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Based on kernel version 3.16. Page generated on 2014-08-06 21:38 EST.

1	<?xml version="1.0" encoding="UTF-8"?>
2	<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
3		"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
4	
5	<book id="kgdbOnLinux">
6	 <bookinfo>
7	  <title>Using kgdb, kdb and the kernel debugger internals</title>
8	
9	  <authorgroup>
10	   <author>
11	    <firstname>Jason</firstname>
12	    <surname>Wessel</surname>
13	    <affiliation>
14	     <address>
15	      <email>jason.wessel@windriver.com</email>
16	     </address>
17	    </affiliation>
18	   </author>
19	  </authorgroup>
20	  <copyright>
21	   <year>2008,2010</year>
22	   <holder>Wind River Systems, Inc.</holder>
23	  </copyright>
24	  <copyright>
25	   <year>2004-2005</year>
26	   <holder>MontaVista Software, Inc.</holder>
27	  </copyright>
28	  <copyright>
29	   <year>2004</year>
30	   <holder>Amit S. Kale</holder>
31	  </copyright>
32	
33	  <legalnotice>
34	   <para>
35	   This file is licensed under the terms of the GNU General Public License
36	   version 2. This program is licensed "as is" without any warranty of any
37	   kind, whether express or implied.
38	   </para>
39	
40	  </legalnotice>
41	 </bookinfo>
42	
43	<toc></toc>
44	  <chapter id="Introduction">
45	    <title>Introduction</title>
46	    <para>
47	    The kernel has two different debugger front ends (kdb and kgdb)
48	    which interface to the debug core.  It is possible to use either
49	    of the debugger front ends and dynamically transition between them
50	    if you configure the kernel properly at compile and runtime.
51	    </para>
52	    <para>
53	    Kdb is simplistic shell-style interface which you can use on a
54	    system console with a keyboard or serial console.  You can use it
55	    to inspect memory, registers, process lists, dmesg, and even set
56	    breakpoints to stop in a certain location.  Kdb is not a source
57	    level debugger, although you can set breakpoints and execute some
58	    basic kernel run control.  Kdb is mainly aimed at doing some
59	    analysis to aid in development or diagnosing kernel problems.  You
60	    can access some symbols by name in kernel built-ins or in kernel
61	    modules if the code was built
62	    with <symbol>CONFIG_KALLSYMS</symbol>.
63	    </para>
64	    <para>
65	    Kgdb is intended to be used as a source level debugger for the
66	    Linux kernel. It is used along with gdb to debug a Linux kernel.
67	    The expectation is that gdb can be used to "break in" to the
68	    kernel to inspect memory, variables and look through call stack
69	    information similar to the way an application developer would use
70	    gdb to debug an application.  It is possible to place breakpoints
71	    in kernel code and perform some limited execution stepping.
72	    </para>
73	    <para>
74	    Two machines are required for using kgdb. One of these machines is
75	    a development machine and the other is the target machine.  The
76	    kernel to be debugged runs on the target machine. The development
77	    machine runs an instance of gdb against the vmlinux file which
78	    contains the symbols (not boot image such as bzImage, zImage,
79	    uImage...).  In gdb the developer specifies the connection
80	    parameters and connects to kgdb.  The type of connection a
81	    developer makes with gdb depends on the availability of kgdb I/O
82	    modules compiled as built-ins or loadable kernel modules in the test
83	    machine's kernel.
84	    </para>
85	  </chapter>
86	  <chapter id="CompilingAKernel">
87	  <title>Compiling a kernel</title>
88	  <para>
89	  <itemizedlist>
90	  <listitem><para>In order to enable compilation of kdb, you must first enable kgdb.</para></listitem>
91	  <listitem><para>The kgdb test compile options are described in the kgdb test suite chapter.</para></listitem>
92	  </itemizedlist>
93	  </para>
94	  <sect1 id="CompileKGDB">
95	    <title>Kernel config options for kgdb</title>
96	    <para>
97	    To enable <symbol>CONFIG_KGDB</symbol> you should look under
98	    "Kernel debugging" and select "KGDB: kernel debugger".
99	    </para>
100	    <para>
101	    While it is not a hard requirement that you have symbols in your
102	    vmlinux file, gdb tends not to be very useful without the symbolic
103	    data, so you will want to turn
104	    on <symbol>CONFIG_DEBUG_INFO</symbol> which is called "Compile the
105	    kernel with debug info" in the config menu.
106	    </para>
107	    <para>
108	    It is advised, but not required that you turn on the
109	    <symbol>CONFIG_FRAME_POINTER</symbol> kernel option which is called "Compile the
110	    kernel with frame pointers" in the config menu.  This option
111	    inserts code to into the compiled executable which saves the frame
112	    information in registers or on the stack at different points which
113	    allows a debugger such as gdb to more accurately construct
114	    stack back traces while debugging the kernel.
115	    </para>
116	    <para>
117	    If the architecture that you are using supports the kernel option
118	    CONFIG_DEBUG_RODATA, you should consider turning it off.  This
119	    option will prevent the use of software breakpoints because it
120	    marks certain regions of the kernel's memory space as read-only.
121	    If kgdb supports it for the architecture you are using, you can
122	    use hardware breakpoints if you desire to run with the
123	    CONFIG_DEBUG_RODATA option turned on, else you need to turn off
124	    this option.
125	    </para>
126	    <para>
127	    Next you should choose one of more I/O drivers to interconnect
128	    debugging host and debugged target.  Early boot debugging requires
129	    a KGDB I/O driver that supports early debugging and the driver
130	    must be built into the kernel directly. Kgdb I/O driver
131	    configuration takes place via kernel or module parameters which
132	    you can learn more about in the in the section that describes the
133	    parameter "kgdboc".
134	    </para>
135	    <para>Here is an example set of .config symbols to enable or
136	    disable for kgdb:
137	    <itemizedlist>
138	    <listitem><para># CONFIG_DEBUG_RODATA is not set</para></listitem>
139	    <listitem><para>CONFIG_FRAME_POINTER=y</para></listitem>
140	    <listitem><para>CONFIG_KGDB=y</para></listitem>
141	    <listitem><para>CONFIG_KGDB_SERIAL_CONSOLE=y</para></listitem>
142	    </itemizedlist>
143	    </para>
144	  </sect1>
145	  <sect1 id="CompileKDB">
146	    <title>Kernel config options for kdb</title>
147	    <para>Kdb is quite a bit more complex than the simple gdbstub
148	    sitting on top of the kernel's debug core.  Kdb must implement a
149	    shell, and also adds some helper functions in other parts of the
150	    kernel, responsible for printing out interesting data such as what
151	    you would see if you ran "lsmod", or "ps".  In order to build kdb
152	    into the kernel you follow the same steps as you would for kgdb.
153	    </para>
154	    <para>The main config option for kdb
155	    is <symbol>CONFIG_KGDB_KDB</symbol> which is called "KGDB_KDB:
156	    include kdb frontend for kgdb" in the config menu.  In theory you
157	    would have already also selected an I/O driver such as the
158	    CONFIG_KGDB_SERIAL_CONSOLE interface if you plan on using kdb on a
159	    serial port, when you were configuring kgdb.
160	    </para>
161	    <para>If you want to use a PS/2-style keyboard with kdb, you would
162	    select CONFIG_KDB_KEYBOARD which is called "KGDB_KDB: keyboard as
163	    input device" in the config menu.  The CONFIG_KDB_KEYBOARD option
164	    is not used for anything in the gdb interface to kgdb.  The
165	    CONFIG_KDB_KEYBOARD option only works with kdb.
166	    </para>
167	    <para>Here is an example set of .config symbols to enable/disable kdb:
168	    <itemizedlist>
169	    <listitem><para># CONFIG_DEBUG_RODATA is not set</para></listitem>
170	    <listitem><para>CONFIG_FRAME_POINTER=y</para></listitem>
171	    <listitem><para>CONFIG_KGDB=y</para></listitem>
172	    <listitem><para>CONFIG_KGDB_SERIAL_CONSOLE=y</para></listitem>
173	    <listitem><para>CONFIG_KGDB_KDB=y</para></listitem>
174	    <listitem><para>CONFIG_KDB_KEYBOARD=y</para></listitem>
175	    </itemizedlist>
176	    </para>
177	  </sect1>
178	  </chapter>
179	  <chapter id="kgdbKernelArgs">
180	  <title>Kernel Debugger Boot Arguments</title>
181	  <para>This section describes the various runtime kernel
182	  parameters that affect the configuration of the kernel debugger.
183	  The following chapter covers using kdb and kgdb as well as
184	  provides some examples of the configuration parameters.</para>
185	   <sect1 id="kgdboc">
186	   <title>Kernel parameter: kgdboc</title>
187	   <para>The kgdboc driver was originally an abbreviation meant to
188	   stand for "kgdb over console".  Today it is the primary mechanism
189	   to configure how to communicate from gdb to kgdb as well as the
190	   devices you want to use to interact with the kdb shell.
191	   </para>
192	   <para>For kgdb/gdb, kgdboc is designed to work with a single serial
193	   port. It is intended to cover the circumstance where you want to
194	   use a serial console as your primary console as well as using it to
195	   perform kernel debugging.  It is also possible to use kgdb on a
196	   serial port which is not designated as a system console.  Kgdboc
197	   may be configured as a kernel built-in or a kernel loadable module.
198	   You can only make use of <constant>kgdbwait</constant> and early
199	   debugging if you build kgdboc into the kernel as a built-in.
200	   <para>Optionally you can elect to activate kms (Kernel Mode
201	   Setting) integration.  When you use kms with kgdboc and you have a
202	   video driver that has atomic mode setting hooks, it is possible to
203	   enter the debugger on the graphics console.  When the kernel
204	   execution is resumed, the previous graphics mode will be restored.
205	   This integration can serve as a useful tool to aid in diagnosing
206	   crashes or doing analysis of memory with kdb while allowing the
207	   full graphics console applications to run.
208	   </para>
209	   </para>
210	   <sect2 id="kgdbocArgs">
211	   <title>kgdboc arguments</title>
212	   <para>Usage: <constant>kgdboc=[kms][[,]kbd][[,]serial_device][,baud]</constant></para>
213	   <para>The order listed above must be observed if you use any of the
214	   optional configurations together.
215	   </para>
216	   <para>Abbreviations:
217	   <itemizedlist>
218	   <listitem><para>kms = Kernel Mode Setting</para></listitem>
219	   <listitem><para>kbd = Keyboard</para></listitem>
220	   </itemizedlist>
221	   </para>
222	   <para>You can configure kgdboc to use the keyboard, and or a serial
223	   device depending on if you are using kdb and or kgdb, in one of the
224	   following scenarios.  The order listed above must be observed if
225	   you use any of the optional configurations together.  Using kms +
226	   only gdb is generally not a useful combination.</para>
227	   <sect3 id="kgdbocArgs1">
228	   <title>Using loadable module or built-in</title>
229	   <para>
230	   <orderedlist>
231	   <listitem><para>As a kernel built-in:</para>
232	   <para>Use the kernel boot argument: <constant>kgdboc=&lt;tty-device&gt;,[baud]</constant></para></listitem>
233	   <listitem>
234	   <para>As a kernel loadable module:</para>
235	   <para>Use the command: <constant>modprobe kgdboc kgdboc=&lt;tty-device&gt;,[baud]</constant></para>
236	   <para>Here are two examples of how you might format the kgdboc
237	   string. The first is for an x86 target using the first serial port.
238	   The second example is for the ARM Versatile AB using the second
239	   serial port.
240	   <orderedlist>
241	   <listitem><para><constant>kgdboc=ttyS0,115200</constant></para></listitem>
242	   <listitem><para><constant>kgdboc=ttyAMA1,115200</constant></para></listitem>
243	   </orderedlist>
244	   </para>
245	   </listitem>
246	   </orderedlist></para>
247	   </sect3>
248	   <sect3 id="kgdbocArgs2">
249	   <title>Configure kgdboc at runtime with sysfs</title>
250	   <para>At run time you can enable or disable kgdboc by echoing a
251	   parameters into the sysfs.  Here are two examples:</para>
252	   <orderedlist>
253	   <listitem><para>Enable kgdboc on ttyS0</para>
254	   <para><constant>echo ttyS0 &gt; /sys/module/kgdboc/parameters/kgdboc</constant></para></listitem>
255	   <listitem><para>Disable kgdboc</para>
256	   <para><constant>echo "" &gt; /sys/module/kgdboc/parameters/kgdboc</constant></para></listitem>
257	   </orderedlist>
258	   <para>NOTE: You do not need to specify the baud if you are
259	   configuring the console on tty which is already configured or
260	   open.</para>
261	   </sect3>
262	   <sect3 id="kgdbocArgs3">
263	   <title>More examples</title>
264	   <para>You can configure kgdboc to use the keyboard, and or a serial
265	   device depending on if you are using kdb and or kgdb, in one of the
266	   following scenarios.</para>
267	   <para>You can configure kgdboc to use the keyboard, and or a serial device
268	   depending on if you are using kdb and or kgdb, in one of the
269	   following scenarios.
270	   <orderedlist>
271	   <listitem><para>kdb and kgdb over only a serial port</para>
272	   <para><constant>kgdboc=&lt;serial_device&gt;[,baud]</constant></para>
273	   <para>Example: <constant>kgdboc=ttyS0,115200</constant></para>
274	   </listitem>
275	   <listitem><para>kdb and kgdb with keyboard and a serial port</para>
276	   <para><constant>kgdboc=kbd,&lt;serial_device&gt;[,baud]</constant></para>
277	   <para>Example: <constant>kgdboc=kbd,ttyS0,115200</constant></para>
278	   </listitem>
279	   <listitem><para>kdb with a keyboard</para>
280	   <para><constant>kgdboc=kbd</constant></para>
281	   </listitem>
282	   <listitem><para>kdb with kernel mode setting</para>
283	   <para><constant>kgdboc=kms,kbd</constant></para>
284	   </listitem>
285	   <listitem><para>kdb with kernel mode setting and kgdb over a serial port</para>
286	   <para><constant>kgdboc=kms,kbd,ttyS0,115200</constant></para>
287	   </listitem>
288	   </orderedlist>
289	   </para>
290	   </sect3>
291	   <para>NOTE: Kgdboc does not support interrupting the target via the
292	   gdb remote protocol.  You must manually send a sysrq-g unless you
293	   have a proxy that splits console output to a terminal program.
294	   A console proxy has a separate TCP port for the debugger and a separate
295	   TCP port for the "human" console.  The proxy can take care of sending
296	   the sysrq-g for you.
297	   </para>
298	   <para>When using kgdboc with no debugger proxy, you can end up
299	    connecting the debugger at one of two entry points.  If an
300	    exception occurs after you have loaded kgdboc, a message should
301	    print on the console stating it is waiting for the debugger.  In
302	    this case you disconnect your terminal program and then connect the
303	    debugger in its place.  If you want to interrupt the target system
304	    and forcibly enter a debug session you have to issue a Sysrq
305	    sequence and then type the letter <constant>g</constant>.  Then
306	    you disconnect the terminal session and connect gdb.  Your options
307	    if you don't like this are to hack gdb to send the sysrq-g for you
308	    as well as on the initial connect, or to use a debugger proxy that
309	    allows an unmodified gdb to do the debugging.
310	   </para>
311	   </sect2>
312	   </sect1>
313	   <sect1 id="kgdbwait">
314	   <title>Kernel parameter: kgdbwait</title>
315	   <para>
316	   The Kernel command line option <constant>kgdbwait</constant> makes
317	   kgdb wait for a debugger connection during booting of a kernel.  You
318	   can only use this option you compiled a kgdb I/O driver into the
319	   kernel and you specified the I/O driver configuration as a kernel
320	   command line option.  The kgdbwait parameter should always follow the
321	   configuration parameter for the kgdb I/O driver in the kernel
322	   command line else the I/O driver will not be configured prior to
323	   asking the kernel to use it to wait.
324	   </para>
325	   <para>
326	   The kernel will stop and wait as early as the I/O driver and
327	   architecture allows when you use this option.  If you build the
328	   kgdb I/O driver as a loadable kernel module kgdbwait will not do
329	   anything.
330	   </para>
331	   </sect1>
332	   <sect1 id="kgdbcon">
333	   <title>Kernel parameter: kgdbcon</title>
334	   <para> The kgdbcon feature allows you to see printk() messages
335	   inside gdb while gdb is connected to the kernel.  Kdb does not make
336	    use of the kgdbcon feature.
337	   </para>
338	   <para>Kgdb supports using the gdb serial protocol to send console
339	   messages to the debugger when the debugger is connected and running.
340	   There are two ways to activate this feature.
341	   <orderedlist>
342	   <listitem><para>Activate with the kernel command line option:</para>
343	   <para><constant>kgdbcon</constant></para>
344	   </listitem>
345	   <listitem><para>Use sysfs before configuring an I/O driver</para>
346	   <para>
347	   <constant>echo 1 &gt; /sys/module/kgdb/parameters/kgdb_use_con</constant>
348	   </para>
349	   <para>
350	   NOTE: If you do this after you configure the kgdb I/O driver, the
351	   setting will not take effect until the next point the I/O is
352	   reconfigured.
353	   </para>
354	   </listitem>
355	   </orderedlist>
356	   <para>IMPORTANT NOTE: You cannot use kgdboc + kgdbcon on a tty that is an
357	   active system console.  An example incorrect usage is <constant>console=ttyS0,115200 kgdboc=ttyS0 kgdbcon</constant>
358	   </para>
359	   <para>It is possible to use this option with kgdboc on a tty that is not a system console.
360	   </para>
361	  </para>
362	  </sect1>
363	   <sect1 id="kgdbreboot">
364	   <title>Run time parameter: kgdbreboot</title>
365	   <para> The kgdbreboot feature allows you to change how the debugger
366	   deals with the reboot notification.  You have 3 choices for the
367	   behavior.  The default behavior is always set to 0.</para>
368	   <orderedlist>
369	   <listitem><para>echo -1 > /sys/module/debug_core/parameters/kgdbreboot</para>
370	   <para>Ignore the reboot notification entirely.</para>
371	   </listitem>
372	   <listitem><para>echo 0 > /sys/module/debug_core/parameters/kgdbreboot</para>
373	   <para>Send the detach message to any attached debugger client.</para>
374	   </listitem>
375	   <listitem><para>echo 1 > /sys/module/debug_core/parameters/kgdbreboot</para>
376	   <para>Enter the debugger on reboot notify.</para>
377	   </listitem>
378	   </orderedlist>
379	  </sect1>
380	  </chapter>
381	  <chapter id="usingKDB">
382	  <title>Using kdb</title>
383	  <para>
384	  </para>
385	  <sect1 id="quickKDBserial">
386	  <title>Quick start for kdb on a serial port</title>
387	  <para>This is a quick example of how to use kdb.</para>
388	  <para><orderedlist>
389	  <listitem><para>Boot kernel with arguments:
390	  <itemizedlist>
391	  <listitem><para><constant>console=ttyS0,115200 kgdboc=ttyS0,115200</constant></para></listitem>
392	  </itemizedlist></para>
393	  <para>OR</para>
394	  <para>Configure kgdboc after the kernel booted; assuming you are using a serial port console:
395	  <itemizedlist>
396	  <listitem><para><constant>echo ttyS0 &gt; /sys/module/kgdboc/parameters/kgdboc</constant></para></listitem>
397	  </itemizedlist>
398	  </para>
399	  </listitem>
400	  <listitem><para>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.</para>
401	  <itemizedlist>
402	  <listitem><para>When logged in as root or with a super user session you can run:</para>
403	   <para><constant>echo g &gt; /proc/sysrq-trigger</constant></para></listitem>
404	  <listitem><para>Example using minicom 2.2</para>
405	  <para>Press: <constant>Control-a</constant></para>
406	  <para>Press: <constant>f</constant></para>
407	  <para>Press: <constant>g</constant></para>
408	  </listitem>
409	  <listitem><para>When you have telneted to a terminal server that supports sending a remote break</para>
410	  <para>Press: <constant>Control-]</constant></para>
411	  <para>Type in:<constant>send break</constant></para>
412	  <para>Press: <constant>Enter</constant></para>
413	  <para>Press: <constant>g</constant></para>
414	  </listitem>
415	  </itemizedlist>
416	  </listitem>
417	  <listitem><para>From the kdb prompt you can run the "help" command to see a complete list of the commands that are available.</para>
418	  <para>Some useful commands in kdb include:
419	  <itemizedlist>
420	  <listitem><para>lsmod  -- Shows where kernel modules are loaded</para></listitem>
421	  <listitem><para>ps -- Displays only the active processes</para></listitem>
422	  <listitem><para>ps A -- Shows all the processes</para></listitem>
423	  <listitem><para>summary -- Shows kernel version info and memory usage</para></listitem>
424	  <listitem><para>bt -- Get a backtrace of the current process using dump_stack()</para></listitem>
425	  <listitem><para>dmesg -- View the kernel syslog buffer</para></listitem>
426	  <listitem><para>go -- Continue the system</para></listitem>
427	  </itemizedlist>
428	  </para>
429	  </listitem>
430	  <listitem>
431	  <para>When you are done using kdb you need to consider rebooting the
432	  system or using the "go" command to resuming normal kernel
433	  execution.  If you have paused the kernel for a lengthy period of
434	  time, applications that rely on timely networking or anything to do
435	  with real wall clock time could be adversely affected, so you
436	  should take this into consideration when using the kernel
437	  debugger.</para>
438	  </listitem>
439	  </orderedlist></para>
440	  </sect1>
441	  <sect1 id="quickKDBkeyboard">
442	  <title>Quick start for kdb using a keyboard connected console</title>
443	  <para>This is a quick example of how to use kdb with a keyboard.</para>
444	  <para><orderedlist>
445	  <listitem><para>Boot kernel with arguments:
446	  <itemizedlist>
447	  <listitem><para><constant>kgdboc=kbd</constant></para></listitem>
448	  </itemizedlist></para>
449	  <para>OR</para>
450	  <para>Configure kgdboc after the kernel booted:
451	  <itemizedlist>
452	  <listitem><para><constant>echo kbd &gt; /sys/module/kgdboc/parameters/kgdboc</constant></para></listitem>
453	  </itemizedlist>
454	  </para>
455	  </listitem>
456	  <listitem><para>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.</para>
457	  <itemizedlist>
458	  <listitem><para>When logged in as root or with a super user session you can run:</para>
459	   <para><constant>echo g &gt; /proc/sysrq-trigger</constant></para></listitem>
460	  <listitem><para>Example using a laptop keyboard</para>
461	  <para>Press and hold down: <constant>Alt</constant></para>
462	  <para>Press and hold down: <constant>Fn</constant></para>
463	  <para>Press and release the key with the label: <constant>SysRq</constant></para>
464	  <para>Release: <constant>Fn</constant></para>
465	  <para>Press and release: <constant>g</constant></para>
466	  <para>Release: <constant>Alt</constant></para>
467	  </listitem>
468	  <listitem><para>Example using a PS/2 101-key keyboard</para>
469	  <para>Press and hold down: <constant>Alt</constant></para>
470	  <para>Press and release the key with the label: <constant>SysRq</constant></para>
471	  <para>Press and release: <constant>g</constant></para>
472	  <para>Release: <constant>Alt</constant></para>
473	  </listitem>
474	  </itemizedlist>
475	  </listitem>
476	  <listitem>
477	  <para>Now type in a kdb command such as "help", "dmesg", "bt" or "go" to continue kernel execution.</para>
478	  </listitem>
479	  </orderedlist></para>
480	  </sect1>
481	  </chapter>
482	  <chapter id="EnableKGDB">
483	   <title>Using kgdb / gdb</title>
484	   <para>In order to use kgdb you must activate it by passing
485	   configuration information to one of the kgdb I/O drivers.  If you
486	   do not pass any configuration information kgdb will not do anything
487	   at all.  Kgdb will only actively hook up to the kernel trap hooks
488	   if a kgdb I/O driver is loaded and configured.  If you unconfigure
489	   a kgdb I/O driver, kgdb will unregister all the kernel hook points.
490	   </para>
491	   <para> All kgdb I/O drivers can be reconfigured at run time, if
492	   <symbol>CONFIG_SYSFS</symbol> and <symbol>CONFIG_MODULES</symbol>
493	   are enabled, by echo'ing a new config string to
494	   <constant>/sys/module/&lt;driver&gt;/parameter/&lt;option&gt;</constant>.
495	   The driver can be unconfigured by passing an empty string.  You cannot
496	   change the configuration while the debugger is attached.  Make sure
497	   to detach the debugger with the <constant>detach</constant> command
498	   prior to trying to unconfigure a kgdb I/O driver.
499	   </para>
500	  <sect1 id="ConnectingGDB">
501	  <title>Connecting with gdb to a serial port</title>
502	  <orderedlist>
503	  <listitem><para>Configure kgdboc</para>
504	   <para>Boot kernel with arguments:
505	   <itemizedlist>
506	    <listitem><para><constant>kgdboc=ttyS0,115200</constant></para></listitem>
507	   </itemizedlist></para>
508	   <para>OR</para>
509	   <para>Configure kgdboc after the kernel booted:
510	   <itemizedlist>
511	    <listitem><para><constant>echo ttyS0 &gt; /sys/module/kgdboc/parameters/kgdboc</constant></para></listitem>
512	   </itemizedlist></para>
513	  </listitem>
514	  <listitem>
515	  <para>Stop kernel execution (break into the debugger)</para>
516	  <para>In order to connect to gdb via kgdboc, the kernel must
517	  first be stopped.  There are several ways to stop the kernel which
518	  include using kgdbwait as a boot argument, via a sysrq-g, or running
519	  the kernel until it takes an exception where it waits for the
520	  debugger to attach.
521	  <itemizedlist>
522	  <listitem><para>When logged in as root or with a super user session you can run:</para>
523	   <para><constant>echo g &gt; /proc/sysrq-trigger</constant></para></listitem>
524	  <listitem><para>Example using minicom 2.2</para>
525	  <para>Press: <constant>Control-a</constant></para>
526	  <para>Press: <constant>f</constant></para>
527	  <para>Press: <constant>g</constant></para>
528	  </listitem>
529	  <listitem><para>When you have telneted to a terminal server that supports sending a remote break</para>
530	  <para>Press: <constant>Control-]</constant></para>
531	  <para>Type in:<constant>send break</constant></para>
532	  <para>Press: <constant>Enter</constant></para>
533	  <para>Press: <constant>g</constant></para>
534	  </listitem>
535	  </itemizedlist>
536	  </para>
537	  </listitem>
538	  <listitem>
539	    <para>Connect from from gdb</para>
540	    <para>
541	    Example (using a directly connected port):
542	    </para>
543	    <programlisting>
544	    % gdb ./vmlinux
545	    (gdb) set remotebaud 115200
546	    (gdb) target remote /dev/ttyS0
547	    </programlisting>
548	    <para>
549	    Example (kgdb to a terminal server on TCP port 2012):
550	    </para>
551	    <programlisting>
552	    % gdb ./vmlinux
553	    (gdb) target remote 192.168.2.2:2012
554	    </programlisting>
555	    <para>
556	    Once connected, you can debug a kernel the way you would debug an
557	    application program.
558	    </para>
559	    <para>
560	    If you are having problems connecting or something is going
561	    seriously wrong while debugging, it will most often be the case
562	    that you want to enable gdb to be verbose about its target
563	    communications.  You do this prior to issuing the <constant>target
564	    remote</constant> command by typing in: <constant>set debug remote 1</constant>
565	    </para>
566	  </listitem>
567	  </orderedlist>
568	  <para>Remember if you continue in gdb, and need to "break in" again,
569	  you need to issue an other sysrq-g.  It is easy to create a simple
570	  entry point by putting a breakpoint at <constant>sys_sync</constant>
571	  and then you can run "sync" from a shell or script to break into the
572	  debugger.</para>
573	  </sect1>
574	  </chapter>
575	  <chapter id="switchKdbKgdb">
576	  <title>kgdb and kdb interoperability</title>
577	  <para>It is possible to transition between kdb and kgdb dynamically.
578	  The debug core will remember which you used the last time and
579	  automatically start in the same mode.</para>
580	  <sect1>
581	  <title>Switching between kdb and kgdb</title>
582	  <sect2>
583	  <title>Switching from kgdb to kdb</title>
584	  <para>
585	  There are two ways to switch from kgdb to kdb: you can use gdb to
586	  issue a maintenance packet, or you can blindly type the command $3#33.
587	  Whenever kernel debugger stops in kgdb mode it will print the
588	  message <constant>KGDB or $3#33 for KDB</constant>.  It is important
589	  to note that you have to type the sequence correctly in one pass.
590	  You cannot type a backspace or delete because kgdb will interpret
591	  that as part of the debug stream.
592	  <orderedlist>
593	  <listitem><para>Change from kgdb to kdb by blindly typing:</para>
594	  <para><constant>$3#33</constant></para></listitem>
595	  <listitem><para>Change from kgdb to kdb with gdb</para>
596	  <para><constant>maintenance packet 3</constant></para>
597	  <para>NOTE: Now you must kill gdb. Typically you press control-z and
598	  issue the command: kill -9 %</para></listitem>
599	  </orderedlist>
600	  </para>
601	  </sect2>
602	  <sect2>
603	  <title>Change from kdb to kgdb</title>
604	  <para>There are two ways you can change from kdb to kgdb.  You can
605	  manually enter kgdb mode by issuing the kgdb command from the kdb
606	  shell prompt, or you can connect gdb while the kdb shell prompt is
607	  active.  The kdb shell looks for the typical first commands that gdb
608	  would issue with the gdb remote protocol and if it sees one of those
609	  commands it automatically changes into kgdb mode.</para>
610	  <orderedlist>
611	  <listitem><para>From kdb issue the command:</para>
612	  <para><constant>kgdb</constant></para>
613	  <para>Now disconnect your terminal program and connect gdb in its place</para></listitem>
614	  <listitem><para>At the kdb prompt, disconnect the terminal program and connect gdb in its place.</para></listitem>
615	  </orderedlist>
616	  </sect2>
617	  </sect1>
618	  <sect1>
619	  <title>Running kdb commands from gdb</title>
620	  <para>It is possible to run a limited set of kdb commands from gdb,
621	  using the gdb monitor command.  You don't want to execute any of the
622	  run control or breakpoint operations, because it can disrupt the
623	  state of the kernel debugger.  You should be using gdb for
624	  breakpoints and run control operations if you have gdb connected.
625	  The more useful commands to run are things like lsmod, dmesg, ps or
626	  possibly some of the memory information commands.  To see all the kdb
627	  commands you can run <constant>monitor help</constant>.</para>
628	  <para>Example:
629	  <informalexample><programlisting>
630	(gdb) monitor ps
631	1 idle process (state I) and
632	27 sleeping system daemon (state M) processes suppressed,
633	use 'ps A' to see all.
634	Task Addr       Pid   Parent [*] cpu State Thread     Command
635	
636	0xc78291d0        1        0  0    0   S  0xc7829404  init
637	0xc7954150      942        1  0    0   S  0xc7954384  dropbear
638	0xc78789c0      944        1  0    0   S  0xc7878bf4  sh
639	(gdb)
640	  </programlisting></informalexample>
641	  </para>
642	  </sect1>
643	  </chapter>
644	  <chapter id="KGDBTestSuite">
645	    <title>kgdb Test Suite</title>
646	    <para>
647	    When kgdb is enabled in the kernel config you can also elect to
648	    enable the config parameter KGDB_TESTS.  Turning this on will
649	    enable a special kgdb I/O module which is designed to test the
650	    kgdb internal functions.
651	    </para>
652	    <para>
653	    The kgdb tests are mainly intended for developers to test the kgdb
654	    internals as well as a tool for developing a new kgdb architecture
655	    specific implementation.  These tests are not really for end users
656	    of the Linux kernel.  The primary source of documentation would be
657	    to look in the drivers/misc/kgdbts.c file.
658	    </para>
659	    <para>
660	    The kgdb test suite can also be configured at compile time to run
661	    the core set of tests by setting the kernel config parameter
662	    KGDB_TESTS_ON_BOOT.  This particular option is aimed at automated
663	    regression testing and does not require modifying the kernel boot
664	    config arguments.  If this is turned on, the kgdb test suite can
665	    be disabled by specifying "kgdbts=" as a kernel boot argument.
666	    </para>
667	  </chapter>
668	  <chapter id="CommonBackEndReq">
669	  <title>Kernel Debugger Internals</title>
670	  <sect1 id="kgdbArchitecture">
671	    <title>Architecture Specifics</title>
672	      <para>
673	      The kernel debugger is organized into a number of components:
674	      <orderedlist>
675	      <listitem><para>The debug core</para>
676	      <para>
677	      The debug core is found in kernel/debugger/debug_core.c.  It contains:
678	      <itemizedlist>
679	      <listitem><para>A generic OS exception handler which includes
680	      sync'ing the processors into a stopped state on an multi-CPU
681	      system.</para></listitem>
682	      <listitem><para>The API to talk to the kgdb I/O drivers</para></listitem>
683	      <listitem><para>The API to make calls to the arch-specific kgdb implementation</para></listitem>
684	      <listitem><para>The logic to perform safe memory reads and writes to memory while using the debugger</para></listitem>
685	      <listitem><para>A full implementation for software breakpoints unless overridden by the arch</para></listitem>
686	      <listitem><para>The API to invoke either the kdb or kgdb frontend to the debug core.</para></listitem>
687	      <listitem><para>The structures and callback API for atomic kernel mode setting.</para>
688	      <para>NOTE: kgdboc is where the kms callbacks are invoked.</para></listitem>
689	      </itemizedlist>
690	      </para>
691	      </listitem>
692	      <listitem><para>kgdb arch-specific implementation</para>
693	      <para>
694	      This implementation is generally found in arch/*/kernel/kgdb.c.
695	      As an example, arch/x86/kernel/kgdb.c contains the specifics to
696	      implement HW breakpoint as well as the initialization to
697	      dynamically register and unregister for the trap handlers on
698	      this architecture.  The arch-specific portion implements:
699	      <itemizedlist>
700	      <listitem><para>contains an arch-specific trap catcher which
701	      invokes kgdb_handle_exception() to start kgdb about doing its
702	      work</para></listitem>
703	      <listitem><para>translation to and from gdb specific packet format to pt_regs</para></listitem>
704	      <listitem><para>Registration and unregistration of architecture specific trap hooks</para></listitem>
705	      <listitem><para>Any special exception handling and cleanup</para></listitem>
706	      <listitem><para>NMI exception handling and cleanup</para></listitem>
707	      <listitem><para>(optional)HW breakpoints</para></listitem>
708	      </itemizedlist>
709	      </para>
710	      </listitem>
711	      <listitem><para>gdbstub frontend (aka kgdb)</para>
712	      <para>The gdbstub is located in kernel/debug/gdbstub.c. It contains:</para>
713	      <itemizedlist>
714	        <listitem><para>All the logic to implement the gdb serial protocol</para></listitem>
715	      </itemizedlist>
716	      </listitem>
717	      <listitem><para>kdb frontend</para>
718	      <para>The kdb debugger shell is broken down into a number of
719	      components.  The kdb core is located in kernel/debug/kdb.  There
720	      are a number of helper functions in some of the other kernel
721	      components to make it possible for kdb to examine and report
722	      information about the kernel without taking locks that could
723	      cause a kernel deadlock.  The kdb core contains implements the following functionality.</para>
724	      <itemizedlist>
725	        <listitem><para>A simple shell</para></listitem>
726	        <listitem><para>The kdb core command set</para></listitem>
727	        <listitem><para>A registration API to register additional kdb shell commands.</para>
728		<itemizedlist>
729	        <listitem><para>A good example of a self-contained kdb module
730	        is the "ftdump" command for dumping the ftrace buffer.  See:
731	        kernel/trace/trace_kdb.c</para></listitem>
732	        <listitem><para>For an example of how to dynamically register
733	        a new kdb command you can build the kdb_hello.ko kernel module
734	        from samples/kdb/kdb_hello.c.  To build this example you can
735	        set CONFIG_SAMPLES=y and CONFIG_SAMPLE_KDB=m in your kernel
736	        config.  Later run "modprobe kdb_hello" and the next time you
737	        enter the kdb shell, you can run the "hello"
738	        command.</para></listitem>
739		</itemizedlist></listitem>
740	        <listitem><para>The implementation for kdb_printf() which
741	        emits messages directly to I/O drivers, bypassing the kernel
742	        log.</para></listitem>
743	        <listitem><para>SW / HW breakpoint management for the kdb shell</para></listitem>
744	      </itemizedlist>
745	      </listitem>
746	      <listitem><para>kgdb I/O driver</para>
747	      <para>
748	      Each kgdb I/O driver has to provide an implementation for the following:
749	      <itemizedlist>
750	      <listitem><para>configuration via built-in or module</para></listitem>
751	      <listitem><para>dynamic configuration and kgdb hook registration calls</para></listitem>
752	      <listitem><para>read and write character interface</para></listitem>
753	      <listitem><para>A cleanup handler for unconfiguring from the kgdb core</para></listitem>
754	      <listitem><para>(optional) Early debug methodology</para></listitem>
755	      </itemizedlist>
756	      Any given kgdb I/O driver has to operate very closely with the
757	      hardware and must do it in such a way that does not enable
758	      interrupts or change other parts of the system context without
759	      completely restoring them. The kgdb core will repeatedly "poll"
760	      a kgdb I/O driver for characters when it needs input.  The I/O
761	      driver is expected to return immediately if there is no data
762	      available.  Doing so allows for the future possibility to touch
763	      watch dog hardware in such a way as to have a target system not
764	      reset when these are enabled.
765	      </para>
766	      </listitem>
767	      </orderedlist>
768	      </para>
769	      <para>
770	      If you are intent on adding kgdb architecture specific support
771	      for a new architecture, the architecture should define
772	      <constant>HAVE_ARCH_KGDB</constant> in the architecture specific
773	      Kconfig file.  This will enable kgdb for the architecture, and
774	      at that point you must create an architecture specific kgdb
775	      implementation.
776	      </para>
777	      <para>
778	      There are a few flags which must be set on every architecture in
779	      their &lt;asm/kgdb.h&gt; file.  These are:
780	      <itemizedlist>
781	        <listitem>
782		  <para>
783		  NUMREGBYTES: The size in bytes of all of the registers, so
784		  that we can ensure they will all fit into a packet.
785		  </para>
786		  <para>
787		  BUFMAX: The size in bytes of the buffer GDB will read into.
788		  This must be larger than NUMREGBYTES.
789		  </para>
790		  <para>
791		  CACHE_FLUSH_IS_SAFE: Set to 1 if it is always safe to call
792		  flush_cache_range or flush_icache_range.  On some architectures,
793		  these functions may not be safe to call on SMP since we keep other
794		  CPUs in a holding pattern.
795		  </para>
796		</listitem>
797	      </itemizedlist>
798	      </para>
799	      <para>
800	      There are also the following functions for the common backend,
801	      found in kernel/kgdb.c, that must be supplied by the
802	      architecture-specific backend unless marked as (optional), in
803	      which case a default function maybe used if the architecture
804	      does not need to provide a specific implementation.
805	      </para>
806	!Iinclude/linux/kgdb.h
807	  </sect1>
808	  <sect1 id="kgdbocDesign">
809	  <title>kgdboc internals</title>
810	  <sect2>
811	  <title>kgdboc and uarts</title>
812	  <para>
813	  The kgdboc driver is actually a very thin driver that relies on the
814	  underlying low level to the hardware driver having "polling hooks"
815	  which the to which the tty driver is attached.  In the initial
816	  implementation of kgdboc it the serial_core was changed to expose a
817	  low level UART hook for doing polled mode reading and writing of a
818	  single character while in an atomic context.  When kgdb makes an I/O
819	  request to the debugger, kgdboc invokes a callback in the serial
820	  core which in turn uses the callback in the UART driver.</para>
821	  <para>
822	  When using kgdboc with a UART, the UART driver must implement two callbacks in the <constant>struct uart_ops</constant>. Example from drivers/8250.c:<programlisting>
823	#ifdef CONFIG_CONSOLE_POLL
824		.poll_get_char = serial8250_get_poll_char,
825		.poll_put_char = serial8250_put_poll_char,
826	#endif
827	  </programlisting>
828	  Any implementation specifics around creating a polling driver use the
829	  <constant>#ifdef CONFIG_CONSOLE_POLL</constant>, as shown above.
830	  Keep in mind that polling hooks have to be implemented in such a way
831	  that they can be called from an atomic context and have to restore
832	  the state of the UART chip on return such that the system can return
833	  to normal when the debugger detaches.  You need to be very careful
834	  with any kind of lock you consider, because failing here is most likely
835	  going to mean pressing the reset button.
836	  </para>
837	  </sect2>
838	  <sect2 id="kgdbocKbd">
839	  <title>kgdboc and keyboards</title>
840	  <para>The kgdboc driver contains logic to configure communications
841	  with an attached keyboard.  The keyboard infrastructure is only
842	  compiled into the kernel when CONFIG_KDB_KEYBOARD=y is set in the
843	  kernel configuration.</para>
844	  <para>The core polled keyboard driver driver for PS/2 type keyboards
845	  is in drivers/char/kdb_keyboard.c.  This driver is hooked into the
846	  debug core when kgdboc populates the callback in the array
847	  called <constant>kdb_poll_funcs[]</constant>.  The
848	  kdb_get_kbd_char() is the top-level function which polls hardware
849	  for single character input.
850	  </para>
851	  </sect2>
852	  <sect2 id="kgdbocKms">
853	  <title>kgdboc and kms</title>
854	  <para>The kgdboc driver contains logic to request the graphics
855	  display to switch to a text context when you are using
856	  "kgdboc=kms,kbd", provided that you have a video driver which has a
857	  frame buffer console and atomic kernel mode setting support.</para>
858	  <para>
859	  Every time the kernel
860	  debugger is entered it calls kgdboc_pre_exp_handler() which in turn
861	  calls con_debug_enter() in the virtual console layer.  On resuming kernel
862	  execution, the kernel debugger calls kgdboc_post_exp_handler() which
863	  in turn calls con_debug_leave().</para>
864	  <para>Any video driver that wants to be compatible with the kernel
865	  debugger and the atomic kms callbacks must implement the
866	  mode_set_base_atomic, fb_debug_enter and fb_debug_leave operations.
867	  For the fb_debug_enter and fb_debug_leave the option exists to use
868	  the generic drm fb helper functions or implement something custom for
869	  the hardware.  The following example shows the initialization of the
870	  .mode_set_base_atomic operation in
871	  drivers/gpu/drm/i915/intel_display.c:
872	  <informalexample>
873	  <programlisting>
874	static const struct drm_crtc_helper_funcs intel_helper_funcs = {
875	[...]
876	        .mode_set_base_atomic = intel_pipe_set_base_atomic,
877	[...]
878	};
879	  </programlisting>
880	  </informalexample>
881	  </para>
882	  <para>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
883	  drivers/gpu/drm/i915/intel_fb.c:
884	  <informalexample>
885	  <programlisting>
886	static struct fb_ops intelfb_ops = {
887	[...]
888	       .fb_debug_enter = drm_fb_helper_debug_enter,
889	       .fb_debug_leave = drm_fb_helper_debug_leave,
890	[...]
891	};
892	  </programlisting>
893	  </informalexample>
894	  </para>
895	  </sect2>
896	  </sect1>
897	  </chapter>
898	  <chapter id="credits">
899	     <title>Credits</title>
900		<para>
901			The following people have contributed to this document:
902			<orderedlist>
903				<listitem><para>Amit Kale<email>amitkale@linsyssoft.com</email></para></listitem>
904				<listitem><para>Tom Rini<email>trini@kernel.crashing.org</email></para></listitem>
905			</orderedlist>
906	                In March 2008 this document was completely rewritten by:
907			<itemizedlist>
908			<listitem><para>Jason Wessel<email>jason.wessel@windriver.com</email></para></listitem>
909			</itemizedlist>
910	                In Jan 2010 this document was updated to include kdb.
911			<itemizedlist>
912			<listitem><para>Jason Wessel<email>jason.wessel@windriver.com</email></para></listitem>
913			</itemizedlist>
914		</para>
915	  </chapter>
916	</book>
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