Merge tag 'trace-v7.3-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-tracegrafted

Pull tracing fixes from Steven Rostedt:

 - Fix error output of boot instance creation failure

   Currently if a boot instance creation fails, instead of printing out
   the name of the instance that failed, it prints "(null)". That is
   because it prints "cur_str" that had already been processed by
   strsep(). Print the saved name instead.

   While at it, print the error code of the failure.

 - Fix use-after-free for same named historgrams

   Histograms can be named so that they can be used in multiple events.
   But if the named histogram has a variable attached, the second event
   that uses the named histogram which duplicates it and needs to free
   the original after duplication leaves the old variable in place and
   still visible. If another histogram uses than variable, it will use
   the stale one which will try to reference the freed duplicate
   histogram and crash the kernel.

   Free the duplicate variables along with the duplicated histogram
   data.

 - Check return value of kthread_run() in event self test

   The events self tests uses a kthread for testing but does not check
   if it succeeded in creating a kthread. If the kthread creation were
   to fail, the code will still try to call kthread_stop() on the error
   returned.

 - Fix race between reading trace_pipe and updating subbuffer size

   If a user is reading the trace_pipe file at the same time they update
   the ring buffer sub-buffer size, can cause the trace_pipe read to
   read stale data. Add trace_access_lock() around updating the ring
   buffer sub-buffer size.

 - Fix eventfs_inode on failure path in creation of the events directory

   In the creation of the "events" directory, if after allocating the
   eventfs_inode a failure is detected, it calls cleanup_ei() which
   calls free_ei(). The free_ei() will test if eventfs_inode being freed
   has no children. It is a bug if it does. But on the failure case of
   the creation of the "events" directory, the children lists have not
   yet been initialized and the free will trigger a warning because
   list_empty() on an uninitialized list returns false.

   Move the initialization into init_ei() where it makes more sense and
   makes sure that a created eventfs_inode has its lists initialized
   upon creation.

 - Check return value of kthread_run() in ftrace direct sample code

   The sample code that shows how to use the ftrace direct calls does
   not test the return of kthread_run() to see if it succeeds. Return a
   failure if the kthread_run() doesn't succeed.

 - Clear user events state on fork in case of alloc failure

   On fork, the child gets a pointer to the parent's user events state.
   It makes a copy of it then updates the child's pointer to it. But if
   the allocation fails, the duplication function leaves the child with
   a pointer to its parent's descriptor. When the child cleans up its
   data, it will free the parent's descriptor while the parent is still
   using it.

   In the duplication function, set the child's user_event_mm to NULL
   before testing if the allocation succeeded, and when it exits it will
   not free the parent's descriptor.

 - Fix retry exhaustion in simple ring buffer reader swap

   simple_ring_buffer_swap_reader_page() starts with retry set to 8 and
   post-decrements it only after a failed link replacement. On the final
   attempt, a successful replacement leaves retry at zero, while a
   failed replacement leaves it at -1.

   But the check for success expects the retry value to be non-zero and
   exits with an error on zero. This is the opposite result. Fix it.

 - Fail nicely when the remote swap_reader_page() returns an error

   Currently, if the swap_reader_page() of a remote buffer fails, it
   triggers a WARN_ON_ONCE() and continues normally. Instead, have it
   exit with an error and a pr_warn() print instead of a full WARNING.

* tag 'trace-v7.3-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace:
  ring-buffer: Stop remote reader update when page swap fails
  tracing: Fix retry exhaustion in simple ring buffer reader swap
  tracing/user_events: Clear copied tracing state before fork duplication
  samples/ftrace: Fix kthread_stop() on ERR_PTR in ftrace-direct-multi-modify
  samples/ftrace: Fix kthread_stop() on ERR_PTR in ftrace-direct-modify
  eventfs: Initialize ei->children and ei->list in init_ei()
  tracing: Fix use-after-free in trace_pipe read on sub-buffer order change
  tracing: Fix crash passing ERR_PTR to kthread_stop()
  tracing: Fix use-after-free with same-name named triggers
  tracing: Fix logged instance name on creation failure

5 files changed, 651 insertions, 0 deletions

diff --git a/Documentation/target/index.rst b/Documentation/target/index.rst
new file mode 100644
index 000000000..51fa8ebc6
--- /dev/null
+++ b/Documentation/target/index.rst
@@ -0,0 +1,12 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==================
+TCM Virtual Device
+==================
+
+.. toctree::
+    :maxdepth: 1
+
+    tcmu-design
+    tcm_mod_builder
+    scripts
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.