, and notifying the kernel
+ that work has been finished
+First, consider instead writing a plugin for tcmu-runner. tcmu-runner
+implements all of this, and provides a higher-level API for plugin
+TCMU is designed so that multiple unrelated processes can manage TCMU
+devices separately. All handlers should make sure to only open their
+devices, based opon a known subtype string.
+a) Discovering and configuring TCMU UIO devices:
+(error checking omitted for brevity)
+int fd, dev_fd;
+char buf[256];
+unsigned long long map_len;
+void *map;
+fd = open("/sys/class/uio/uio0/name", O_RDONLY);
+ret = read(fd, buf, sizeof(buf));
+buf[ret-1] = '\0'; /* null-terminate and chop off the \n */
+/* we only want uio devices whose name is a format we expect */
+if (strncmp(buf, "tcm-user", 8))
+ exit(-1);
+/* Further checking for subtype also needed here */
+fd = open(/sys/class/uio/%s/maps/map0/size, O_RDONLY);
+ret = read(fd, buf, sizeof(buf));
+str_buf[ret-1] = '\0'; /* null-terminate and chop off the \n */
+map_len = strtoull(buf, NULL, 0);
+dev_fd = open("/dev/uio0", O_RDWR);
+map = mmap(NULL, map_len, PROT_READ|PROT_WRITE, MAP_SHARED, dev_fd, 0);
+b) Waiting for events on the device(s)
+while (1) {
+ char buf[4];
+ int ret = read(dev_fd, buf, 4); /* will block */
+ handle_device_events(dev_fd, map);
+c) Managing the command ring
+#include <linux/target_core_user.h>
+int handle_device_events(int fd, void *map)
+ struct tcmu_mailbox *mb = map;
+ struct tcmu_cmd_entry *ent = (void *) mb + mb->cmdr_off + mb->cmd_tail;
+ int did_some_work = 0;
+ /* Process events from cmd ring until we catch up with cmd_head */
+ while (ent != (void *)mb + mb->cmdr_off + mb->cmd_head) {
+ if (tcmu_hdr_get_op(&ent->hdr) == TCMU_OP_CMD) {
+ uint8_t *cdb = (void *)mb + ent->req.cdb_off;
+ bool success = true;
+ /* Handle command here. */
+ printf("SCSI opcode: 0x%x\n", cdb[0]);
+ /* Set response fields */
+ if (success)
+ ent->rsp.scsi_status = SCSI_NO_SENSE;
+ else {
+ /* Also fill in rsp->sense_buffer here */
+ ent->rsp.scsi_status = SCSI_CHECK_CONDITION;
+ }
+ }
+ else {
+ /* Do nothing for PAD entries */
+ }
+ /* update cmd_tail */
+ mb->cmd_tail = (mb->cmd_tail + tcmu_hdr_get_len(&ent->hdr)) % mb->cmdr_size;
+ ent = (void *) mb + mb->cmdr_off + mb->cmd_tail;
+ did_some_work = 1;
+ }
+ /* Notify the kernel that work has been finished */
+ if (did_some_work) {
+ uint32_t buf = 0;
+ write(fd, &buf, 4);
+ }
+ return 0;
+Command filtering and pass_level
+TCMU supports a "pass_level" option with valid values of 0 or 1. When
+the value is 0 (the default), nearly all SCSI commands received for
+the device are passed through to the handler. This allows maximum
+flexibility but increases the amount of code required by the handler,
+to support all mandatory SCSI commands. If pass_level is set to 1,
+then only IO-related commands are presented, and the rest are handled
+by LIO's in-kernel command emulation. The commands presented at level
+1 include all versions of:
+A final note
+Please be careful to return codes as defined by the SCSI
+specifications. These are different than some values defined in the
+scsi/scsi.h include file. For example, CHECK CONDITION's status code
+is 2, not 1.