Merge tag 'trace-v7.3-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-tracegrafted

Pull tracing fixes from Steven Rostedt:

 - Don't destroy user event fields when removal fails

   User event fields are destroyed before the event is removed from
   visibility. But that can fail leaving the still visible event with no
   fields. Move the destroying of the fields to after the event is
   successfully removed from visibility.

 - Initialize function graph state is fork before calling
   copy_exec_state()

   For non-CLONE_VM forks, copy_exec_state() allocates a new
   task_exec_state. If that allocation fails, ftrace_graph_exit_task()
   will free the tasks ret_stack pointer. Since that pointer is still
   using the parent's ret_stack, it mistakenly frees the parent's
   pointer too.

   Call ftrace_graph_init() on the task first which will NULL out the
   new tasks's ret_stack and if the copy fails, it will not free
   anything.

 - Remove FGRAPH_MAX_INDEX

   The macro FGRAPH_MAX_INDEX was added but never used. Remove it.

 - Save ent_size in function graph printing of nested functions

   The function graph tracer needs to look at the next event to see if
   the next event is the return of the current function entry. If it is,
   it prints a single line:

	ktime_get();

   Otherwise it prints it like a nested function:

	tick_nohz_irq_exit() {
	    ktime_get();
	    kcpustat_irq_exit();
	}

   In order to look at the next event, it must save the current event so
   that it has the information to print from it. It saves the event in
   the iterator descriptor called "ent". What it doesn't save is the
   ent_size of the event which is now used to know if the function graph
   arguments are to be printed. The peek doesn't save the size so the
   size used happens to be that of the size of the last event that was
   seen.

   Save the entry event size in the iterator descriptor so that the
   correct size is used.

 - Fix several errors with freeing data in the histogram code

   The histogram code had a lot of leaked or or incorrect accounting
   when failures happen. Correct them.

 - Fix histogram regression of .percent and .graph modifiers

   Up until 6.3 histogram values could have "percent" or "graph"
   modifiers that changed how they were printed. But a change that added
   restricting histograms values from being strings, stack traces and
   other modifiers inadvertently prevented them from using the percent
   and graph modifiers, which were legal use cases for values.

   Put back the percent and graph modifiers.

 - Fix various typos in the comments

 - Set the trace_clock before initializing a histogram with clock
   argument

   The histogram API allows the user to specific which trace clock to
   use via a "clock=" string. The histogram is set up first before the
   clock is checked. If the passed in clock is not valid, it exits
   without fully fixing up the histogram leaving it on the list and a
   use-after-free can trigger.

   Update the clock argument first and if it fails then exit gracefully
   before the histogram trigger is placed on any lists.

 - Restore :mod: trailer after parsing in ftrace_set_clr_event

   The function ftrace_set_clr_event() modifies the parse string and
   needs to put it back to what was passed in. It searches for ":mod:"
   via a strsep() but fails to put back the first ':' in the string.

   Add back the ':' in the passed in string.

 - Take trace_array reference when opening a tracer options file

   The options files are dynamically created and some tracers add their
   own options. When a tracer adds their own list of options, the
   trace_array holding them has an array to hold the list of options for
   each tracer. This array increases in size via a krealloc(), and the
   new entry gets a newly allocated array to hold the options of the new
   tracer being added.

   The element in each entry of the tracer's option array holds a
   pointer back to the trace_array, a pointer to the tracer it is
   associated to, a pointer to the flags of the option.

   The issue is that these arrays are freed when the trace_array is
   freed when its instance it represents is removed from the instances
   directory. There's a race that an open of one of these options files
   can happen when the instance is being removed.

   Add a new helper function to be called by the open function of the
   options file to iterate all existing trace_arrays under a lock and
   find the one that has the given option element in one of it's tracer
   arrays. If found, then update the associated trace_array's reference
   counter to keep it from being freed. If not found, have the open call
   return -ENODEV.

 - Disable interrupts when acquiring the lock in rb_wake_up_waiters()

   The function rb_wake_up_waiters() assumes it will be called in
   interrupt context and does not disable irqs when taking
   cpu_buffer->reader_lock, which can be called in hard interrupt
   context. The issue is in PREEMPT_RT, this function is called in
   thread context leaving this lock open to a deadlock.

   Take the lock with interrupts disabled.

 - Use rcu_assign_pointer() for tmp_ops filter hash

   The tmp_ops used in update_ftrace_direct_mod() assigns its
   filter_hash field directly, but that field is annotated as __rcu and
   sparse complains. Assign it with rcu_assign_pointer()

 - Fix use-after-free in enable_trigger_private_data_free()

   The trace_event_call is accessed through the event_trigger_data's
   trace_event_file pointer to put the trace_event_call on freeing. The
   issue is that the trace_event_file data may have been freed already
   causing a use-after-free. Add a field to the event_trigger_data that
   points directly to the trace_event_call so that it can decrement its
   reference directly without needing to go through the
   trace_event_file.

 - Fix accounting of buffer data remote headers

   trace_buffer_desc_size() and trace_remote_alloc_buffer() undercount
   the number of pages is needed for the asked for size as it doesn't
   take into account the meta data on each page. Add a helper function
   to do the calculation properly and use that in these functions.

 - Catch nr_page_va overflow in ring_buffer_desc sizing

   The number of pages per remote ring buffer is capped by
   ring_buffer_desc::nr_page_va (32 bits). A buffer_size large enough to
   overflow that field would silently allocate a descriptor smaller than
   what was asked for.

 - Do not resize the subbuf order if any per_cpu buffer is disabled

   The mmapping of ring buffers disables resizing the subbuffers, but it
   is done per-cpu whereas the subbuf size change is done for all the
   per_cpu buffers under the buffer->mutex. It could change the size of
   some while the mapping is happening on others. Have the resize of the
   subbuf order check all the per_cpu buffers under the lock to see if
   any of them is disabled before starting and causing an inconsistency
   between buffers that are being mapped.

* tag 'trace-v7.3-rc2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace: (25 commits)
  ring-buffer: Check resize_disabled before publishing the new subbuf order
  tracing/remotes: Catch nr_page_va overflow in ring_buffer_desc sizing
  tracing/remotes: Account for ring buffer page header in size calculation
  tracing: Don't dereference trace_event_file in deferred trigger free
  ftrace: Use rcu_assign_pointer() for tmp_ops filter hash
  ring-buffer: Acquire the lock with irqsave in rb_wake_up_waiters()
  tracing: Take trace_array reference when opening a tracer options file
  tracing: Fix ring_buffer_read_page_size() kernel-doc
  tracing: Restore :mod: trailer after parsing in ftrace_set_clr_event()
  tracing: Fix memory corruption from a "STACKTRACE" histogram key
  tracing: Fix memory corruption from the histogram stacktrace modifier
  tracing: Undo the registration when enabling the histogram trigger fails
  tracing: Take the reference before publishing the named histogram trigger
  tracing: Set the trace clock before registering the histogram trigger
  tracing: Fix typo "preceeded" in comment
  tracing: Fix typo "availabe" in comment
  tracing: Let histogram values keep the percent and graph modifiers
  tracing: Keep the entry count when the histogram stats allocation fails
  tracing: Free histogram the field rejected for a bad modifier
  tracing: Free histogram the var ref when its initialization fails
  ...

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.