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+=================================================
+Using kgdb, kdb and the kernel debugger internals
+=================================================
+
+:Author: Jason Wessel
+
+Introduction
+============
+
+The kernel has two different debugger front ends (kdb and kgdb) which
+interface to the debug core. It is possible to use either of the
+debugger front ends and dynamically transition between them if you
+configure the kernel properly at compile and runtime.
+
+Kdb is simplistic shell-style interface which you can use on a system
+console with a keyboard or serial console. You can use it to inspect
+memory, registers, process lists, dmesg, and even set breakpoints to
+stop in a certain location. Kdb is not a source level debugger, although
+you can set breakpoints and execute some basic kernel run control. Kdb
+is mainly aimed at doing some analysis to aid in development or
+diagnosing kernel problems. You can access some symbols by name in
+kernel built-ins or in kernel modules if the code was built with
+``CONFIG_KALLSYMS``.
+
+Kgdb is intended to be used as a source level debugger for the Linux
+kernel. It is used along with gdb to debug a Linux kernel. The
+expectation is that gdb can be used to "break in" to the kernel to
+inspect memory, variables and look through call stack information
+similar to the way an application developer would use gdb to debug an
+application. It is possible to place breakpoints in kernel code and
+perform some limited execution stepping.
+
+Two machines are required for using kgdb. One of these machines is a
+development machine and the other is the target machine. The kernel to
+be debugged runs on the target machine. The development machine runs an
+instance of gdb against the vmlinux file which contains the symbols (not
+a boot image such as bzImage, zImage, uImage...). In gdb the developer
+specifies the connection parameters and connects to kgdb. The type of
+connection a developer makes with gdb depends on the availability of
+kgdb I/O modules compiled as built-ins or loadable kernel modules in the
+test machine's kernel.
+
+Compiling a kernel
+==================
+
+-  In order to enable compilation of kdb, you must first enable kgdb.
+
+-  The kgdb test compile options are described in the kgdb test suite
+   chapter.
+
+Kernel config options for kgdb
+------------------------------
+
+To enable ``CONFIG_KGDB`` you should look under
+:menuselection:`Kernel hacking --> Kernel debugging` and select
+:menuselection:`KGDB: kernel debugger`.
+
+While it is not a hard requirement that you have symbols in your vmlinux
+file, gdb tends not to be very useful without the symbolic data, so you
+will want to turn on ``CONFIG_DEBUG_INFO`` which is called
+:menuselection:`Compile the kernel with debug info` in the config menu.
+
+It is advised, but not required, that you turn on the
+``CONFIG_FRAME_POINTER`` kernel option which is called :menuselection:`Compile
+the kernel with frame pointers` in the config menu. This option inserts code
+to into the compiled executable which saves the frame information in
+registers or on the stack at different points which allows a debugger
+such as gdb to more accurately construct stack back traces while
+debugging the kernel.
+
+If the architecture that you are using supports the kernel option
+``CONFIG_STRICT_KERNEL_RWX``, you should consider turning it off. This
+option will prevent the use of software breakpoints because it marks
+certain regions of the kernel's memory space as read-only. If kgdb
+supports it for the architecture you are using, you can use hardware
+breakpoints if you desire to run with the ``CONFIG_STRICT_KERNEL_RWX``
+option turned on, else you need to turn off this option.
+
+Next you should choose one of more I/O drivers to interconnect debugging
+host and debugged target. Early boot debugging requires a KGDB I/O
+driver that supports early debugging and the driver must be built into
+the kernel directly. Kgdb I/O driver configuration takes place via
+kernel or module parameters which you can learn more about in the in the
+section that describes the parameter kgdboc.
+
+Here is an example set of ``.config`` symbols to enable or disable for kgdb::
+
+  # CONFIG_STRICT_KERNEL_RWX is not set
+  CONFIG_FRAME_POINTER=y
+  CONFIG_KGDB=y
+  CONFIG_KGDB_SERIAL_CONSOLE=y
+
+Kernel config options for kdb
+-----------------------------
+
+Kdb is quite a bit more complex than the simple gdbstub sitting on top
+of the kernel's debug core. Kdb must implement a shell, and also adds
+some helper functions in other parts of the kernel, responsible for
+printing out interesting data such as what you would see if you ran
+``lsmod``, or ``ps``. In order to build kdb into the kernel you follow the
+same steps as you would for kgdb.
+
+The main config option for kdb is ``CONFIG_KGDB_KDB`` which is called
+:menuselection:`KGDB_KDB: include kdb frontend for kgdb` in the config menu.
+In theory you would have already also selected an I/O driver such as the
+``CONFIG_KGDB_SERIAL_CONSOLE`` interface if you plan on using kdb on a
+serial port, when you were configuring kgdb.
+
+If you want to use a PS/2-style keyboard with kdb, you would select
+``CONFIG_KDB_KEYBOARD`` which is called :menuselection:`KGDB_KDB: keyboard as
+input device` in the config menu. The ``CONFIG_KDB_KEYBOARD`` option is not
+used for anything in the gdb interface to kgdb. The ``CONFIG_KDB_KEYBOARD``
+option only works with kdb.
+
+Here is an example set of ``.config`` symbols to enable/disable kdb::
+
+  # CONFIG_STRICT_KERNEL_RWX is not set
+  CONFIG_FRAME_POINTER=y
+  CONFIG_KGDB=y
+  CONFIG_KGDB_SERIAL_CONSOLE=y
+  CONFIG_KGDB_KDB=y
+  CONFIG_KDB_KEYBOARD=y
+
+Kernel Debugger Boot Arguments
+==============================
+
+This section describes the various runtime kernel parameters that affect
+the configuration of the kernel debugger. The following chapter covers
+using kdb and kgdb as well as providing some examples of the
+configuration parameters.
+
+Kernel parameter: kgdboc
+------------------------
+
+The kgdboc driver was originally an abbreviation meant to stand for
+"kgdb over console". Today it is the primary mechanism to configure how
+to communicate from gdb to kgdb as well as the devices you want to use
+to interact with the kdb shell.
+
+For kgdb/gdb, kgdboc is designed to work with a single serial port. It
+is intended to cover the circumstance where you want to use a serial
+console as your primary console as well as using it to perform kernel
+debugging. It is also possible to use kgdb on a serial port which is not
+designated as a system console. Kgdboc may be configured as a kernel
+built-in or a kernel loadable module. You can only make use of
+``kgdbwait`` and early debugging if you build kgdboc into the kernel as
+a built-in.
+
+Optionally you can elect to activate kms (Kernel Mode Setting)
+integration. When you use kms with kgdboc and you have a video driver
+that has atomic mode setting hooks, it is possible to enter the debugger
+on the graphics console. When the kernel execution is resumed, the
+previous graphics mode will be restored. This integration can serve as a
+useful tool to aid in diagnosing crashes or doing analysis of memory
+with kdb while allowing the full graphics console applications to run.
+
+kgdboc arguments
+~~~~~~~~~~~~~~~~
+
+Usage::
+
+	kgdboc=[kms][[,]kbd][[,]serial_device][,baud]
+
+The order listed above must be observed if you use any of the optional
+configurations together.
+
+Abbreviations:
+
+-  kms = Kernel Mode Setting
+
+-  kbd = Keyboard
+
+You can configure kgdboc to use the keyboard, and/or a serial device
+depending on if you are using kdb and/or kgdb, in one of the following
+scenarios. The order listed above must be observed if you use any of the
+optional configurations together. Using kms + only gdb is generally not
+a useful combination.
+
+Using loadable module or built-in
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+1. As a kernel built-in:
+
+   Use the kernel boot argument::
+
+	kgdboc=<tty-device>,[baud]
+
+2. As a kernel loadable module:
+
+   Use the command::
+
+	modprobe kgdboc kgdboc=<tty-device>,[baud]
+
+   Here are two examples of how you might format the kgdboc string. The
+   first is for an x86 target using the first serial port. The second
+   example is for the ARM Versatile AB using the second serial port.
+
+   1. ``kgdboc=ttyS0,115200``
+
+   2. ``kgdboc=ttyAMA1,115200``
+
+Configure kgdboc at runtime with sysfs
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+At run time you can enable or disable kgdboc by echoing a parameters
+into the sysfs. Here are two examples:
+
+1. Enable kgdboc on ttyS0::
+
+	echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc
+
+2. Disable kgdboc::
+
+	echo "" > /sys/module/kgdboc/parameters/kgdboc
+
+.. note::
+
+   You do not need to specify the baud if you are configuring the
+   console on tty which is already configured or open.
+
+More examples
+^^^^^^^^^^^^^
+
+You can configure kgdboc to use the keyboard, and/or a serial device
+depending on if you are using kdb and/or kgdb, in one of the following
+scenarios.
+
+1. kdb and kgdb over only a serial port::
+
+	kgdboc=<serial_device>[,baud]
+
+   Example::
+
+	kgdboc=ttyS0,115200
+
+2. kdb and kgdb with keyboard and a serial port::
+
+	kgdboc=kbd,<serial_device>[,baud]
+
+   Example::
+
+	kgdboc=kbd,ttyS0,115200
+
+3. kdb with a keyboard::
+
+	kgdboc=kbd
+
+4. kdb with kernel mode setting::
+
+	kgdboc=kms,kbd
+
+5. kdb with kernel mode setting and kgdb over a serial port::
+
+	kgdboc=kms,kbd,ttyS0,115200
+
+.. note::
+
+   Kgdboc does not support interrupting the target via the gdb remote
+   protocol. You must manually send a :kbd:`SysRq-G` unless you have a proxy
+   that splits console output to a terminal program. A console proxy has a
+   separate TCP port for the debugger and a separate TCP port for the
+   "human" console. The proxy can take care of sending the :kbd:`SysRq-G`
+   for you.
+
+When using kgdboc with no debugger proxy, you can end up connecting the
+debugger at one of two entry points. If an exception occurs after you
+have loaded kgdboc, a message should print on the console stating it is
+waiting for the debugger. In this case you disconnect your terminal
+program and then connect the debugger in its place. If you want to
+interrupt the target system and forcibly enter a debug session you have
+to issue a :kbd:`Sysrq` sequence and then type the letter :kbd:`g`. Then you
+disconnect the terminal session and connect gdb. Your options if you
+don't like this are to hack gdb to send the :kbd:`SysRq-G` for you as well as
+on the initial connect, or to use a debugger proxy that allows an
+unmodified gdb to do the debugging.
+
+Kernel parameter: ``kgdbwait``
+------------------------------
+
+The Kernel command line option ``kgdbwait`` makes kgdb wait for a
+debugger connection during booting of a kernel. You can only use this
+option if you compiled a kgdb I/O driver into the kernel and you
+specified the I/O driver configuration as a kernel command line option.
+The kgdbwait parameter should always follow the configuration parameter
+for the kgdb I/O driver in the kernel command line else the I/O driver
+will not be configured prior to asking the kernel to use it to wait.
+
+The kernel will stop and wait as early as the I/O driver and
+architecture allows when you use this option. If you build the kgdb I/O
+driver as a loadable kernel module kgdbwait will not do anything.
+
+Kernel parameter: ``kgdbcon``
+-----------------------------
+
+The ``kgdbcon`` feature allows you to see :c:func:`printk` messages inside gdb
+while gdb is connected to the kernel. Kdb does not make use of the kgdbcon
+feature.
+
+Kgdb supports using the gdb serial protocol to send console messages to
+the debugger when the debugger is connected and running. There are two
+ways to activate this feature.
+
+1. Activate with the kernel command line option::
+
+	kgdbcon
+
+2. Use sysfs before configuring an I/O driver::
+
+	echo 1 > /sys/module/kgdb/parameters/kgdb_use_con
+
+.. note::
+
+   If you do this after you configure the kgdb I/O driver, the
+   setting will not take effect until the next point the I/O is
+   reconfigured.
+
+.. important::
+
+   You cannot use kgdboc + kgdbcon on a tty that is an
+   active system console. An example of incorrect usage is::
+
+	console=ttyS0,115200 kgdboc=ttyS0 kgdbcon
+
+It is possible to use this option with kgdboc on a tty that is not a
+system console.
+
+Run time parameter: ``kgdbreboot``
+----------------------------------
+
+The kgdbreboot feature allows you to change how the debugger deals with
+the reboot notification. You have 3 choices for the behavior. The
+default behavior is always set to 0.
+
+.. tabularcolumns:: |p{0.4cm}|p{11.5cm}|p{5.6cm}|
+
+.. flat-table::
+  :widths: 1 10 8
+
+  * - 1
+    - ``echo -1 > /sys/module/debug_core/parameters/kgdbreboot``
+    - Ignore the reboot notification entirely.
+
+  * - 2
+    - ``echo 0 > /sys/module/debug_core/parameters/kgdbreboot``
+    - Send the detach message to any attached debugger client.
+
+  * - 3
+    - ``echo 1 > /sys/module/debug_core/parameters/kgdbreboot``
+    - Enter the debugger on reboot notify.
+
+Kernel parameter: ``nokaslr``
+-----------------------------
+
+If the architecture that you are using enable KASLR by default,
+you should consider turning it off.  KASLR randomizes the
+virtual address where the kernel image is mapped and confuse
+gdb which resolve kernel symbol address from symbol table
+of vmlinux.
+
+Using kdb
+=========
+
+Quick start for kdb on a serial port
+------------------------------------
+
+This is a quick example of how to use kdb.
+
+1. Configure kgdboc at boot using kernel parameters::
+
+	console=ttyS0,115200 kgdboc=ttyS0,115200 nokaslr
+
+   OR
+
+   Configure kgdboc after the kernel has booted; assuming you are using
+   a serial port console::
+
+	echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc
+
+2. Enter the kernel debugger manually or by waiting for an oops or
+   fault. There are several ways you can enter the kernel debugger
+   manually; all involve using the :kbd:`SysRq-G`, which means you must have
+   enabled ``CONFIG_MAGIC_SysRq=y`` in your kernel config.
+
+   -  When logged in as root or with a super user session you can run::
+
+	echo g > /proc/sysrq-trigger
+
+   -  Example using minicom 2.2
+
+      Press: :kbd:`CTRL-A` :kbd:`f` :kbd:`g`
+
+   -  When you have telneted to a terminal server that supports sending
+      a remote break
+
+      Press: :kbd:`CTRL-]`
+
+      Type in: ``send break``
+
+      Press: :kbd:`Enter` :kbd:`g`
+
+3. From the kdb prompt you can run the ``help`` command to see a complete
+   list of the commands that are available.
+
+   Some useful commands in kdb include:
+
+   =========== =================================================================
+   ``lsmod``   Shows where kernel modules are loaded
+   ``ps``      Displays only the active processes
+   ``ps A``    Shows all the processes
+   ``summary`` Shows kernel version info and memory usage
+   ``bt``      Get a backtrace of the current process using :c:func:`dump_stack`
+   ``dmesg``   View the kernel syslog buffer
+   ``go``      Continue the system
+   =========== =================================================================
+
+4. When you are done using kdb you need to consider rebooting the system
+   or using the ``go`` command to resuming normal kernel execution. If you
+   have paused the kernel for a lengthy period of time, applications
+   that rely on timely networking or anything to do with real wall clock
+   time could be adversely affected, so you should take this into
+   consideration when using the kernel debugger.
+
+Quick start for kdb using a keyboard connected console
+------------------------------------------------------
+
+This is a quick example of how to use kdb with a keyboard.
+
+1. Configure kgdboc at boot using kernel parameters::
+
+	kgdboc=kbd
+
+   OR
+
+   Configure kgdboc after the kernel has booted::
+
+	echo kbd > /sys/module/kgdboc/parameters/kgdboc
+
+2. Enter the kernel debugger manually or by waiting for an oops or
+   fault. There are several ways you can enter the kernel debugger
+   manually; all involve using the :kbd:`SysRq-G`, which means you must have
+   enabled ``CONFIG_MAGIC_SysRq=y`` in your kernel config.
+
+   -  When logged in as root or with a super user session you can run::
+
+	echo g > /proc/sysrq-trigger
+
+   -  Example using a laptop keyboard:
+
+      Press and hold down: :kbd:`Alt`
+
+      Press and hold down: :kbd:`Fn`
+
+      Press and release the key with the label: :kbd:`SysRq`
+
+      Release: :kbd:`Fn`
+
+      Press and release: :kbd:`g`
+
+      Release: :kbd:`Alt`
+
+   -  Example using a PS/2 101-key keyboard
+
+      Press and hold down: :kbd:`Alt`
+
+      Press and release the key with the label: :kbd:`SysRq`
+
+      Press and release: :kbd:`g`
+
+      Release: :kbd:`Alt`
+
+3. Now type in a kdb command such as ``help``, ``dmesg``, ``bt`` or ``go`` to
+   continue kernel execution.
+
+Using kgdb / gdb
+================
+
+In order to use kgdb you must activate it by passing configuration
+information to one of the kgdb I/O drivers. If you do not pass any
+configuration information kgdb will not do anything at all. Kgdb will
+only actively hook up to the kernel trap hooks if a kgdb I/O driver is
+loaded and configured. If you unconfigure a kgdb I/O driver, kgdb will
+unregister all the kernel hook points.
+
+All kgdb I/O drivers can be reconfigured at run time, if
+``CONFIG_SYSFS`` and ``CONFIG_MODULES`` are enabled, by echo'ing a new
+config string to ``/sys/module/<driver>/parameter/<option>``. The driver
+can be unconfigured by passing an empty string. You cannot change the
+configuration while the debugger is attached. Make sure to detach the
+debugger with the ``detach`` command prior to trying to unconfigure a
+kgdb I/O driver.
+
+Connecting with gdb to a serial port
+------------------------------------
+
+1. Configure kgdboc
+
+   Configure kgdboc at boot using kernel parameters::
+
+	kgdboc=ttyS0,115200
+
+   OR
+
+   Configure kgdboc after the kernel has booted::
+
+	echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc
+
+2. Stop kernel execution (break into the debugger)
+
+   In order to connect to gdb via kgdboc, the kernel must first be
+   stopped. There are several ways to stop the kernel which include
+   using kgdbwait as a boot argument, via a :kbd:`SysRq-G`, or running the
+   kernel until it takes an exception where it waits for the debugger to
+   attach.
+
+   -  When logged in as root or with a super user session you can run::
+
+	echo g > /proc/sysrq-trigger
+
+   -  Example using minicom 2.2
+
+      Press: :kbd:`CTRL-A` :kbd:`f` :kbd:`g`
+
+   -  When you have telneted to a terminal server that supports sending
+      a remote break
+
+      Press: :kbd:`CTRL-]`
+
+      Type in: ``send break``
+
+      Press: :kbd:`Enter` :kbd:`g`
+
+3. Connect from gdb
+
+   Example (using a directly connected port)::
+
+           % gdb ./vmlinux
+           (gdb) set remotebaud 115200
+           (gdb) target remote /dev/ttyS0
+
+
+   Example (kgdb to a terminal server on TCP port 2012)::
+
+           % gdb ./vmlinux
+           (gdb) target remote 192.168.2.2:2012
+
+
+   Once connected, you can debug a kernel the way you would debug an
+   application program.
+
+   If you are having problems connecting or something is going seriously
+   wrong while debugging, it will most often be the case that you want
+   to enable gdb to be verbose about its target communications. You do
+   this prior to issuing the ``target remote`` command by typing in::
+
+	set debug remote 1
+
+Remember if you continue in gdb, and need to "break in" again, you need
+to issue an other :kbd:`SysRq-G`. It is easy to create a simple entry point by
+putting a breakpoint at ``sys_sync`` and then you can run ``sync`` from a
+shell or script to break into the debugger.
+
+kgdb and kdb interoperability
+=============================
+
+It is possible to transition between kdb and kgdb dynamically. The debug
+core will remember which you used the last time and automatically start
+in the same mode.
+
+Switching between kdb and kgdb
+------------------------------
+
+Switching from kgdb to kdb
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+There are two ways to switch from kgdb to kdb: you can use gdb to issue
+a maintenance packet, or you can blindly type the command ``$3#33``.
+Whenever the kernel debugger stops in kgdb mode it will print the
+message ``KGDB or $3#33 for KDB``. It is important to note that you have
+to type the sequence correctly in one pass. You cannot type a backspace
+or delete because kgdb will interpret that as part of the debug stream.
+
+1. Change from kgdb to kdb by blindly typing::
+
+	$3#33
+
+2. Change from kgdb to kdb with gdb::
+
+	maintenance packet 3
+
+   .. note::
+
+     Now you must kill gdb. Typically you press :kbd:`CTRL-Z` and issue
+     the command::
+
+	kill -9 %
+
+Change from kdb to kgdb
+~~~~~~~~~~~~~~~~~~~~~~~
+
+There are two ways you can change from kdb to kgdb. You can manually
+enter kgdb mode by issuing the kgdb command from the kdb shell prompt,
+or you can connect gdb while the kdb shell prompt is active. The kdb
+shell looks for the typical first commands that gdb would issue with the
+gdb remote protocol and if it sees one of those commands it
+automatically changes into kgdb mode.
+
+1. From kdb issue the command::
+
+	kgdb
+
+   Now disconnect your terminal program and connect gdb in its place
+
+2. At the kdb prompt, disconnect the terminal program and connect gdb in
+   its place.
+
+Running kdb commands from gdb
+-----------------------------
+
+It is possible to run a limited set of kdb commands from gdb, using the
+gdb monitor command. You don't want to execute any of the run control or
+breakpoint operations, because it can disrupt the state of the kernel
+debugger. You should be using gdb for breakpoints and run control
+operations if you have gdb connected. The more useful commands to run
+are things like lsmod, dmesg, ps or possibly some of the memory
+information commands. To see all the kdb commands you can run
+``monitor help``.
+
+Example::
+
+    (gdb) monitor ps
+    1 idle process (state I) and
+    27 sleeping system daemon (state M) processes suppressed,
+    use 'ps A' to see all.
+    Task Addr       Pid   Parent [*] cpu State Thread     Command
+
+    0xc78291d0        1        0  0    0   S  0xc7829404  init
+    0xc7954150      942        1  0    0   S  0xc7954384  dropbear
+    0xc78789c0      944        1  0    0   S  0xc7878bf4  sh
+    (gdb)
+
+kgdb Test Suite
+===============
+
+When kgdb is enabled in the kernel config you can also elect to enable
+the config parameter ``KGDB_TESTS``. Turning this on will enable a special
+kgdb I/O module which is designed to test the kgdb internal functions.
+
+The kgdb tests are mainly intended for developers to test the kgdb
+internals as well as a tool for developing a new kgdb architecture
+specific implementation. These tests are not really for end users of the
+Linux kernel. The primary source of documentation would be to look in
+the ``drivers/misc/kgdbts.c`` file.
+
+The kgdb test suite can also be configured at compile time to run the
+core set of tests by setting the kernel config parameter
+``KGDB_TESTS_ON_BOOT``. This particular option is aimed at automated
+regression testing and does not require modifying the kernel boot config
+arguments. If this is turned on, the kgdb test suite can be disabled by
+specifying ``kgdbts=`` as a kernel boot argument.
+
+Kernel Debugger Internals
+=========================
+
+Architecture Specifics
+----------------------
+
+The kernel debugger is organized into a number of components:
+
+1. The debug core
+
+   The debug core is found in ``kernel/debugger/debug_core.c``. It
+   contains:
+
+   -  A generic OS exception handler which includes sync'ing the
+      processors into a stopped state on an multi-CPU system.
+
+   -  The API to talk to the kgdb I/O drivers
+
+   -  The API to make calls to the arch-specific kgdb implementation
+
+   -  The logic to perform safe memory reads and writes to memory while
+      using the debugger
+
+   -  A full implementation for software breakpoints unless overridden
+      by the arch
+
+   -  The API to invoke either the kdb or kgdb frontend to the debug
+      core.
+
+   -  The structures and callback API for atomic kernel mode setting.
+
+      .. note:: kgdboc is where the kms callbacks are invoked.
+
+2. kgdb arch-specific implementation
+
+   This implementation is generally found in ``arch/*/kernel/kgdb.c``. As
+   an example, ``arch/x86/kernel/kgdb.c`` contains the specifics to
+   implement HW breakpoint as well as the initialization to dynamically
+   register and unregister for the trap handlers on this architecture.
+   The arch-specific portion implements:
+
+   -  contains an arch-specific trap catcher which invokes
+      :c:func:`kgdb_handle_exception` to start kgdb about doing its work
+
+   -  translation to and from gdb specific packet format to :c:type:`pt_regs`
+
+   -  Registration and unregistration of architecture specific trap
+      hooks
+
+   -  Any special exception handling and cleanup
+
+   -  NMI exception handling and cleanup
+
+   -  (optional) HW breakpoints
+
+3. gdbstub frontend (aka kgdb)
+
+   The gdbstub is located in ``kernel/debug/gdbstub.c``. It contains:
+
+   -  All the logic to implement the gdb serial protocol
+
+4. kdb frontend
+
+   The kdb debugger shell is broken down into a number of components.
+   The kdb core is located in kernel/debug/kdb. There are a number of
+   helper functions in some of the other kernel components to make it
+   possible for kdb to examine and report information about the kernel
+   without taking locks that could cause a kernel deadlock. The kdb core
+   contains implements the following functionality.
+
+   -  A simple shell
+
+   -  The kdb core command set
+
+   -  A registration API to register additional kdb shell commands.
+
+      -  A good example of a self-contained kdb module is the ``ftdump``
+         command for dumping the ftrace buffer. See:
+         ``kernel/trace/trace_kdb.c``
+
+      -  For an example of how to dynamically register a new kdb command
+         you can build the kdb_hello.ko kernel module from
+         ``samples/kdb/kdb_hello.c``. To build this example you can set
+         ``CONFIG_SAMPLES=y`` and ``CONFIG_SAMPLE_KDB=m`` in your kernel
+         config. Later run ``modprobe kdb_hello`` and the next time you
+         enter the kdb shell, you can run the ``hello`` command.
+
+   -  The implementation for :c:func:`kdb_printf` which emits messages directly
+      to I/O drivers, bypassing the kernel log.
+
+   -  SW / HW breakpoint management for the kdb shell
+
+5. kgdb I/O driver
+
+   Each kgdb I/O driver has to provide an implementation for the
+   following:
+
+   -  configuration via built-in or module
+
+   -  dynamic configuration and kgdb hook registration calls
+
+   -  read and write character interface
+
+   -  A cleanup handler for unconfiguring from the kgdb core
+
+   -  (optional) Early debug methodology
+
+   Any given kgdb I/O driver has to operate very closely with the
+   hardware and must do it in such a way that does not enable interrupts
+   or change other parts of the system context without completely
+   restoring them. The kgdb core will repeatedly "poll" a kgdb I/O
+   driver for characters when it needs input. The I/O driver is expected
+   to return immediately if there is no data available. Doing so allows
+   for the future possibility to touch watchdog hardware in such a way
+   as to have a target system not reset when these are enabled.
+
+If you are intent on adding kgdb architecture specific support for a new
+architecture, the architecture should define ``HAVE_ARCH_KGDB`` in the
+architecture specific Kconfig file. This will enable kgdb for the
+architecture, and at that point you must create an architecture specific
+kgdb implementation.
+
+There are a few flags which must be set on every architecture in their
+``asm/kgdb.h`` file. These are:
+
+-  ``NUMREGBYTES``:
+     The size in bytes of all of the registers, so that we
+     can ensure they will all fit into a packet.
+
+-  ``BUFMAX``:
+     The size in bytes of the buffer GDB will read into. This must
+     be larger than NUMREGBYTES.
+
+-  ``CACHE_FLUSH_IS_SAFE``:
+     Set to 1 if it is always safe to call
+     flush_cache_range or flush_icache_range. On some architectures,
+     these functions may not be safe to call on SMP since we keep other
+     CPUs in a holding pattern.
+
+There are also the following functions for the common backend, found in
+``kernel/kgdb.c``, that must be supplied by the architecture-specific
+backend unless marked as (optional), in which case a default function
+maybe used if the architecture does not need to provide a specific
+implementation.
+
+.. kernel-doc:: include/linux/kgdb.h
+   :internal:
+
+kgdboc internals
+----------------
+
+kgdboc and uarts
+~~~~~~~~~~~~~~~~
+
+The kgdboc driver is actually a very thin driver that relies on the
+underlying low level to the hardware driver having "polling hooks" to
+which the tty driver is attached. In the initial implementation of
+kgdboc the serial_core was changed to expose a low level UART hook for
+doing polled mode reading and writing of a single character while in an
+atomic context. When kgdb makes an I/O request to the debugger, kgdboc
+invokes a callback in the serial core which in turn uses the callback in
+the UART driver.
+
+When using kgdboc with a UART, the UART driver must implement two
+callbacks in the :c:type:`struct uart_ops <uart_ops>`.
+Example from ``drivers/8250.c``::
+
+
+    #ifdef CONFIG_CONSOLE_POLL
+        .poll_get_char = serial8250_get_poll_char,
+        .poll_put_char = serial8250_put_poll_char,
+    #endif
+
+
+Any implementation specifics around creating a polling driver use the
+``#ifdef CONFIG_CONSOLE_POLL``, as shown above. Keep in mind that
+polling hooks have to be implemented in such a way that they can be
+called from an atomic context and have to restore the state of the UART
+chip on return such that the system can return to normal when the
+debugger detaches. You need to be very careful with any kind of lock you
+consider, because failing here is most likely going to mean pressing the
+reset button.
+
+kgdboc and keyboards
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+The kgdboc driver contains logic to configure communications with an
+attached keyboard. The keyboard infrastructure is only compiled into the
+kernel when ``CONFIG_KDB_KEYBOARD=y`` is set in the kernel configuration.
+
+The core polled keyboard driver driver for PS/2 type keyboards is in
+``drivers/char/kdb_keyboard.c``. This driver is hooked into the debug core
+when kgdboc populates the callback in the array called
+:c:type:`kdb_poll_funcs[]`. The :c:func:`kdb_get_kbd_char` is the top-level
+function which polls hardware for single character input.
+
+kgdboc and kms
+~~~~~~~~~~~~~~~~~~
+
+The kgdboc driver contains logic to request the graphics display to
+switch to a text context when you are using ``kgdboc=kms,kbd``, provided
+that you have a video driver which has a frame buffer console and atomic
+kernel mode setting support.
+
+Every time the kernel debugger is entered it calls
+:c:func:`kgdboc_pre_exp_handler` which in turn calls :c:func:`con_debug_enter`
+in the virtual console layer. On resuming kernel execution, the kernel
+debugger calls :c:func:`kgdboc_post_exp_handler` which in turn calls
+:c:func:`con_debug_leave`.
+
+Any video driver that wants to be compatible with the kernel debugger
+and the atomic kms callbacks must implement the ``mode_set_base_atomic``,
+``fb_debug_enter`` and ``fb_debug_leave operations``. For the
+``fb_debug_enter`` and ``fb_debug_leave`` the option exists to use the
+generic drm fb helper functions or implement something custom for the
+hardware. The following example shows the initialization of the
+.mode_set_base_atomic operation in
+drivers/gpu/drm/i915/intel_display.c::
+
+
+    static const struct drm_crtc_helper_funcs intel_helper_funcs = {
+    [...]
+            .mode_set_base_atomic = intel_pipe_set_base_atomic,
+    [...]
+    };
+
+
+Here is an example of how the i915 driver initializes the
+fb_debug_enter and fb_debug_leave functions to use the generic drm
+helpers in ``drivers/gpu/drm/i915/intel_fb.c``::
+
+
+    static struct fb_ops intelfb_ops = {
+    [...]
+           .fb_debug_enter = drm_fb_helper_debug_enter,
+           .fb_debug_leave = drm_fb_helper_debug_leave,
+    [...]
+    };
+
+
+Credits
+=======
+
+The following people have contributed to this document:
+
+1. Amit Kale <amitkale@linsyssoft.com>
+
+2. Tom Rini <trini@kernel.crashing.org>
+
+In March 2008 this document was completely rewritten by:
+
+-  Jason Wessel <jason.wessel@windriver.com>
+
+In Jan 2010 this document was updated to include kdb.
+
+-  Jason Wessel <jason.wessel@windriver.com>