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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 and the kgdb 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
21 <authorgroup>
22 <author>
23 <firstname>Tom</firstname>
24 <surname>Rini</surname>
25 <affiliation>
26 <address>
27 <email>trini@kernel.crashing.org</email>
28 </address>
29 </affiliation>
30 </author>
31 </authorgroup>
32
33 <authorgroup>
34 <author>
35 <firstname>Amit S.</firstname>
36 <surname>Kale</surname>
37 <affiliation>
38 <address>
39 <email>amitkale@linsyssoft.com</email>
40 </address>
41 </affiliation>
42 </author>
43 </authorgroup>
44
45 <copyright>
46 <year>2008</year>
47 <holder>Wind River Systems, Inc.</holder>
48 </copyright>
49 <copyright>
50 <year>2004-2005</year>
51 <holder>MontaVista Software, Inc.</holder>
52 </copyright>
53 <copyright>
54 <year>2004</year>
55 <holder>Amit S. Kale</holder>
56 </copyright>
57
58 <legalnotice>
59 <para>
60 This file is licensed under the terms of the GNU General Public License
61 version 2. This program is licensed "as is" without any warranty of any
62 kind, whether express or implied.
63 </para>
64
65 </legalnotice>
66 </bookinfo>
67
68<toc></toc>
69 <chapter id="Introduction">
70 <title>Introduction</title>
71 <para>
72 kgdb is a source level debugger for linux kernel. It is used along
73 with gdb to debug a linux kernel. The expectation is that gdb can
74 be used to "break in" to the kernel to inspect memory, variables
75 and look through a cal stack information similar to what an
76 application developer would use gdb for. It is possible to place
77 breakpoints in kernel code and perform some limited execution
78 stepping.
79 </para>
80 <para>
81 Two machines are required for using kgdb. One of these machines is a
82 development machine and the other is a test machine. The kernel
83 to be debugged runs on the test machine. The development machine
84 runs an instance of gdb against the vmlinux file which contains
85 the symbols (not boot image such as bzImage, zImage, uImage...).
86 In gdb the developer specifies the connection parameters and
87 connects to kgdb. Depending on which kgdb I/O modules exist in
88 the kernel for a given architecture, it may be possible to debug
89 the test machine's kernel with the development machine using a
90 rs232 or ethernet connection.
91 </para>
92 </chapter>
93 <chapter id="CompilingAKernel">
94 <title>Compiling a kernel</title>
95 <para>
96 To enable <symbol>CONFIG_KGDB</symbol>, look under the "Kernel debugging"
97 and then select "KGDB: kernel debugging with remote gdb".
98 </para>
99 <para>
100 Next you should choose one of more I/O drivers to interconnect debugging
101 host and debugged target. Early boot debugging requires a KGDB
102 I/O driver that supports early debugging and the driver must be
103 built into the kernel directly. Kgdb I/O driver configuration
104 takes place via kernel or module parameters, see following
105 chapter.
106 </para>
107 <para>
108 The kgdb test compile options are described in the kgdb test suite chapter.
109 </para>
110
111 </chapter>
112 <chapter id="EnableKGDB">
113 <title>Enable kgdb for debugging</title>
114 <para>
115 In order to use kgdb you must activate it by passing configuration
116 information to one of the kgdb I/O drivers. If you do not pass any
117 configuration information kgdb will not do anything at all. Kgdb
118 will only actively hook up to the kernel trap hooks if a kgdb I/O
119 driver is loaded and configured. If you unconfigure a kgdb I/O
120 driver, kgdb will unregister all the kernel hook points.
121 </para>
122 <para>
123 All drivers can be reconfigured at run time, if
124 <symbol>CONFIG_SYSFS</symbol> and <symbol>CONFIG_MODULES</symbol>
125 are enabled, by echo'ing a new config string to
126 <constant>/sys/module/&lt;driver&gt;/parameter/&lt;option&gt;</constant>.
127 The driver can be unconfigured by passing an empty string. You cannot
128 change the configuration while the debugger is attached. Make sure
129 to detach the debugger with the <constant>detach</constant> command
130 prior to trying unconfigure a kgdb I/O driver.
131 </para>
132 <sect1 id="kgdbwait">
133 <title>Kernel parameter: kgdbwait</title>
134 <para>
135 The Kernel command line option <constant>kgdbwait</constant> makes
136 kgdb wait for a debugger connection during booting of a kernel. You
137 can only use this option you compiled a kgdb I/O driver into the
138 kernel and you specified the I/O driver configuration as a kernel
139 command line option. The kgdbwait parameter should always follow the
140 configuration parameter for the kgdb I/O driver in the kernel
141 command line else the I/O driver will not be configured prior to
142 asking the kernel to use it to wait.
143 </para>
144 <para>
145 The kernel will stop and wait as early as the I/O driver and
146 architecture will allow when you use this option. If you build the
147 kgdb I/O driver as a kernel module kgdbwait will not do anything.
148 </para>
149 </sect1>
150 <sect1 id="kgdboc">
151 <title>Kernel parameter: kgdboc</title>
152 <para>
153 The kgdboc driver was originally an abbreviation meant to stand for
154 "kgdb over console". Kgdboc is designed to work with a single
225a4424 155 serial port. It was meant to cover the circumstance
e3e2aaf7 156 where you wanted to use a serial console as your primary console as
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157 well as using it to perform kernel debugging. Of course you can
158 also use kgdboc without assigning a console to the same port.
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159 </para>
160 <sect2 id="UsingKgdboc">
161 <title>Using kgdboc</title>
162 <para>
163 You can configure kgdboc via sysfs or a module or kernel boot line
164 parameter depending on if you build with CONFIG_KGDBOC as a module
165 or built-in.
166 <orderedlist>
167 <listitem><para>From the module load or build-in</para>
168 <para><constant>kgdboc=&lt;tty-device&gt;,[baud]</constant></para>
169 <para>
170 The example here would be if your console port was typically ttyS0, you would use something like <constant>kgdboc=ttyS0,115200</constant> or on the ARM Versatile AB you would likely use <constant>kgdboc=ttyAMA0,115200</constant>
171 </para>
172 </listitem>
173 <listitem><para>From sysfs</para>
174 <para><constant>echo ttyS0 &gt; /sys/module/kgdboc/parameters/kgdboc</constant></para>
175 </listitem>
176 </orderedlist>
177 </para>
178 <para>
179 NOTE: Kgdboc does not support interrupting the target via the
180 gdb remote protocol. You must manually send a sysrq-g unless you
181 have a proxy that splits console output to a terminal problem and
182 has a separate port for the debugger to connect to that sends the
183 sysrq-g for you.
184 </para>
185 <para>When using kgdboc with no debugger proxy, you can end up
186 connecting the debugger for one of two entry points. If an
187 exception occurs after you have loaded kgdboc a message should print
188 on the console stating it is waiting for the debugger. In case you
189 disconnect your terminal program and then connect the debugger in
190 its place. If you want to interrupt the target system and forcibly
191 enter a debug session you have to issue a Sysrq sequence and then
192 type the letter <constant>g</constant>. Then you disconnect the
193 terminal session and connect gdb. Your options if you don't like
194 this are to hack gdb to send the sysrq-g for you as well as on the
195 initial connect, or to use a debugger proxy that allows an
196 unmodified gdb to do the debugging.
197 </para>
198 </sect2>
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199 </sect1>
200 <sect1 id="kgdbcon">
201 <title>Kernel parameter: kgdbcon</title>
202 <para>
203 Kgdb supports using the gdb serial protocol to send console messages
204 to the debugger when the debugger is connected and running. There
205 are two ways to activate this feature.
206 <orderedlist>
207 <listitem><para>Activate with the kernel command line option:</para>
208 <para><constant>kgdbcon</constant></para>
209 </listitem>
210 <listitem><para>Use sysfs before configuring an io driver</para>
211 <para>
212 <constant>echo 1 &gt; /sys/module/kgdb/parameters/kgdb_use_con</constant>
213 </para>
214 <para>
215 NOTE: If you do this after you configure the kgdb I/O driver, the
216 setting will not take effect until the next point the I/O is
217 reconfigured.
218 </para>
219 </listitem>
220 </orderedlist>
221 </para>
222 <para>
223 IMPORTANT NOTE: Using this option with kgdb over the console
224 (kgdboc) or kgdb over ethernet (kgdboe) is not supported.
225 </para>
226 </sect1>
227 </chapter>
228 <chapter id="ConnectingGDB">
229 <title>Connecting gdb</title>
230 <para>
231 If you are using kgdboc, you need to have used kgdbwait as a boot
232 argument, issued a sysrq-g, or the system you are going to debug
233 has already taken an exception and is waiting for the debugger to
234 attach before you can connect gdb.
235 </para>
236 <para>
237 If you are not using different kgdb I/O driver other than kgdboc,
238 you should be able to connect and the target will automatically
239 respond.
240 </para>
241 <para>
242 Example (using a serial port):
243 </para>
244 <programlisting>
245 % gdb ./vmlinux
246 (gdb) set remotebaud 115200
247 (gdb) target remote /dev/ttyS0
248 </programlisting>
249 <para>
250 Example (kgdb to a terminal server):
251 </para>
252 <programlisting>
253 % gdb ./vmlinux
254 (gdb) target remote udp:192.168.2.2:6443
255 </programlisting>
256 <para>
257 Example (kgdb over ethernet):
258 </para>
259 <programlisting>
260 % gdb ./vmlinux
261 (gdb) target remote udp:192.168.2.2:6443
262 </programlisting>
263 <para>
264 Once connected, you can debug a kernel the way you would debug an
265 application program.
266 </para>
267 <para>
268 If you are having problems connecting or something is going
269 seriously wrong while debugging, it will most often be the case
270 that you want to enable gdb to be verbose about its target
271 communications. You do this prior to issuing the <constant>target
272 remote</constant> command by typing in: <constant>set remote debug 1</constant>
273 </para>
274 </chapter>
275 <chapter id="KGDBTestSuite">
276 <title>kgdb Test Suite</title>
277 <para>
278 When kgdb is enabled in the kernel config you can also elect to
279 enable the config parameter KGDB_TESTS. Turning this on will
280 enable a special kgdb I/O module which is designed to test the
281 kgdb internal functions.
282 </para>
283 <para>
284 The kgdb tests are mainly intended for developers to test the kgdb
285 internals as well as a tool for developing a new kgdb architecture
286 specific implementation. These tests are not really for end users
287 of the Linux kernel. The primary source of documentation would be
288 to look in the drivers/misc/kgdbts.c file.
289 </para>
290 <para>
291 The kgdb test suite can also be configured at compile time to run
292 the core set of tests by setting the kernel config parameter
293 KGDB_TESTS_ON_BOOT. This particular option is aimed at automated
294 regression testing and does not require modifying the kernel boot
295 config arguments. If this is turned on, the kgdb test suite can
296 be disabled by specifying "kgdbts=" as a kernel boot argument.
297 </para>
298 </chapter>
299 <chapter id="CommonBackEndReq">
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300 <title>KGDB Internals</title>
301 <sect1 id="kgdbArchitecture">
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302 <title>Architecture Specifics</title>
303 <para>
304 Kgdb is organized into three basic components:
305 <orderedlist>
306 <listitem><para>kgdb core</para>
307 <para>
308 The kgdb core is found in kernel/kgdb.c. It contains:
309 <itemizedlist>
310 <listitem><para>All the logic to implement the gdb serial protocol</para></listitem>
311 <listitem><para>A generic OS exception handler which includes sync'ing the processors into a stopped state on an multi cpu system.</para></listitem>
312 <listitem><para>The API to talk to the kgdb I/O drivers</para></listitem>
313 <listitem><para>The API to make calls to the arch specific kgdb implementation</para></listitem>
314 <listitem><para>The logic to perform safe memory reads and writes to memory while using the debugger</para></listitem>
315 <listitem><para>A full implementation for software breakpoints unless overridden by the arch</para></listitem>
316 </itemizedlist>
317 </para>
318 </listitem>
319 <listitem><para>kgdb arch specific implementation</para>
320 <para>
321 This implementation is generally found in arch/*/kernel/kgdb.c.
322 As an example, arch/x86/kernel/kgdb.c contains the specifics to
323 implement HW breakpoint as well as the initialization to
324 dynamically register and unregister for the trap handlers on
325 this architecture. The arch specific portion implements:
326 <itemizedlist>
327 <listitem><para>contains an arch specific trap catcher which
328 invokes kgdb_handle_exception() to start kgdb about doing its
329 work</para></listitem>
330 <listitem><para>translation to and from gdb specific packet format to pt_regs</para></listitem>
331 <listitem><para>Registration and unregistration of architecture specific trap hooks</para></listitem>
332 <listitem><para>Any special exception handling and cleanup</para></listitem>
333 <listitem><para>NMI exception handling and cleanup</para></listitem>
334 <listitem><para>(optional)HW breakpoints</para></listitem>
335 </itemizedlist>
336 </para>
337 </listitem>
338 <listitem><para>kgdb I/O driver</para>
339 <para>
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340 Each kgdb I/O driver has to provide an implemenation for the following:
341 <itemizedlist>
342 <listitem><para>configuration via builtin or module</para></listitem>
343 <listitem><para>dynamic configuration and kgdb hook registration calls</para></listitem>
344 <listitem><para>read and write character interface</para></listitem>
345 <listitem><para>A cleanup handler for unconfiguring from the kgdb core</para></listitem>
346 <listitem><para>(optional) Early debug methodology</para></listitem>
347 </itemizedlist>
348 Any given kgdb I/O driver has to operate very closely with the
349 hardware and must do it in such a way that does not enable
350 interrupts or change other parts of the system context without
351 completely restoring them. The kgdb core will repeatedly "poll"
352 a kgdb I/O driver for characters when it needs input. The I/O
353 driver is expected to return immediately if there is no data
354 available. Doing so allows for the future possibility to touch
355 watch dog hardware in such a way as to have a target system not
356 reset when these are enabled.
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357 </para>
358 </listitem>
359 </orderedlist>
360 </para>
361 <para>
362 If you are intent on adding kgdb architecture specific support
363 for a new architecture, the architecture should define
364 <constant>HAVE_ARCH_KGDB</constant> in the architecture specific
365 Kconfig file. This will enable kgdb for the architecture, and
366 at that point you must create an architecture specific kgdb
367 implementation.
368 </para>
369 <para>
370 There are a few flags which must be set on every architecture in
371 their &lt;asm/kgdb.h&gt; file. These are:
372 <itemizedlist>
373 <listitem>
374 <para>
375 NUMREGBYTES: The size in bytes of all of the registers, so
376 that we can ensure they will all fit into a packet.
377 </para>
378 <para>
379 BUFMAX: The size in bytes of the buffer GDB will read into.
380 This must be larger than NUMREGBYTES.
381 </para>
382 <para>
383 CACHE_FLUSH_IS_SAFE: Set to 1 if it is always safe to call
384 flush_cache_range or flush_icache_range. On some architectures,
385 these functions may not be safe to call on SMP since we keep other
386 CPUs in a holding pattern.
387 </para>
388 </listitem>
389 </itemizedlist>
390 </para>
391 <para>
392 There are also the following functions for the common backend,
393 found in kernel/kgdb.c, that must be supplied by the
394 architecture-specific backend unless marked as (optional), in
395 which case a default function maybe used if the architecture
396 does not need to provide a specific implementation.
397 </para>
398!Iinclude/linux/kgdb.h
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399 </sect1>
400 <sect1 id="kgdbocDesign">
401 <title>kgdboc internals</title>
402 <para>
403 The kgdboc driver is actually a very thin driver that relies on the
404 underlying low level to the hardware driver having "polling hooks"
405 which the to which the tty driver is attached. In the initial
406 implementation of kgdboc it the serial_core was changed to expose a
407 low level uart hook for doing polled mode reading and writing of a
408 single character while in an atomic context. When kgdb makes an I/O
409 request to the debugger, kgdboc invokes a call back in the serial
410 core which in turn uses the call back in the uart driver. It is
411 certainly possible to extend kgdboc to work with non-uart based
412 consoles in the future.
413 </para>
414 <para>
415 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>
416#ifdef CONFIG_CONSOLE_POLL
417 .poll_get_char = serial8250_get_poll_char,
418 .poll_put_char = serial8250_put_poll_char,
419#endif
420 </programlisting>
421 Any implementation specifics around creating a polling driver use the
422 <constant>#ifdef CONFIG_CONSOLE_POLL</constant>, as shown above.
423 Keep in mind that polling hooks have to be implemented in such a way
424 that they can be called from an atomic context and have to restore
425 the state of the uart chip on return such that the system can return
426 to normal when the debugger detaches. You need to be very careful
427 with any kind of lock you consider, because failing here is most
428 going to mean pressing the reset button.
429 </para>
430 </sect1>
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431 </chapter>
432 <chapter id="credits">
433 <title>Credits</title>
434 <para>
435 The following people have contributed to this document:
436 <orderedlist>
437 <listitem><para>Amit Kale<email>amitkale@linsyssoft.com</email></para></listitem>
438 <listitem><para>Tom Rini<email>trini@kernel.crashing.org</email></para></listitem>
e3e2aaf7 439 </orderedlist>
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440 In March 2008 this document was completely rewritten by:
441 <itemizedlist>
442 <listitem><para>Jason Wessel<email>jason.wessel@windriver.com</email></para></listitem>
443 </itemizedlist>
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444 </para>
445 </chapter>
446</book>
447