1=================================================
2Using kgdb, kdb and the kernel debugger internals
3=================================================
4
5:Author: Jason Wessel
6
7Introduction
8============
9
10The kernel has two different debugger front ends (kdb and kgdb) which
11interface to the debug core. It is possible to use either of the
12debugger front ends and dynamically transition between them if you
13configure the kernel properly at compile and runtime.
14
15Kdb is simplistic shell-style interface which you can use on a system
16console with a keyboard or serial console. You can use it to inspect
17memory, registers, process lists, dmesg, and even set breakpoints to
18stop in a certain location. Kdb is not a source level debugger, although
19you can set breakpoints and execute some basic kernel run control. Kdb
20is mainly aimed at doing some analysis to aid in development or
21diagnosing kernel problems. You can access some symbols by name in
22kernel built-ins or in kernel modules if the code was built with
23``CONFIG_KALLSYMS``.
24
25Kgdb is intended to be used as a source level debugger for the Linux
26kernel. It is used along with gdb to debug a Linux kernel. The
27expectation is that gdb can be used to "break in" to the kernel to
28inspect memory, variables and look through call stack information
29similar to the way an application developer would use gdb to debug an
30application. It is possible to place breakpoints in kernel code and
31perform some limited execution stepping.
32
33Two machines are required for using kgdb. One of these machines is a
34development machine and the other is the target machine. The kernel to
35be debugged runs on the target machine. The development machine runs an
36instance of gdb against the vmlinux file which contains the symbols (not
37a boot image such as bzImage, zImage, uImage...). In gdb the developer
38specifies the connection parameters and connects to kgdb. The type of
39connection a developer makes with gdb depends on the availability of
40kgdb I/O modules compiled as built-ins or loadable kernel modules in the
41test machine's kernel.
42
43Compiling a kernel
44==================
45
46-  In order to enable compilation of kdb, you must first enable kgdb.
47
48-  The kgdb test compile options are described in the kgdb test suite
49   chapter.
50
51Kernel config options for kgdb
52------------------------------
53
54To enable ``CONFIG_KGDB`` you should look under
55:menuselection:`Kernel hacking --> Kernel debugging` and select
56:menuselection:`KGDB: kernel debugger`.
57
58While it is not a hard requirement that you have symbols in your vmlinux
59file, gdb tends not to be very useful without the symbolic data, so you
60will want to turn on ``CONFIG_DEBUG_INFO`` which is called
61:menuselection:`Compile the kernel with debug info` in the config menu.
62
63It is advised, but not required, that you turn on the
64``CONFIG_FRAME_POINTER`` kernel option which is called :menuselection:`Compile
65the kernel with frame pointers` in the config menu. This option inserts code
66into the compiled executable which saves the frame information in registers
67or on the stack at different points which allows a debugger such as gdb to
68more accurately construct stack back traces while debugging the kernel.
69
70If the architecture that you are using supports the kernel option
71``CONFIG_STRICT_KERNEL_RWX``, you should consider turning it off. This
72option will prevent the use of software breakpoints because it marks
73certain regions of the kernel's memory space as read-only. If kgdb
74supports it for the architecture you are using, you can use hardware
75breakpoints if you desire to run with the ``CONFIG_STRICT_KERNEL_RWX``
76option turned on, else you need to turn off this option.
77
78Next you should choose one of more I/O drivers to interconnect debugging
79host and debugged target. Early boot debugging requires a KGDB I/O
80driver that supports early debugging and the driver must be built into
81the kernel directly. Kgdb I/O driver configuration takes place via
82kernel or module parameters which you can learn more about in the in the
83section that describes the parameter kgdboc.
84
85Here is an example set of ``.config`` symbols to enable or disable for kgdb::
86
87  # CONFIG_STRICT_KERNEL_RWX is not set
88  CONFIG_FRAME_POINTER=y
89  CONFIG_KGDB=y
90  CONFIG_KGDB_SERIAL_CONSOLE=y
91
92Kernel config options for kdb
93-----------------------------
94
95Kdb is quite a bit more complex than the simple gdbstub sitting on top
96of the kernel's debug core. Kdb must implement a shell, and also adds
97some helper functions in other parts of the kernel, responsible for
98printing out interesting data such as what you would see if you ran
99``lsmod``, or ``ps``. In order to build kdb into the kernel you follow the
100same steps as you would for kgdb.
101
102The main config option for kdb is ``CONFIG_KGDB_KDB`` which is called
103:menuselection:`KGDB_KDB: include kdb frontend for kgdb` in the config menu.
104In theory you would have already also selected an I/O driver such as the
105``CONFIG_KGDB_SERIAL_CONSOLE`` interface if you plan on using kdb on a
106serial port, when you were configuring kgdb.
107
108If you want to use a PS/2-style keyboard with kdb, you would select
109``CONFIG_KDB_KEYBOARD`` which is called :menuselection:`KGDB_KDB: keyboard as
110input device` in the config menu. The ``CONFIG_KDB_KEYBOARD`` option is not
111used for anything in the gdb interface to kgdb. The ``CONFIG_KDB_KEYBOARD``
112option only works with kdb.
113
114Here is an example set of ``.config`` symbols to enable/disable kdb::
115
116  # CONFIG_STRICT_KERNEL_RWX is not set
117  CONFIG_FRAME_POINTER=y
118  CONFIG_KGDB=y
119  CONFIG_KGDB_SERIAL_CONSOLE=y
120  CONFIG_KGDB_KDB=y
121  CONFIG_KDB_KEYBOARD=y
122
123Kernel Debugger Boot Arguments
124==============================
125
126This section describes the various runtime kernel parameters that affect
127the configuration of the kernel debugger. The following chapter covers
128using kdb and kgdb as well as providing some examples of the
129configuration parameters.
130
131Kernel parameter: kgdboc
132------------------------
133
134The kgdboc driver was originally an abbreviation meant to stand for
135"kgdb over console". Today it is the primary mechanism to configure how
136to communicate from gdb to kgdb as well as the devices you want to use
137to interact with the kdb shell.
138
139For kgdb/gdb, kgdboc is designed to work with a single serial port. It
140is intended to cover the circumstance where you want to use a serial
141console as your primary console as well as using it to perform kernel
142debugging. It is also possible to use kgdb on a serial port which is not
143designated as a system console. Kgdboc may be configured as a kernel
144built-in or a kernel loadable module. You can only make use of
145``kgdbwait`` and early debugging if you build kgdboc into the kernel as
146a built-in.
147
148Optionally you can elect to activate kms (Kernel Mode Setting)
149integration. When you use kms with kgdboc and you have a video driver
150that has atomic mode setting hooks, it is possible to enter the debugger
151on the graphics console. When the kernel execution is resumed, the
152previous graphics mode will be restored. This integration can serve as a
153useful tool to aid in diagnosing crashes or doing analysis of memory
154with kdb while allowing the full graphics console applications to run.
155
156kgdboc arguments
157~~~~~~~~~~~~~~~~
158
159Usage::
160
161	kgdboc=[kms][[,]kbd][[,]serial_device][,baud]
162
163The order listed above must be observed if you use any of the optional
164configurations together.
165
166Abbreviations:
167
168-  kms = Kernel Mode Setting
169
170-  kbd = Keyboard
171
172You can configure kgdboc to use the keyboard, and/or a serial device
173depending on if you are using kdb and/or kgdb, in one of the following
174scenarios. The order listed above must be observed if you use any of the
175optional configurations together. Using kms + only gdb is generally not
176a useful combination.
177
178Using loadable module or built-in
179^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
180
1811. As a kernel built-in:
182
183   Use the kernel boot argument::
184
185	kgdboc=<tty-device>,[baud]
186
1872. As a kernel loadable module:
188
189   Use the command::
190
191	modprobe kgdboc kgdboc=<tty-device>,[baud]
192
193   Here are two examples of how you might format the kgdboc string. The
194   first is for an x86 target using the first serial port. The second
195   example is for the ARM Versatile AB using the second serial port.
196
197   1. ``kgdboc=ttyS0,115200``
198
199   2. ``kgdboc=ttyAMA1,115200``
200
201Configure kgdboc at runtime with sysfs
202^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
203
204At run time you can enable or disable kgdboc by echoing a parameters
205into the sysfs. Here are two examples:
206
2071. Enable kgdboc on ttyS0::
208
209	echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc
210
2112. Disable kgdboc::
212
213	echo "" > /sys/module/kgdboc/parameters/kgdboc
214
215.. note::
216
217   You do not need to specify the baud if you are configuring the
218   console on tty which is already configured or open.
219
220More examples
221^^^^^^^^^^^^^
222
223You can configure kgdboc to use the keyboard, and/or a serial device
224depending on if you are using kdb and/or kgdb, in one of the following
225scenarios.
226
2271. kdb and kgdb over only a serial port::
228
229	kgdboc=<serial_device>[,baud]
230
231   Example::
232
233	kgdboc=ttyS0,115200
234
2352. kdb and kgdb with keyboard and a serial port::
236
237	kgdboc=kbd,<serial_device>[,baud]
238
239   Example::
240
241	kgdboc=kbd,ttyS0,115200
242
2433. kdb with a keyboard::
244
245	kgdboc=kbd
246
2474. kdb with kernel mode setting::
248
249	kgdboc=kms,kbd
250
2515. kdb with kernel mode setting and kgdb over a serial port::
252
253	kgdboc=kms,kbd,ttyS0,115200
254
255.. note::
256
257   Kgdboc does not support interrupting the target via the gdb remote
258   protocol. You must manually send a :kbd:`SysRq-G` unless you have a proxy
259   that splits console output to a terminal program. A console proxy has a
260   separate TCP port for the debugger and a separate TCP port for the
261   "human" console. The proxy can take care of sending the :kbd:`SysRq-G`
262   for you.
263
264When using kgdboc with no debugger proxy, you can end up connecting the
265debugger at one of two entry points. If an exception occurs after you
266have loaded kgdboc, a message should print on the console stating it is
267waiting for the debugger. In this case you disconnect your terminal
268program and then connect the debugger in its place. If you want to
269interrupt the target system and forcibly enter a debug session you have
270to issue a :kbd:`Sysrq` sequence and then type the letter :kbd:`g`. Then you
271disconnect the terminal session and connect gdb. Your options if you
272don't like this are to hack gdb to send the :kbd:`SysRq-G` for you as well as
273on the initial connect, or to use a debugger proxy that allows an
274unmodified gdb to do the debugging.
275
276Kernel parameter: ``kgdboc_earlycon``
277-------------------------------------
278
279If you specify the kernel parameter ``kgdboc_earlycon`` and your serial
280driver registers a boot console that supports polling (doesn't need
281interrupts and implements a nonblocking read() function) kgdb will attempt
282to work using the boot console until it can transition to the regular
283tty driver specified by the ``kgdboc`` parameter.
284
285Normally there is only one boot console (especially that implements the
286read() function) so just adding ``kgdboc_earlycon`` on its own is
287sufficient to make this work. If you have more than one boot console you
288can add the boot console's name to differentiate. Note that names that
289are registered through the boot console layer and the tty layer are not
290the same for the same port.
291
292For instance, on one board to be explicit you might do::
293
294   kgdboc_earlycon=qcom_geni kgdboc=ttyMSM0
295
296If the only boot console on the device was "qcom_geni", you could simplify::
297
298   kgdboc_earlycon kgdboc=ttyMSM0
299
300Kernel parameter: ``kgdbwait``
301------------------------------
302
303The Kernel command line option ``kgdbwait`` makes kgdb wait for a
304debugger connection during booting of a kernel. You can only use this
305option if you compiled a kgdb I/O driver into the kernel and you
306specified the I/O driver configuration as a kernel command line option.
307The kgdbwait parameter should always follow the configuration parameter
308for the kgdb I/O driver in the kernel command line else the I/O driver
309will not be configured prior to asking the kernel to use it to wait.
310
311The kernel will stop and wait as early as the I/O driver and
312architecture allows when you use this option. If you build the kgdb I/O
313driver as a loadable kernel module kgdbwait will not do anything.
314
315Kernel parameter: ``kgdbcon``
316-----------------------------
317
318The ``kgdbcon`` feature allows you to see printk() messages inside gdb
319while gdb is connected to the kernel. Kdb does not make use of the kgdbcon
320feature.
321
322Kgdb supports using the gdb serial protocol to send console messages to
323the debugger when the debugger is connected and running. There are two
324ways to activate this feature.
325
3261. Activate with the kernel command line option::
327
328	kgdbcon
329
3302. Use sysfs before configuring an I/O driver::
331
332	echo 1 > /sys/module/kgdb/parameters/kgdb_use_con
333
334.. note::
335
336   If you do this after you configure the kgdb I/O driver, the
337   setting will not take effect until the next point the I/O is
338   reconfigured.
339
340.. important::
341
342   You cannot use kgdboc + kgdbcon on a tty that is an
343   active system console. An example of incorrect usage is::
344
345	console=ttyS0,115200 kgdboc=ttyS0 kgdbcon
346
347It is possible to use this option with kgdboc on a tty that is not a
348system console.
349
350Run time parameter: ``kgdbreboot``
351----------------------------------
352
353The kgdbreboot feature allows you to change how the debugger deals with
354the reboot notification. You have 3 choices for the behavior. The
355default behavior is always set to 0.
356
357.. tabularcolumns:: |p{0.4cm}|p{11.5cm}|p{5.6cm}|
358
359.. flat-table::
360  :widths: 1 10 8
361
362  * - 1
363    - ``echo -1 > /sys/module/debug_core/parameters/kgdbreboot``
364    - Ignore the reboot notification entirely.
365
366  * - 2
367    - ``echo 0 > /sys/module/debug_core/parameters/kgdbreboot``
368    - Send the detach message to any attached debugger client.
369
370  * - 3
371    - ``echo 1 > /sys/module/debug_core/parameters/kgdbreboot``
372    - Enter the debugger on reboot notify.
373
374Kernel parameter: ``nokaslr``
375-----------------------------
376
377If the architecture that you are using enable KASLR by default,
378you should consider turning it off.  KASLR randomizes the
379virtual address where the kernel image is mapped and confuse
380gdb which resolve kernel symbol address from symbol table
381of vmlinux.
382
383Using kdb
384=========
385
386Quick start for kdb on a serial port
387------------------------------------
388
389This is a quick example of how to use kdb.
390
3911. Configure kgdboc at boot using kernel parameters::
392
393	console=ttyS0,115200 kgdboc=ttyS0,115200 nokaslr
394
395   OR
396
397   Configure kgdboc after the kernel has booted; assuming you are using
398   a serial port console::
399
400	echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc
401
4022. Enter the kernel debugger manually or by waiting for an oops or
403   fault. There are several ways you can enter the kernel debugger
404   manually; all involve using the :kbd:`SysRq-G`, which means you must have
405   enabled ``CONFIG_MAGIC_SysRq=y`` in your kernel config.
406
407   -  When logged in as root or with a super user session you can run::
408
409	echo g > /proc/sysrq-trigger
410
411   -  Example using minicom 2.2
412
413      Press: :kbd:`CTRL-A` :kbd:`f` :kbd:`g`
414
415   -  When you have telneted to a terminal server that supports sending
416      a remote break
417
418      Press: :kbd:`CTRL-]`
419
420      Type in: ``send break``
421
422      Press: :kbd:`Enter` :kbd:`g`
423
4243. From the kdb prompt you can run the ``help`` command to see a complete
425   list of the commands that are available.
426
427   Some useful commands in kdb include:
428
429   =========== =================================================================
430   ``lsmod``   Shows where kernel modules are loaded
431   ``ps``      Displays only the active processes
432   ``ps A``    Shows all the processes
433   ``summary`` Shows kernel version info and memory usage
434   ``bt``      Get a backtrace of the current process using dump_stack()
435   ``dmesg``   View the kernel syslog buffer
436   ``go``      Continue the system
437   =========== =================================================================
438
4394. When you are done using kdb you need to consider rebooting the system
440   or using the ``go`` command to resuming normal kernel execution. If you
441   have paused the kernel for a lengthy period of time, applications
442   that rely on timely networking or anything to do with real wall clock
443   time could be adversely affected, so you should take this into
444   consideration when using the kernel debugger.
445
446Quick start for kdb using a keyboard connected console
447------------------------------------------------------
448
449This is a quick example of how to use kdb with a keyboard.
450
4511. Configure kgdboc at boot using kernel parameters::
452
453	kgdboc=kbd
454
455   OR
456
457   Configure kgdboc after the kernel has booted::
458
459	echo kbd > /sys/module/kgdboc/parameters/kgdboc
460
4612. Enter the kernel debugger manually or by waiting for an oops or
462   fault. There are several ways you can enter the kernel debugger
463   manually; all involve using the :kbd:`SysRq-G`, which means you must have
464   enabled ``CONFIG_MAGIC_SysRq=y`` in your kernel config.
465
466   -  When logged in as root or with a super user session you can run::
467
468	echo g > /proc/sysrq-trigger
469
470   -  Example using a laptop keyboard:
471
472      Press and hold down: :kbd:`Alt`
473
474      Press and hold down: :kbd:`Fn`
475
476      Press and release the key with the label: :kbd:`SysRq`
477
478      Release: :kbd:`Fn`
479
480      Press and release: :kbd:`g`
481
482      Release: :kbd:`Alt`
483
484   -  Example using a PS/2 101-key keyboard
485
486      Press and hold down: :kbd:`Alt`
487
488      Press and release the key with the label: :kbd:`SysRq`
489
490      Press and release: :kbd:`g`
491
492      Release: :kbd:`Alt`
493
4943. Now type in a kdb command such as ``help``, ``dmesg``, ``bt`` or ``go`` to
495   continue kernel execution.
496
497Using kgdb / gdb
498================
499
500In order to use kgdb you must activate it by passing configuration
501information to one of the kgdb I/O drivers. If you do not pass any
502configuration information kgdb will not do anything at all. Kgdb will
503only actively hook up to the kernel trap hooks if a kgdb I/O driver is
504loaded and configured. If you unconfigure a kgdb I/O driver, kgdb will
505unregister all the kernel hook points.
506
507All kgdb I/O drivers can be reconfigured at run time, if
508``CONFIG_SYSFS`` and ``CONFIG_MODULES`` are enabled, by echo'ing a new
509config string to ``/sys/module/<driver>/parameter/<option>``. The driver
510can be unconfigured by passing an empty string. You cannot change the
511configuration while the debugger is attached. Make sure to detach the
512debugger with the ``detach`` command prior to trying to unconfigure a
513kgdb I/O driver.
514
515Connecting with gdb to a serial port
516------------------------------------
517
5181. Configure kgdboc
519
520   Configure kgdboc at boot using kernel parameters::
521
522	kgdboc=ttyS0,115200
523
524   OR
525
526   Configure kgdboc after the kernel has booted::
527
528	echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc
529
5302. Stop kernel execution (break into the debugger)
531
532   In order to connect to gdb via kgdboc, the kernel must first be
533   stopped. There are several ways to stop the kernel which include
534   using kgdbwait as a boot argument, via a :kbd:`SysRq-G`, or running the
535   kernel until it takes an exception where it waits for the debugger to
536   attach.
537
538   -  When logged in as root or with a super user session you can run::
539
540	echo g > /proc/sysrq-trigger
541
542   -  Example using minicom 2.2
543
544      Press: :kbd:`CTRL-A` :kbd:`f` :kbd:`g`
545
546   -  When you have telneted to a terminal server that supports sending
547      a remote break
548
549      Press: :kbd:`CTRL-]`
550
551      Type in: ``send break``
552
553      Press: :kbd:`Enter` :kbd:`g`
554
5553. Connect from gdb
556
557   Example (using a directly connected port)::
558
559           % gdb ./vmlinux
560           (gdb) set remotebaud 115200
561           (gdb) target remote /dev/ttyS0
562
563
564   Example (kgdb to a terminal server on TCP port 2012)::
565
566           % gdb ./vmlinux
567           (gdb) target remote 192.168.2.2:2012
568
569
570   Once connected, you can debug a kernel the way you would debug an
571   application program.
572
573   If you are having problems connecting or something is going seriously
574   wrong while debugging, it will most often be the case that you want
575   to enable gdb to be verbose about its target communications. You do
576   this prior to issuing the ``target remote`` command by typing in::
577
578	set debug remote 1
579
580Remember if you continue in gdb, and need to "break in" again, you need
581to issue an other :kbd:`SysRq-G`. It is easy to create a simple entry point by
582putting a breakpoint at ``sys_sync`` and then you can run ``sync`` from a
583shell or script to break into the debugger.
584
585kgdb and kdb interoperability
586=============================
587
588It is possible to transition between kdb and kgdb dynamically. The debug
589core will remember which you used the last time and automatically start
590in the same mode.
591
592Switching between kdb and kgdb
593------------------------------
594
595Switching from kgdb to kdb
596~~~~~~~~~~~~~~~~~~~~~~~~~~
597
598There are two ways to switch from kgdb to kdb: you can use gdb to issue
599a maintenance packet, or you can blindly type the command ``$3#33``.
600Whenever the kernel debugger stops in kgdb mode it will print the
601message ``KGDB or $3#33 for KDB``. It is important to note that you have
602to type the sequence correctly in one pass. You cannot type a backspace
603or delete because kgdb will interpret that as part of the debug stream.
604
6051. Change from kgdb to kdb by blindly typing::
606
607	$3#33
608
6092. Change from kgdb to kdb with gdb::
610
611	maintenance packet 3
612
613   .. note::
614
615     Now you must kill gdb. Typically you press :kbd:`CTRL-Z` and issue
616     the command::
617
618	kill -9 %
619
620Change from kdb to kgdb
621~~~~~~~~~~~~~~~~~~~~~~~
622
623There are two ways you can change from kdb to kgdb. You can manually
624enter kgdb mode by issuing the kgdb command from the kdb shell prompt,
625or you can connect gdb while the kdb shell prompt is active. The kdb
626shell looks for the typical first commands that gdb would issue with the
627gdb remote protocol and if it sees one of those commands it
628automatically changes into kgdb mode.
629
6301. From kdb issue the command::
631
632	kgdb
633
634   Now disconnect your terminal program and connect gdb in its place
635
6362. At the kdb prompt, disconnect the terminal program and connect gdb in
637   its place.
638
639Running kdb commands from gdb
640-----------------------------
641
642It is possible to run a limited set of kdb commands from gdb, using the
643gdb monitor command. You don't want to execute any of the run control or
644breakpoint operations, because it can disrupt the state of the kernel
645debugger. You should be using gdb for breakpoints and run control
646operations if you have gdb connected. The more useful commands to run
647are things like lsmod, dmesg, ps or possibly some of the memory
648information commands. To see all the kdb commands you can run
649``monitor help``.
650
651Example::
652
653    (gdb) monitor ps
654    1 idle process (state I) and
655    27 sleeping system daemon (state M) processes suppressed,
656    use 'ps A' to see all.
657    Task Addr       Pid   Parent [*] cpu State Thread     Command
658
659    0xc78291d0        1        0  0    0   S  0xc7829404  init
660    0xc7954150      942        1  0    0   S  0xc7954384  dropbear
661    0xc78789c0      944        1  0    0   S  0xc7878bf4  sh
662    (gdb)
663
664kgdb Test Suite
665===============
666
667When kgdb is enabled in the kernel config you can also elect to enable
668the config parameter ``KGDB_TESTS``. Turning this on will enable a special
669kgdb I/O module which is designed to test the kgdb internal functions.
670
671The kgdb tests are mainly intended for developers to test the kgdb
672internals as well as a tool for developing a new kgdb architecture
673specific implementation. These tests are not really for end users of the
674Linux kernel. The primary source of documentation would be to look in
675the ``drivers/misc/kgdbts.c`` file.
676
677The kgdb test suite can also be configured at compile time to run the
678core set of tests by setting the kernel config parameter
679``KGDB_TESTS_ON_BOOT``. This particular option is aimed at automated
680regression testing and does not require modifying the kernel boot config
681arguments. If this is turned on, the kgdb test suite can be disabled by
682specifying ``kgdbts=`` as a kernel boot argument.
683
684Kernel Debugger Internals
685=========================
686
687Architecture Specifics
688----------------------
689
690The kernel debugger is organized into a number of components:
691
6921. The debug core
693
694   The debug core is found in ``kernel/debugger/debug_core.c``. It
695   contains:
696
697   -  A generic OS exception handler which includes sync'ing the
698      processors into a stopped state on an multi-CPU system.
699
700   -  The API to talk to the kgdb I/O drivers
701
702   -  The API to make calls to the arch-specific kgdb implementation
703
704   -  The logic to perform safe memory reads and writes to memory while
705      using the debugger
706
707   -  A full implementation for software breakpoints unless overridden
708      by the arch
709
710   -  The API to invoke either the kdb or kgdb frontend to the debug
711      core.
712
713   -  The structures and callback API for atomic kernel mode setting.
714
715      .. note:: kgdboc is where the kms callbacks are invoked.
716
7172. kgdb arch-specific implementation
718
719   This implementation is generally found in ``arch/*/kernel/kgdb.c``. As
720   an example, ``arch/x86/kernel/kgdb.c`` contains the specifics to
721   implement HW breakpoint as well as the initialization to dynamically
722   register and unregister for the trap handlers on this architecture.
723   The arch-specific portion implements:
724
725   -  contains an arch-specific trap catcher which invokes
726      kgdb_handle_exception() to start kgdb about doing its work
727
728   -  translation to and from gdb specific packet format to struct pt_regs
729
730   -  Registration and unregistration of architecture specific trap
731      hooks
732
733   -  Any special exception handling and cleanup
734
735   -  NMI exception handling and cleanup
736
737   -  (optional) HW breakpoints
738
7393. gdbstub frontend (aka kgdb)
740
741   The gdbstub is located in ``kernel/debug/gdbstub.c``. It contains:
742
743   -  All the logic to implement the gdb serial protocol
744
7454. kdb frontend
746
747   The kdb debugger shell is broken down into a number of components.
748   The kdb core is located in kernel/debug/kdb. There are a number of
749   helper functions in some of the other kernel components to make it
750   possible for kdb to examine and report information about the kernel
751   without taking locks that could cause a kernel deadlock. The kdb core
752   contains implements the following functionality.
753
754   -  A simple shell
755
756   -  The kdb core command set
757
758   -  A registration API to register additional kdb shell commands.
759
760      -  A good example of a self-contained kdb module is the ``ftdump``
761         command for dumping the ftrace buffer. See:
762         ``kernel/trace/trace_kdb.c``
763
764      -  For an example of how to dynamically register a new kdb command
765         you can build the kdb_hello.ko kernel module from
766         ``samples/kdb/kdb_hello.c``. To build this example you can set
767         ``CONFIG_SAMPLES=y`` and ``CONFIG_SAMPLE_KDB=m`` in your kernel
768         config. Later run ``modprobe kdb_hello`` and the next time you
769         enter the kdb shell, you can run the ``hello`` command.
770
771   -  The implementation for kdb_printf() which emits messages directly
772      to I/O drivers, bypassing the kernel log.
773
774   -  SW / HW breakpoint management for the kdb shell
775
7765. kgdb I/O driver
777
778   Each kgdb I/O driver has to provide an implementation for the
779   following:
780
781   -  configuration via built-in or module
782
783   -  dynamic configuration and kgdb hook registration calls
784
785   -  read and write character interface
786
787   -  A cleanup handler for unconfiguring from the kgdb core
788
789   -  (optional) Early debug methodology
790
791   Any given kgdb I/O driver has to operate very closely with the
792   hardware and must do it in such a way that does not enable interrupts
793   or change other parts of the system context without completely
794   restoring them. The kgdb core will repeatedly "poll" a kgdb I/O
795   driver for characters when it needs input. The I/O driver is expected
796   to return immediately if there is no data available. Doing so allows
797   for the future possibility to touch watchdog hardware in such a way
798   as to have a target system not reset when these are enabled.
799
800If you are intent on adding kgdb architecture specific support for a new
801architecture, the architecture should define ``HAVE_ARCH_KGDB`` in the
802architecture specific Kconfig file. This will enable kgdb for the
803architecture, and at that point you must create an architecture specific
804kgdb implementation.
805
806There are a few flags which must be set on every architecture in their
807``asm/kgdb.h`` file. These are:
808
809-  ``NUMREGBYTES``:
810     The size in bytes of all of the registers, so that we
811     can ensure they will all fit into a packet.
812
813-  ``BUFMAX``:
814     The size in bytes of the buffer GDB will read into. This must
815     be larger than NUMREGBYTES.
816
817-  ``CACHE_FLUSH_IS_SAFE``:
818     Set to 1 if it is always safe to call
819     flush_cache_range or flush_icache_range. On some architectures,
820     these functions may not be safe to call on SMP since we keep other
821     CPUs in a holding pattern.
822
823There are also the following functions for the common backend, found in
824``kernel/kgdb.c``, that must be supplied by the architecture-specific
825backend unless marked as (optional), in which case a default function
826maybe used if the architecture does not need to provide a specific
827implementation.
828
829.. kernel-doc:: include/linux/kgdb.h
830   :internal:
831
832kgdboc internals
833----------------
834
835kgdboc and uarts
836~~~~~~~~~~~~~~~~
837
838The kgdboc driver is actually a very thin driver that relies on the
839underlying low level to the hardware driver having "polling hooks" to
840which the tty driver is attached. In the initial implementation of
841kgdboc the serial_core was changed to expose a low level UART hook for
842doing polled mode reading and writing of a single character while in an
843atomic context. When kgdb makes an I/O request to the debugger, kgdboc
844invokes a callback in the serial core which in turn uses the callback in
845the UART driver.
846
847When using kgdboc with a UART, the UART driver must implement two
848callbacks in the struct uart_ops.
849Example from ``drivers/8250.c``::
850
851
852    #ifdef CONFIG_CONSOLE_POLL
853        .poll_get_char = serial8250_get_poll_char,
854        .poll_put_char = serial8250_put_poll_char,
855    #endif
856
857
858Any implementation specifics around creating a polling driver use the
859``#ifdef CONFIG_CONSOLE_POLL``, as shown above. Keep in mind that
860polling hooks have to be implemented in such a way that they can be
861called from an atomic context and have to restore the state of the UART
862chip on return such that the system can return to normal when the
863debugger detaches. You need to be very careful with any kind of lock you
864consider, because failing here is most likely going to mean pressing the
865reset button.
866
867kgdboc and keyboards
868~~~~~~~~~~~~~~~~~~~~~~~~
869
870The kgdboc driver contains logic to configure communications with an
871attached keyboard. The keyboard infrastructure is only compiled into the
872kernel when ``CONFIG_KDB_KEYBOARD=y`` is set in the kernel configuration.
873
874The core polled keyboard driver for PS/2 type keyboards is in
875``drivers/char/kdb_keyboard.c``. This driver is hooked into the debug core
876when kgdboc populates the callback in the array called
877:c:expr:`kdb_poll_funcs[]`. The kdb_get_kbd_char() is the top-level
878function which polls hardware for single character input.
879
880kgdboc and kms
881~~~~~~~~~~~~~~~~~~
882
883The kgdboc driver contains logic to request the graphics display to
884switch to a text context when you are using ``kgdboc=kms,kbd``, provided
885that you have a video driver which has a frame buffer console and atomic
886kernel mode setting support.
887
888Every time the kernel debugger is entered it calls
889kgdboc_pre_exp_handler() which in turn calls con_debug_enter()
890in the virtual console layer. On resuming kernel execution, the kernel
891debugger calls kgdboc_post_exp_handler() which in turn calls
892con_debug_leave().
893
894Any video driver that wants to be compatible with the kernel debugger
895and the atomic kms callbacks must implement the ``mode_set_base_atomic``,
896``fb_debug_enter`` and ``fb_debug_leave operations``. For the
897``fb_debug_enter`` and ``fb_debug_leave`` the option exists to use the
898generic drm fb helper functions or implement something custom for the
899hardware. The following example shows the initialization of the
900.mode_set_base_atomic operation in
901drivers/gpu/drm/i915/intel_display.c::
902
903
904    static const struct drm_crtc_helper_funcs intel_helper_funcs = {
905    [...]
906            .mode_set_base_atomic = intel_pipe_set_base_atomic,
907    [...]
908    };
909
910
911Here is an example of how the i915 driver initializes the
912fb_debug_enter and fb_debug_leave functions to use the generic drm
913helpers in ``drivers/gpu/drm/i915/intel_fb.c``::
914
915
916    static struct fb_ops intelfb_ops = {
917    [...]
918           .fb_debug_enter = drm_fb_helper_debug_enter,
919           .fb_debug_leave = drm_fb_helper_debug_leave,
920    [...]
921    };
922
923
924Credits
925=======
926
927The following people have contributed to this document:
928
9291. Amit Kale <amitkale@linsyssoft.com>
930
9312. Tom Rini <trini@kernel.crashing.org>
932
933In March 2008 this document was completely rewritten by:
934
935-  Jason Wessel <jason.wessel@windriver.com>
936
937In Jan 2010 this document was updated to include kdb.
938
939-  Jason Wessel <jason.wessel@windriver.com>
940