Adding upstream version 6.6.15.upstream/6.6.15

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

5 files changed, 658 insertions, 0 deletions

diff --git a/Documentation/target/index.rst b/Documentation/target/index.rst
new file mode 100644
index 000000000..4b24f81f7
--- /dev/null
+++ b/Documentation/target/index.rst
@@ -0,0 +1,19 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==================
+TCM Virtual Device
+==================
+
+.. toctree::
+    :maxdepth: 1
+
+    tcmu-design
+    tcm_mod_builder
+    scripts
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`
diff --git a/Documentation/target/scripts.rst b/Documentation/target/scripts.rst
new file mode 100644
index 000000000..aa7b9c62c
--- /dev/null
+++ b/Documentation/target/scripts.rst
@@ -0,0 +1,5 @@
+Target export device script
+---------------------------
+
+.. literalinclude:: target-export-device
+    :language: shell
diff --git a/Documentation/target/target-export-device b/Documentation/target/target-export-device
new file mode 100755
index 000000000..b803f4f88
--- /dev/null
+++ b/Documentation/target/target-export-device
@@ -0,0 +1,80 @@
+#!/bin/sh
+#
+# This script illustrates the sequence of operations in configfs to
+# create a very simple LIO iSCSI target with a file or block device
+# backstore.
+#
+# (C) Copyright 2014 Christophe Vu-Brugier <cvubrugier@fastmail.fm>
+#
+
+print_usage() {
+    cat <<EOF
+Usage: $(basename $0) [-p PORTAL] DEVICE|FILE
+Export a block device or a file as an iSCSI target with a single LUN
+EOF
+}
+
+die() {
+    echo $1
+    exit 1
+}
+
+while getopts "hp:" arg; do
+    case $arg in
+        h) print_usage; exit 0;;
+        p) PORTAL=${OPTARG};;
+    esac
+done
+shift $(($OPTIND - 1))
+
+DEVICE=$1
+[ -n "$DEVICE" ] || die "Missing device or file argument"
+[ -b $DEVICE -o -f $DEVICE ] || die "Invalid device or file: ${DEVICE}"
+IQN="iqn.2003-01.org.linux-iscsi.$(hostname):$(basename $DEVICE)"
+[ -n "$PORTAL" ] || PORTAL="0.0.0.0:3260"
+
+CONFIGFS=/sys/kernel/config
+CORE_DIR=$CONFIGFS/target/core
+ISCSI_DIR=$CONFIGFS/target/iscsi
+
+# Load the target modules and mount the config file system
+lsmod | grep -q configfs || modprobe configfs
+lsmod | grep -q target_core_mod || modprobe target_core_mod
+mount | grep -q ^configfs || mount -t configfs none $CONFIGFS
+mkdir -p $ISCSI_DIR
+
+# Create a backstore
+if [ -b $DEVICE ]; then
+    BACKSTORE_DIR=$CORE_DIR/iblock_0/data
+    mkdir -p $BACKSTORE_DIR
+    echo "udev_path=${DEVICE}" > $BACKSTORE_DIR/control
+else
+    BACKSTORE_DIR=$CORE_DIR/fileio_0/data
+    mkdir -p $BACKSTORE_DIR
+    DEVICE_SIZE=$(du -b $DEVICE | cut -f1)
+    echo "fd_dev_name=${DEVICE}" > $BACKSTORE_DIR/control
+    echo "fd_dev_size=${DEVICE_SIZE}" > $BACKSTORE_DIR/control
+    echo 1 > $BACKSTORE_DIR/attrib/emulate_write_cache
+fi
+echo 1 > $BACKSTORE_DIR/enable
+
+# Create an iSCSI target and a target portal group (TPG)
+mkdir $ISCSI_DIR/$IQN
+mkdir $ISCSI_DIR/$IQN/tpgt_1/
+
+# Create a LUN
+mkdir $ISCSI_DIR/$IQN/tpgt_1/lun/lun_0
+ln -s $BACKSTORE_DIR $ISCSI_DIR/$IQN/tpgt_1/lun/lun_0/data
+echo 1 > $ISCSI_DIR/$IQN/tpgt_1/enable
+
+# Create a network portal
+mkdir $ISCSI_DIR/$IQN/tpgt_1/np/$PORTAL
+
+# Disable authentication
+echo 0 > $ISCSI_DIR/$IQN/tpgt_1/attrib/authentication
+echo 1 > $ISCSI_DIR/$IQN/tpgt_1/attrib/generate_node_acls
+
+# Allow write access for non authenticated initiators
+echo 0 > $ISCSI_DIR/$IQN/tpgt_1/attrib/demo_mode_write_protect
+
+echo "Target ${IQN}, portal ${PORTAL} has been created"
diff --git a/Documentation/target/tcm_mod_builder.rst b/Documentation/target/tcm_mod_builder.rst
new file mode 100644
index 000000000..9bfc9822e
--- /dev/null
+++ b/Documentation/target/tcm_mod_builder.rst
@@ -0,0 +1,149 @@
+=========================================
+The TCM v4 fabric module script generator
+=========================================
+
+Greetings all,
+
+This document is intended to be a mini-HOWTO for using the tcm_mod_builder.py
+script to generate a brand new functional TCM v4 fabric .ko module of your very own,
+that once built can be immediately be loaded to start access the new TCM/ConfigFS
+fabric skeleton, by simply using::
+
+	modprobe $TCM_NEW_MOD
+	mkdir -p /sys/kernel/config/target/$TCM_NEW_MOD
+
+This script will create a new drivers/target/$TCM_NEW_MOD/, and will do the following
+
+	1) Generate new API callers for drivers/target/target_core_fabric_configs.c logic
+	   ->make_tpg(), ->drop_tpg(), ->make_wwn(), ->drop_wwn().  These are created
+	   into $TCM_NEW_MOD/$TCM_NEW_MOD_configfs.c
+	2) Generate basic infrastructure for loading/unloading LKMs and TCM/ConfigFS fabric module
+	   using a skeleton struct target_core_fabric_ops API template.
+	3) Based on user defined T10 Proto_Ident for the new fabric module being built,
+	   the TransportID / Initiator and Target WWPN related handlers for
+	   SPC-3 persistent reservation are automatically generated in $TCM_NEW_MOD/$TCM_NEW_MOD_fabric.c
+	   using drivers/target/target_core_fabric_lib.c logic.
+	4) NOP API calls for all other Data I/O path and fabric dependent attribute logic
+	   in $TCM_NEW_MOD/$TCM_NEW_MOD_fabric.c
+
+tcm_mod_builder.py depends upon the mandatory '-p $PROTO_IDENT' and '-m
+$FABRIC_MOD_name' parameters, and actually running the script looks like::
+
+  target:/mnt/sdb/lio-core-2.6.git/Documentation/target# python tcm_mod_builder.py -p iSCSI -m tcm_nab5000
+  tcm_dir: /mnt/sdb/lio-core-2.6.git/Documentation/target/../../
+  Set fabric_mod_name: tcm_nab5000
+  Set fabric_mod_dir:
+  /mnt/sdb/lio-core-2.6.git/Documentation/target/../../drivers/target/tcm_nab5000
+  Using proto_ident: iSCSI
+  Creating fabric_mod_dir:
+  /mnt/sdb/lio-core-2.6.git/Documentation/target/../../drivers/target/tcm_nab5000
+  Writing file:
+  /mnt/sdb/lio-core-2.6.git/Documentation/target/../../drivers/target/tcm_nab5000/tcm_nab5000_base.h
+  Using tcm_mod_scan_fabric_ops:
+  /mnt/sdb/lio-core-2.6.git/Documentation/target/../../include/target/target_core_fabric_ops.h
+  Writing file:
+  /mnt/sdb/lio-core-2.6.git/Documentation/target/../../drivers/target/tcm_nab5000/tcm_nab5000_fabric.c
+  Writing file:
+  /mnt/sdb/lio-core-2.6.git/Documentation/target/../../drivers/target/tcm_nab5000/tcm_nab5000_fabric.h
+  Writing file:
+  /mnt/sdb/lio-core-2.6.git/Documentation/target/../../drivers/target/tcm_nab5000/tcm_nab5000_configfs.c
+  Writing file:
+  /mnt/sdb/lio-core-2.6.git/Documentation/target/../../drivers/target/tcm_nab5000/Kbuild
+  Writing file:
+  /mnt/sdb/lio-core-2.6.git/Documentation/target/../../drivers/target/tcm_nab5000/Kconfig
+  Would you like to add tcm_nab5000to drivers/target/Kbuild..? [yes,no]: yes
+  Would you like to add tcm_nab5000to drivers/target/Kconfig..? [yes,no]: yes
+
+At the end of tcm_mod_builder.py. the script will ask to add the following
+line to drivers/target/Kbuild::
+
+	obj-$(CONFIG_TCM_NAB5000)       += tcm_nab5000/
+
+and the same for drivers/target/Kconfig::
+
+	source "drivers/target/tcm_nab5000/Kconfig"
+
+#) Run 'make menuconfig' and select the new CONFIG_TCM_NAB5000 item::
+
+	<M>   TCM_NAB5000 fabric module
+
+#) Build using 'make modules', once completed you will have::
+
+    target:/mnt/sdb/lio-core-2.6.git# ls -la drivers/target/tcm_nab5000/
+    total 1348
+    drwxr-xr-x 2 root root   4096 2010-10-05 03:23 .
+    drwxr-xr-x 9 root root   4096 2010-10-05 03:22 ..
+    -rw-r--r-- 1 root root    282 2010-10-05 03:22 Kbuild
+    -rw-r--r-- 1 root root    171 2010-10-05 03:22 Kconfig
+    -rw-r--r-- 1 root root     49 2010-10-05 03:23 modules.order
+    -rw-r--r-- 1 root root    738 2010-10-05 03:22 tcm_nab5000_base.h
+    -rw-r--r-- 1 root root   9096 2010-10-05 03:22 tcm_nab5000_configfs.c
+    -rw-r--r-- 1 root root 191200 2010-10-05 03:23 tcm_nab5000_configfs.o
+    -rw-r--r-- 1 root root  40504 2010-10-05 03:23 .tcm_nab5000_configfs.o.cmd
+    -rw-r--r-- 1 root root   5414 2010-10-05 03:22 tcm_nab5000_fabric.c
+    -rw-r--r-- 1 root root   2016 2010-10-05 03:22 tcm_nab5000_fabric.h
+    -rw-r--r-- 1 root root 190932 2010-10-05 03:23 tcm_nab5000_fabric.o
+    -rw-r--r-- 1 root root  40713 2010-10-05 03:23 .tcm_nab5000_fabric.o.cmd
+    -rw-r--r-- 1 root root 401861 2010-10-05 03:23 tcm_nab5000.ko
+    -rw-r--r-- 1 root root    265 2010-10-05 03:23 .tcm_nab5000.ko.cmd
+    -rw-r--r-- 1 root root    459 2010-10-05 03:23 tcm_nab5000.mod.c
+    -rw-r--r-- 1 root root  23896 2010-10-05 03:23 tcm_nab5000.mod.o
+    -rw-r--r-- 1 root root  22655 2010-10-05 03:23 .tcm_nab5000.mod.o.cmd
+    -rw-r--r-- 1 root root 379022 2010-10-05 03:23 tcm_nab5000.o
+    -rw-r--r-- 1 root root    211 2010-10-05 03:23 .tcm_nab5000.o.cmd
+
+#) Load the new module, create a lun_0 configfs group, and add new TCM Core
+   IBLOCK backstore symlink to port::
+
+    target:/mnt/sdb/lio-core-2.6.git# insmod drivers/target/tcm_nab5000.ko
+    target:/mnt/sdb/lio-core-2.6.git# mkdir -p /sys/kernel/config/target/nab5000/iqn.foo/tpgt_1/lun/lun_0
+    target:/mnt/sdb/lio-core-2.6.git# cd /sys/kernel/config/target/nab5000/iqn.foo/tpgt_1/lun/lun_0/
+    target:/sys/kernel/config/target/nab5000/iqn.foo/tpgt_1/lun/lun_0# ln -s /sys/kernel/config/target/core/iblock_0/lvm_test0 nab5000_port
+
+    target:/sys/kernel/config/target/nab5000/iqn.foo/tpgt_1/lun/lun_0# cd -
+    target:/mnt/sdb/lio-core-2.6.git# tree /sys/kernel/config/target/nab5000/
+    /sys/kernel/config/target/nab5000/
+    |-- discovery_auth
+    |-- iqn.foo
+    |   `-- tpgt_1
+    |       |-- acls
+    |       |-- attrib
+    |       |-- lun
+    |       |   `-- lun_0
+    |       |       |-- alua_tg_pt_gp
+    |       |       |-- alua_tg_pt_offline
+    |       |       |-- alua_tg_pt_status
+    |       |       |-- alua_tg_pt_write_md
+    |	|	`-- nab5000_port -> ../../../../../../target/core/iblock_0/lvm_test0
+    |       |-- np
+    |       `-- param
+    `-- version
+
+    target:/mnt/sdb/lio-core-2.6.git# lsmod
+    Module                  Size  Used by
+    tcm_nab5000             3935  4
+    iscsi_target_mod      193211  0
+    target_core_stgt        8090  0
+    target_core_pscsi      11122  1
+    target_core_file        9172  2
+    target_core_iblock      9280  1
+    target_core_mod       228575  31
+    tcm_nab5000,iscsi_target_mod,target_core_stgt,target_core_pscsi,target_core_file,target_core_iblock
+    libfc                  73681  0
+    scsi_debug             56265  0
+    scsi_tgt                8666  1 target_core_stgt
+    configfs               20644  2 target_core_mod
+
+----------------------------------------------------------------------
+
+Future TODO items
+=================
+
+	1) Add more T10 proto_idents
+	2) Make tcm_mod_dump_fabric_ops() smarter and generate function pointer
+	   defs directly from include/target/target_core_fabric_ops.h:struct target_core_fabric_ops
+	   structure members.
+
+October 5th, 2010
+
+Nicholas A. Bellinger <nab@linux-iscsi.org>
diff --git a/Documentation/target/tcmu-design.rst b/Documentation/target/tcmu-design.rst
new file mode 100644
index 000000000..eff3da1d2
--- /dev/null
+++ b/Documentation/target/tcmu-design.rst
@@ -0,0 +1,405 @@
+====================
+TCM Userspace Design
+====================
+
+
+.. Contents:
+
+   1) 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) A final note
+
+
+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.
+
+Background
+----------
+
+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".
+
+
+Benefits
+--------
+
+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:
+    - TCMU_MAILBOX_FLAG_CAP_OOOC:
+	indicates out-of-order completion is supported.
+	See "The Command Ring" for details.
+
+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 are 8-byte aligned. They 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. It also contains cmd_id and
+flags fields for setting by the kernel (kflags) and userspace
+(uflags).
+
+Currently only two opcodes are defined, TCMU_OP_CMD and TCMU_OP_PAD.
+
+When the opcode is CMD, the entry in the command ring is a struct
+tcmu_cmd_entry. Userspace 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 the req.iov[] array. iov_cnt contains the number of
+entries in iov[] needed to describe either the Data-In or Data-Out
+buffers. For bidirectional commands, iov_cnt specifies how many iovec
+entries cover the Data-Out area, and iov_bidi_cnt specifies how many
+iovec entries immediately after that in iov[] cover the Data-In
+area. Just like other fields, iov.iov_base is an offset from the start
+of the region.
+
+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.
+
+If TCMU_MAILBOX_FLAG_CAP_OOOC is set for mailbox->flags, kernel is
+capable of handling out-of-order completions. In this case, userspace can
+handle command in different order other than original. Since kernel would
+still process the commands in the same order it appeared in the command
+ring, userspace need to update the cmd->id when completing the
+command(a.k.a steal the original command's entry).
+
+When the opcode is PAD, userspace only updates cmd_tail as above --
+it's a no-op. (The kernel inserts PAD entries to ensure each CMD entry
+is contiguous within the command ring.)
+
+More opcodes may be added in the future. If userspace encounters an
+opcode it does not handle, it must set UNKNOWN_OP bit (bit 0) in
+hdr.uflags, update cmd_tail, and proceed with processing additional
+commands, if any.
+
+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
+format::
+
+	tcm-user/<hba_num>/<device_name>/<subtype>/<path>
+
+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::
+
+	/sys/kernel/config/target/core/user_<hba_num>/<device_name>
+
+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()::
+
+	mmap(NULL, size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0)
+
+where size must be equal to the value read from
+/sys/class/uio/uioX/maps/map0/size.
+
+
+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
+authors.
+
+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));
+      close(fd);
+      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));
+      close(fd);
+      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.len_op) == 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 if (tcmu_hdr_get_op(ent->hdr.len_op) != TCMU_OP_PAD) {
+            /* Tell the kernel we didn't handle unknown opcodes */
+            ent->hdr.uflags |= TCMU_UFLAG_UNKNOWN_OP;
+          }
+          else {
+            /* Do nothing for PAD entries except update cmd_tail */
+          }
+
+          /* 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;
+      }
+
+
+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.