xref: /linux-6.15/Documentation/process/debugging/kgdb.rst (revision d5af79c0)

=================================================
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
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 or more I/O drivers to interconnect the 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
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 writing parameters
into 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 `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 `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 `Sysrq` sequence and then type the letter `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 `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: ``kgdboc_earlycon``
-------------------------------------

If you specify the kernel parameter ``kgdboc_earlycon`` and your serial
driver registers a boot console that supports polling (doesn't need
interrupts and implements a nonblocking read() function) kgdb will attempt
to work using the boot console until it can transition to the regular
tty driver specified by the ``kgdboc`` parameter.

Normally there is only one boot console (especially that implements the
read() function) so just adding ``kgdboc_earlycon`` on its own is
sufficient to make this work. If you have more than one boot console you
can add the boot console's name to differentiate. Note that names that
are registered through the boot console layer and the tty layer are not
the same for the same port.

For instance, on one board to be explicit you might do::

   kgdboc_earlycon=qcom_geni kgdboc=ttyMSM0

If the only boot console on the device was "qcom_geni", you could simplify::

   kgdboc_earlycon kgdboc=ttyMSM0

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 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/debug_core/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 enables KASLR by default,
you should consider turning it off.  KASLR randomizes the
virtual address where the kernel image is mapped and confuses
gdb which resolves addresses of kernel symbols from the 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 `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: `CTRL-A` `f` `g`

   -  When you have telneted to a terminal server that supports sending
      a remote break

      Press: `CTRL-]`

      Type in: ``send break``

      Press: `Enter` `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 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 `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: `Alt`

      Press and hold down: `Fn`

      Press and release the key with the label: `SysRq`

      Release: `Fn`

      Press and release: `g`

      Release: `Alt`

   -  Example using a PS/2 101-key keyboard

      Press and hold down: `Alt`

      Press and release the key with the label: `SysRq`

      Press and release: `g`

      Release: `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 `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: `CTRL-A` `f` `g`

   -  When you have telneted to a terminal server that supports sending
      a remote break

      Press: `CTRL-]`

      Type in: ``send break``

      Press: `Enter` `g`

3. Connect from gdb

   Example (using a directly connected port)::

           % gdb ./vmlinux
           (gdb) set serial baud 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 `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 `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

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
      kgdb_handle_exception() to start kgdb about doing its work

   -  translation to and from gdb specific packet format to struct 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
   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 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 struct 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 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:expr:`kdb_poll_funcs[]`. The 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
kgdboc_pre_exp_handler() which in turn calls con_debug_enter()
in the virtual console layer. On resuming kernel execution, the kernel
debugger calls kgdboc_post_exp_handler() which in turn calls
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 <[email protected]>

2. Tom Rini <[email protected]>

In March 2008 this document was completely rewritten by:

-  Jason Wessel <[email protected]>

In Jan 2010 this document was updated to include kdb.

-  Jason Wessel <[email protected]>