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1=================================================2Using kgdb, kdb and the kernel debugger internals3=================================================4 5:Author: Jason Wessel6 7Introduction8============9 10The kernel has two different debugger front ends (kdb and kgdb) which11interface to the debug core. It is possible to use either of the12debugger front ends and dynamically transition between them if you13configure the kernel properly at compile and runtime.14 15Kdb is simplistic shell-style interface which you can use on a system16console with a keyboard or serial console. You can use it to inspect17memory, registers, process lists, dmesg, and even set breakpoints to18stop in a certain location. Kdb is not a source level debugger, although19you can set breakpoints and execute some basic kernel run control. Kdb20is mainly aimed at doing some analysis to aid in development or21diagnosing kernel problems. You can access some symbols by name in22kernel built-ins or in kernel modules if the code was built with23``CONFIG_KALLSYMS``.24 25Kgdb is intended to be used as a source level debugger for the Linux26kernel. It is used along with gdb to debug a Linux kernel. The27expectation is that gdb can be used to "break in" to the kernel to28inspect memory, variables and look through call stack information29similar to the way an application developer would use gdb to debug an30application. It is possible to place breakpoints in kernel code and31perform some limited execution stepping.32 33Two machines are required for using kgdb. One of these machines is a34development machine and the other is the target machine. The kernel to35be debugged runs on the target machine. The development machine runs an36instance of gdb against the vmlinux file which contains the symbols (not37a boot image such as bzImage, zImage, uImage...). In gdb the developer38specifies the connection parameters and connects to kgdb. The type of39connection a developer makes with gdb depends on the availability of40kgdb I/O modules compiled as built-ins or loadable kernel modules in the41test machine's kernel.42 43Compiling a kernel44==================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 suite49 chapter.50 51Kernel config options for kgdb52------------------------------53 54To enable ``CONFIG_KGDB`` you should look under55:menuselection:`Kernel hacking --> Kernel debugging` and select56:menuselection:`KGDB: kernel debugger`.57 58While it is not a hard requirement that you have symbols in your vmlinux59file, gdb tends not to be very useful without the symbolic data, so you60will want to turn on ``CONFIG_DEBUG_INFO`` which is called61:menuselection:`Compile the kernel with debug info` in the config menu.62 63It is advised, but not required, that you turn on the64``CONFIG_FRAME_POINTER`` kernel option which is called :menuselection:`Compile65the kernel with frame pointers` in the config menu. This option inserts code66into the compiled executable which saves the frame information in registers67or on the stack at different points which allows a debugger such as gdb to68more accurately construct stack back traces while debugging the kernel.69 70If the architecture that you are using supports the kernel option71``CONFIG_STRICT_KERNEL_RWX``, you should consider turning it off. This72option will prevent the use of software breakpoints because it marks73certain regions of the kernel's memory space as read-only. If kgdb74supports it for the architecture you are using, you can use hardware75breakpoints 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 debugging79host and debugged target. Early boot debugging requires a KGDB I/O80driver that supports early debugging and the driver must be built into81the kernel directly. Kgdb I/O driver configuration takes place via82kernel or module parameters which you can learn more about in the in the83section 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 set88 CONFIG_FRAME_POINTER=y89 CONFIG_KGDB=y90 CONFIG_KGDB_SERIAL_CONSOLE=y91 92Kernel config options for kdb93-----------------------------94 95Kdb is quite a bit more complex than the simple gdbstub sitting on top96of the kernel's debug core. Kdb must implement a shell, and also adds97some helper functions in other parts of the kernel, responsible for98printing out interesting data such as what you would see if you ran99``lsmod``, or ``ps``. In order to build kdb into the kernel you follow the100same steps as you would for kgdb.101 102The main config option for kdb is ``CONFIG_KGDB_KDB`` which is called103: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 the105``CONFIG_KGDB_SERIAL_CONSOLE`` interface if you plan on using kdb on a106serial port, when you were configuring kgdb.107 108If you want to use a PS/2-style keyboard with kdb, you would select109``CONFIG_KDB_KEYBOARD`` which is called :menuselection:`KGDB_KDB: keyboard as110input device` in the config menu. The ``CONFIG_KDB_KEYBOARD`` option is not111used 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 set117 CONFIG_FRAME_POINTER=y118 CONFIG_KGDB=y119 CONFIG_KGDB_SERIAL_CONSOLE=y120 CONFIG_KGDB_KDB=y121 CONFIG_KDB_KEYBOARD=y122 123Kernel Debugger Boot Arguments124==============================125 126This section describes the various runtime kernel parameters that affect127the configuration of the kernel debugger. The following chapter covers128using kdb and kgdb as well as providing some examples of the129configuration parameters.130 131Kernel parameter: kgdboc132------------------------133 134The kgdboc driver was originally an abbreviation meant to stand for135"kgdb over console". Today it is the primary mechanism to configure how136to communicate from gdb to kgdb as well as the devices you want to use137to interact with the kdb shell.138 139For kgdb/gdb, kgdboc is designed to work with a single serial port. It140is intended to cover the circumstance where you want to use a serial141console as your primary console as well as using it to perform kernel142debugging. It is also possible to use kgdb on a serial port which is not143designated as a system console. Kgdboc may be configured as a kernel144built-in or a kernel loadable module. You can only make use of145``kgdbwait`` and early debugging if you build kgdboc into the kernel as146a 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 driver150that has atomic mode setting hooks, it is possible to enter the debugger151on the graphics console. When the kernel execution is resumed, the152previous graphics mode will be restored. This integration can serve as a153useful tool to aid in diagnosing crashes or doing analysis of memory154with kdb while allowing the full graphics console applications to run.155 156kgdboc arguments157~~~~~~~~~~~~~~~~158 159Usage::160 161 kgdboc=[kms][[,]kbd][[,]serial_device][,baud]162 163The order listed above must be observed if you use any of the optional164configurations together.165 166Abbreviations:167 168- kms = Kernel Mode Setting169 170- kbd = Keyboard171 172You can configure kgdboc to use the keyboard, and/or a serial device173depending on if you are using kdb and/or kgdb, in one of the following174scenarios. The order listed above must be observed if you use any of the175optional configurations together. Using kms + only gdb is generally not176a useful combination.177 178Using loadable module or built-in179^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^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. The194 first is for an x86 target using the first serial port. The second195 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 sysfs202^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^203 204At run time you can enable or disable kgdboc by echoing a parameters205into the sysfs. Here are two examples:206 2071. Enable kgdboc on ttyS0::208 209 echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc210 2112. Disable kgdboc::212 213 echo "" > /sys/module/kgdboc/parameters/kgdboc214 215.. note::216 217 You do not need to specify the baud if you are configuring the218 console on tty which is already configured or open.219 220More examples221^^^^^^^^^^^^^222 223You can configure kgdboc to use the keyboard, and/or a serial device224depending on if you are using kdb and/or kgdb, in one of the following225scenarios.226 2271. kdb and kgdb over only a serial port::228 229 kgdboc=<serial_device>[,baud]230 231 Example::232 233 kgdboc=ttyS0,115200234 2352. kdb and kgdb with keyboard and a serial port::236 237 kgdboc=kbd,<serial_device>[,baud]238 239 Example::240 241 kgdboc=kbd,ttyS0,115200242 2433. kdb with a keyboard::244 245 kgdboc=kbd246 2474. kdb with kernel mode setting::248 249 kgdboc=kms,kbd250 2515. kdb with kernel mode setting and kgdb over a serial port::252 253 kgdboc=kms,kbd,ttyS0,115200254 255.. note::256 257 Kgdboc does not support interrupting the target via the gdb remote258 protocol. You must manually send a :kbd:`SysRq-G` unless you have a proxy259 that splits console output to a terminal program. A console proxy has a260 separate TCP port for the debugger and a separate TCP port for the261 "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 the265debugger at one of two entry points. If an exception occurs after you266have loaded kgdboc, a message should print on the console stating it is267waiting for the debugger. In this case you disconnect your terminal268program and then connect the debugger in its place. If you want to269interrupt the target system and forcibly enter a debug session you have270to issue a :kbd:`Sysrq` sequence and then type the letter :kbd:`g`. Then you271disconnect the terminal session and connect gdb. Your options if you272don't like this are to hack gdb to send the :kbd:`SysRq-G` for you as well as273on the initial connect, or to use a debugger proxy that allows an274unmodified gdb to do the debugging.275 276Kernel parameter: ``kgdboc_earlycon``277-------------------------------------278 279If you specify the kernel parameter ``kgdboc_earlycon`` and your serial280driver registers a boot console that supports polling (doesn't need281interrupts and implements a nonblocking read() function) kgdb will attempt282to work using the boot console until it can transition to the regular283tty driver specified by the ``kgdboc`` parameter.284 285Normally there is only one boot console (especially that implements the286read() function) so just adding ``kgdboc_earlycon`` on its own is287sufficient to make this work. If you have more than one boot console you288can add the boot console's name to differentiate. Note that names that289are registered through the boot console layer and the tty layer are not290the same for the same port.291 292For instance, on one board to be explicit you might do::293 294 kgdboc_earlycon=qcom_geni kgdboc=ttyMSM0295 296If the only boot console on the device was "qcom_geni", you could simplify::297 298 kgdboc_earlycon kgdboc=ttyMSM0299 300Kernel parameter: ``kgdbwait``301------------------------------302 303The Kernel command line option ``kgdbwait`` makes kgdb wait for a304debugger connection during booting of a kernel. You can only use this305option if you compiled a kgdb I/O driver into the kernel and you306specified the I/O driver configuration as a kernel command line option.307The kgdbwait parameter should always follow the configuration parameter308for the kgdb I/O driver in the kernel command line else the I/O driver309will 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 and312architecture allows when you use this option. If you build the kgdb I/O313driver 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 gdb319while gdb is connected to the kernel. Kdb does not make use of the kgdbcon320feature.321 322Kgdb supports using the gdb serial protocol to send console messages to323the debugger when the debugger is connected and running. There are two324ways to activate this feature.325 3261. Activate with the kernel command line option::327 328 kgdbcon329 3302. Use sysfs before configuring an I/O driver::331 332 echo 1 > /sys/module/kgdb/parameters/kgdb_use_con333 334.. note::335 336 If you do this after you configure the kgdb I/O driver, the337 setting will not take effect until the next point the I/O is338 reconfigured.339 340.. important::341 342 You cannot use kgdboc + kgdbcon on a tty that is an343 active system console. An example of incorrect usage is::344 345 console=ttyS0,115200 kgdboc=ttyS0 kgdbcon346 347It is possible to use this option with kgdboc on a tty that is not a348system console.349 350Run time parameter: ``kgdbreboot``351----------------------------------352 353The kgdbreboot feature allows you to change how the debugger deals with354the reboot notification. You have 3 choices for the behavior. The355default 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 8361 362 * - 1363 - ``echo -1 > /sys/module/debug_core/parameters/kgdbreboot``364 - Ignore the reboot notification entirely.365 366 * - 2367 - ``echo 0 > /sys/module/debug_core/parameters/kgdbreboot``368 - Send the detach message to any attached debugger client.369 370 * - 3371 - ``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 the379virtual address where the kernel image is mapped and confuse380gdb which resolve kernel symbol address from symbol table381of vmlinux.382 383Using kdb384=========385 386Quick start for kdb on a serial port387------------------------------------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 nokaslr394 395 OR396 397 Configure kgdboc after the kernel has booted; assuming you are using398 a serial port console::399 400 echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc401 4022. Enter the kernel debugger manually or by waiting for an oops or403 fault. There are several ways you can enter the kernel debugger404 manually; all involve using the :kbd:`SysRq-G`, which means you must have405 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-trigger410 411 - Example using minicom 2.2412 413 Press: :kbd:`CTRL-A` :kbd:`f` :kbd:`g`414 415 - When you have telneted to a terminal server that supports sending416 a remote break417 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 complete425 list of the commands that are available.426 427 Some useful commands in kdb include:428 429 =========== =================================================================430 ``lsmod`` Shows where kernel modules are loaded431 ``ps`` Displays only the active processes432 ``ps A`` Shows all the processes433 ``summary`` Shows kernel version info and memory usage434 ``bt`` Get a backtrace of the current process using dump_stack()435 ``dmesg`` View the kernel syslog buffer436 ``go`` Continue the system437 =========== =================================================================438 4394. When you are done using kdb you need to consider rebooting the system440 or using the ``go`` command to resuming normal kernel execution. If you441 have paused the kernel for a lengthy period of time, applications442 that rely on timely networking or anything to do with real wall clock443 time could be adversely affected, so you should take this into444 consideration when using the kernel debugger.445 446Quick start for kdb using a keyboard connected console447------------------------------------------------------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=kbd454 455 OR456 457 Configure kgdboc after the kernel has booted::458 459 echo kbd > /sys/module/kgdboc/parameters/kgdboc460 4612. Enter the kernel debugger manually or by waiting for an oops or462 fault. There are several ways you can enter the kernel debugger463 manually; all involve using the :kbd:`SysRq-G`, which means you must have464 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-trigger469 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 keyboard485 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`` to495 continue kernel execution.496 497Using kgdb / gdb498================499 500In order to use kgdb you must activate it by passing configuration501information to one of the kgdb I/O drivers. If you do not pass any502configuration information kgdb will not do anything at all. Kgdb will503only actively hook up to the kernel trap hooks if a kgdb I/O driver is504loaded and configured. If you unconfigure a kgdb I/O driver, kgdb will505unregister all the kernel hook points.506 507All kgdb I/O drivers can be reconfigured at run time, if508``CONFIG_SYSFS`` and ``CONFIG_MODULES`` are enabled, by echo'ing a new509config string to ``/sys/module/<driver>/parameter/<option>``. The driver510can be unconfigured by passing an empty string. You cannot change the511configuration while the debugger is attached. Make sure to detach the512debugger with the ``detach`` command prior to trying to unconfigure a513kgdb I/O driver.514 515Connecting with gdb to a serial port516------------------------------------517 5181. Configure kgdboc519 520 Configure kgdboc at boot using kernel parameters::521 522 kgdboc=ttyS0,115200523 524 OR525 526 Configure kgdboc after the kernel has booted::527 528 echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc529 5302. Stop kernel execution (break into the debugger)531 532 In order to connect to gdb via kgdboc, the kernel must first be533 stopped. There are several ways to stop the kernel which include534 using kgdbwait as a boot argument, via a :kbd:`SysRq-G`, or running the535 kernel until it takes an exception where it waits for the debugger to536 attach.537 538 - When logged in as root or with a super user session you can run::539 540 echo g > /proc/sysrq-trigger541 542 - Example using minicom 2.2543 544 Press: :kbd:`CTRL-A` :kbd:`f` :kbd:`g`545 546 - When you have telneted to a terminal server that supports sending547 a remote break548 549 Press: :kbd:`CTRL-]`550 551 Type in: ``send break``552 553 Press: :kbd:`Enter` :kbd:`g`554 5553. Connect from gdb556 557 Example (using a directly connected port)::558 559 % gdb ./vmlinux560 (gdb) set serial baud 115200561 (gdb) target remote /dev/ttyS0562 563 564 Example (kgdb to a terminal server on TCP port 2012)::565 566 % gdb ./vmlinux567 (gdb) target remote 192.168.2.2:2012568 569 570 Once connected, you can debug a kernel the way you would debug an571 application program.572 573 If you are having problems connecting or something is going seriously574 wrong while debugging, it will most often be the case that you want575 to enable gdb to be verbose about its target communications. You do576 this prior to issuing the ``target remote`` command by typing in::577 578 set debug remote 1579 580Remember if you continue in gdb, and need to "break in" again, you need581to issue an other :kbd:`SysRq-G`. It is easy to create a simple entry point by582putting a breakpoint at ``sys_sync`` and then you can run ``sync`` from a583shell or script to break into the debugger.584 585kgdb and kdb interoperability586=============================587 588It is possible to transition between kdb and kgdb dynamically. The debug589core will remember which you used the last time and automatically start590in the same mode.591 592Switching between kdb and kgdb593------------------------------594 595Switching from kgdb to kdb596~~~~~~~~~~~~~~~~~~~~~~~~~~597 598There are two ways to switch from kgdb to kdb: you can use gdb to issue599a maintenance packet, or you can blindly type the command ``$3#33``.600Whenever the kernel debugger stops in kgdb mode it will print the601message ``KGDB or $3#33 for KDB``. It is important to note that you have602to type the sequence correctly in one pass. You cannot type a backspace603or 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#33608 6092. Change from kgdb to kdb with gdb::610 611 maintenance packet 3612 613 .. note::614 615 Now you must kill gdb. Typically you press :kbd:`CTRL-Z` and issue616 the command::617 618 kill -9 %619 620Change from kdb to kgdb621~~~~~~~~~~~~~~~~~~~~~~~622 623There are two ways you can change from kdb to kgdb. You can manually624enter 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 kdb626shell looks for the typical first commands that gdb would issue with the627gdb remote protocol and if it sees one of those commands it628automatically changes into kgdb mode.629 6301. From kdb issue the command::631 632 kgdb633 634 Now disconnect your terminal program and connect gdb in its place635 6362. At the kdb prompt, disconnect the terminal program and connect gdb in637 its place.638 639Running kdb commands from gdb640-----------------------------641 642It is possible to run a limited set of kdb commands from gdb, using the643gdb monitor command. You don't want to execute any of the run control or644breakpoint operations, because it can disrupt the state of the kernel645debugger. You should be using gdb for breakpoints and run control646operations if you have gdb connected. The more useful commands to run647are things like lsmod, dmesg, ps or possibly some of the memory648information commands. To see all the kdb commands you can run649``monitor help``.650 651Example::652 653 (gdb) monitor ps654 1 idle process (state I) and655 27 sleeping system daemon (state M) processes suppressed,656 use 'ps A' to see all.657 Task Addr Pid Parent [*] cpu State Thread Command658 659 0xc78291d0 1 0 0 0 S 0xc7829404 init660 0xc7954150 942 1 0 0 S 0xc7954384 dropbear661 0xc78789c0 944 1 0 0 S 0xc7878bf4 sh662 (gdb)663 664kgdb Test Suite665===============666 667When kgdb is enabled in the kernel config you can also elect to enable668the config parameter ``KGDB_TESTS``. Turning this on will enable a special669kgdb I/O module which is designed to test the kgdb internal functions.670 671The kgdb tests are mainly intended for developers to test the kgdb672internals as well as a tool for developing a new kgdb architecture673specific implementation. These tests are not really for end users of the674Linux kernel. The primary source of documentation would be to look in675the ``drivers/misc/kgdbts.c`` file.676 677The kgdb test suite can also be configured at compile time to run the678core set of tests by setting the kernel config parameter679``KGDB_TESTS_ON_BOOT``. This particular option is aimed at automated680regression testing and does not require modifying the kernel boot config681arguments. If this is turned on, the kgdb test suite can be disabled by682specifying ``kgdbts=`` as a kernel boot argument.683 684Kernel Debugger Internals685=========================686 687Architecture Specifics688----------------------689 690The kernel debugger is organized into a number of components:691 6921. The debug core693 694 The debug core is found in ``kernel/debugger/debug_core.c``. It695 contains:696 697 - A generic OS exception handler which includes sync'ing the698 processors into a stopped state on an multi-CPU system.699 700 - The API to talk to the kgdb I/O drivers701 702 - The API to make calls to the arch-specific kgdb implementation703 704 - The logic to perform safe memory reads and writes to memory while705 using the debugger706 707 - A full implementation for software breakpoints unless overridden708 by the arch709 710 - The API to invoke either the kdb or kgdb frontend to the debug711 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 implementation718 719 This implementation is generally found in ``arch/*/kernel/kgdb.c``. As720 an example, ``arch/x86/kernel/kgdb.c`` contains the specifics to721 implement HW breakpoint as well as the initialization to dynamically722 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 invokes726 kgdb_handle_exception() to start kgdb about doing its work727 728 - translation to and from gdb specific packet format to struct pt_regs729 730 - Registration and unregistration of architecture specific trap731 hooks732 733 - Any special exception handling and cleanup734 735 - NMI exception handling and cleanup736 737 - (optional) HW breakpoints738 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 protocol744 7454. kdb frontend746 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 of749 helper functions in some of the other kernel components to make it750 possible for kdb to examine and report information about the kernel751 without taking locks that could cause a kernel deadlock. The kdb core752 contains implements the following functionality.753 754 - A simple shell755 756 - The kdb core command set757 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 command765 you can build the kdb_hello.ko kernel module from766 ``samples/kdb/kdb_hello.c``. To build this example you can set767 ``CONFIG_SAMPLES=y`` and ``CONFIG_SAMPLE_KDB=m`` in your kernel768 config. Later run ``modprobe kdb_hello`` and the next time you769 enter the kdb shell, you can run the ``hello`` command.770 771 - The implementation for kdb_printf() which emits messages directly772 to I/O drivers, bypassing the kernel log.773 774 - SW / HW breakpoint management for the kdb shell775 7765. kgdb I/O driver777 778 Each kgdb I/O driver has to provide an implementation for the779 following:780 781 - configuration via built-in or module782 783 - dynamic configuration and kgdb hook registration calls784 785 - read and write character interface786 787 - A cleanup handler for unconfiguring from the kgdb core788 789 - (optional) Early debug methodology790 791 Any given kgdb I/O driver has to operate very closely with the792 hardware and must do it in such a way that does not enable interrupts793 or change other parts of the system context without completely794 restoring them. The kgdb core will repeatedly "poll" a kgdb I/O795 driver for characters when it needs input. The I/O driver is expected796 to return immediately if there is no data available. Doing so allows797 for the future possibility to touch watchdog hardware in such a way798 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 new801architecture, the architecture should define ``HAVE_ARCH_KGDB`` in the802architecture specific Kconfig file. This will enable kgdb for the803architecture, and at that point you must create an architecture specific804kgdb implementation.805 806There are a few flags which must be set on every architecture in their807``asm/kgdb.h`` file. These are:808 809- ``NUMREGBYTES``:810 The size in bytes of all of the registers, so that we811 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 must815 be larger than NUMREGBYTES.816 817- ``CACHE_FLUSH_IS_SAFE``:818 Set to 1 if it is always safe to call819 flush_cache_range or flush_icache_range. On some architectures,820 these functions may not be safe to call on SMP since we keep other821 CPUs in a holding pattern.822 823There are also the following functions for the common backend, found in824``kernel/kgdb.c``, that must be supplied by the architecture-specific825backend unless marked as (optional), in which case a default function826maybe used if the architecture does not need to provide a specific827implementation.828 829.. kernel-doc:: include/linux/kgdb.h830 :internal:831 832kgdboc internals833----------------834 835kgdboc and uarts836~~~~~~~~~~~~~~~~837 838The kgdboc driver is actually a very thin driver that relies on the839underlying low level to the hardware driver having "polling hooks" to840which the tty driver is attached. In the initial implementation of841kgdboc the serial_core was changed to expose a low level UART hook for842doing polled mode reading and writing of a single character while in an843atomic context. When kgdb makes an I/O request to the debugger, kgdboc844invokes a callback in the serial core which in turn uses the callback in845the UART driver.846 847When using kgdboc with a UART, the UART driver must implement two848callbacks in the struct uart_ops.849Example from ``drivers/8250.c``::850 851 852 #ifdef CONFIG_CONSOLE_POLL853 .poll_get_char = serial8250_get_poll_char,854 .poll_put_char = serial8250_put_poll_char,855 #endif856 857 858Any implementation specifics around creating a polling driver use the859``#ifdef CONFIG_CONSOLE_POLL``, as shown above. Keep in mind that860polling hooks have to be implemented in such a way that they can be861called from an atomic context and have to restore the state of the UART862chip on return such that the system can return to normal when the863debugger detaches. You need to be very careful with any kind of lock you864consider, because failing here is most likely going to mean pressing the865reset button.866 867kgdboc and keyboards868~~~~~~~~~~~~~~~~~~~~~~~~869 870The kgdboc driver contains logic to configure communications with an871attached keyboard. The keyboard infrastructure is only compiled into the872kernel when ``CONFIG_KDB_KEYBOARD=y`` is set in the kernel configuration.873 874The core polled keyboard driver for PS/2 type keyboards is in875``drivers/char/kdb_keyboard.c``. This driver is hooked into the debug core876when kgdboc populates the callback in the array called877:c:expr:`kdb_poll_funcs[]`. The kdb_get_kbd_char() is the top-level878function which polls hardware for single character input.879 880kgdboc and kms881~~~~~~~~~~~~~~~~~~882 883The kgdboc driver contains logic to request the graphics display to884switch to a text context when you are using ``kgdboc=kms,kbd``, provided885that you have a video driver which has a frame buffer console and atomic886kernel mode setting support.887 888Every time the kernel debugger is entered it calls889kgdboc_pre_exp_handler() which in turn calls con_debug_enter()890in the virtual console layer. On resuming kernel execution, the kernel891debugger calls kgdboc_post_exp_handler() which in turn calls892con_debug_leave().893 894Any video driver that wants to be compatible with the kernel debugger895and the atomic kms callbacks must implement the ``mode_set_base_atomic``,896``fb_debug_enter`` and ``fb_debug_leave operations``. For the897``fb_debug_enter`` and ``fb_debug_leave`` the option exists to use the898generic drm fb helper functions or implement something custom for the899hardware. The following example shows the initialization of the900.mode_set_base_atomic operation in901drivers/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 the912fb_debug_enter and fb_debug_leave functions to use the generic drm913helpers 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 924Credits925=======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