Documentation/DocBook/kgdb.tmpl

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   2 <!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
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   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 call 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> you should first turn on
  97     "Prompt for development and/or incomplete code/drivers"
  98     (CONFIG_EXPERIMENTAL) in  "General setup", then under the
  99     "Kernel debugging" select "KGDB: kernel debugging with remote gdb".
 100     </para>
 101     <para>
 102     Next you should choose one of more I/O drivers to interconnect debugging
 103     host and debugged target.  Early boot debugging requires a KGDB
 104     I/O driver that supports early debugging and the driver must be
 105     built into the kernel directly. Kgdb I/O driver configuration
 106     takes place via kernel or module parameters, see following
 107     chapter.
 108     </para>
 109     <para>
 110     The kgdb test compile options are described in the kgdb test suite chapter.
 111     </para>
 112
 113   </chapter>
 114   <chapter id="EnableKGDB">
 115    <title>Enable kgdb for debugging</title>
 116    <para>
 117    In order to use kgdb you must activate it by passing configuration
 118    information to one of the kgdb I/O drivers.  If you do not pass any
 119    configuration information kgdb will not do anything at all.  Kgdb
 120    will only actively hook up to the kernel trap hooks if a kgdb I/O
 121    driver is loaded and configured.  If you unconfigure a kgdb I/O
 122    driver, kgdb will unregister all the kernel hook points.
 123    </para>
 124    <para>
 125    All drivers can be reconfigured at run time, if
 126    <symbol>CONFIG_SYSFS</symbol> and <symbol>CONFIG_MODULES</symbol>
 127    are enabled, by echo'ing a new config string to
 128    <constant>/sys/module/&lt;driver&gt;/parameter/&lt;option&gt;</constant>.
 129    The driver can be unconfigured by passing an empty string.  You cannot
 130    change the configuration while the debugger is attached.  Make sure
 131    to detach the debugger with the <constant>detach</constant> command
 132    prior to trying unconfigure a kgdb I/O driver.
 133    </para>
 134    <sect1 id="kgdbwait">
 135    <title>Kernel parameter: kgdbwait</title>
 136    <para>
 137    The Kernel command line option <constant>kgdbwait</constant> makes
 138    kgdb wait for a debugger connection during booting of a kernel.  You
 139    can only use this option you compiled a kgdb I/O driver into the
 140    kernel and you specified the I/O driver configuration as a kernel
 141    command line option.  The kgdbwait parameter should always follow the
 142    configuration parameter for the kgdb I/O driver in the kernel
 143    command line else the I/O driver will not be configured prior to
 144    asking the kernel to use it to wait.
 145    </para>
 146    <para>
 147    The kernel will stop and wait as early as the I/O driver and
 148    architecture will allow when you use this option.  If you build the
 149    kgdb I/O driver as a kernel module kgdbwait will not do anything.
 150    </para>
 151    </sect1>
 152   <sect1 id="kgdboc">
 153   <title>Kernel parameter: kgdboc</title>
 154   <para>
 155   The kgdboc driver was originally an abbreviation meant to stand for
 156   "kgdb over console".  Kgdboc is designed to work with a single
 157   serial port. It was meant to cover the circumstance
 158   where you wanted to use a serial console as your primary console as
 159   well as using it to perform kernel debugging.  Of course you can
 160   also use kgdboc without assigning a console to the same port.
 161   </para>
 162   <sect2 id="UsingKgdboc">
 163   <title>Using kgdboc</title>
 164   <para>
 165   You can configure kgdboc via sysfs or a module or kernel boot line
 166   parameter depending on if you build with CONFIG_KGDBOC as a module
 167   or built-in.
 168   <orderedlist>
 169   <listitem><para>From the module load or build-in</para>
 170   <para><constant>kgdboc=&lt;tty-device&gt;,[baud]</constant></para>
 171   <para>
 172   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>
 173   </para>
 174   </listitem>
 175   <listitem><para>From sysfs</para>
 176   <para><constant>echo ttyS0 &gt; /sys/module/kgdboc/parameters/kgdboc</constant></para>
 177   </listitem>
 178   </orderedlist>
 179   </para>
 180   <para>
 181   NOTE: Kgdboc does not support interrupting the target via the
 182   gdb remote protocol.  You must manually send a sysrq-g unless you
 183   have a proxy that splits console output to a terminal problem and
 184   has a separate port for the debugger to connect to that sends the
 185   sysrq-g for you.
 186   </para>
 187   <para>When using kgdboc with no debugger proxy, you can end up
 188   connecting the debugger for one of two entry points.  If an
 189   exception occurs after you have loaded kgdboc a message should print
 190   on the console stating it is waiting for the debugger.  In case you
 191   disconnect your terminal program and then connect the debugger in
 192   its place.  If you want to interrupt the target system and forcibly
 193   enter a debug session you have to issue a Sysrq sequence and then
 194   type the letter <constant>g</constant>.  Then you disconnect the
 195   terminal session and connect gdb.  Your options if you don't like
 196   this are to hack gdb to send the sysrq-g for you as well as on the
 197   initial connect, or to use a debugger proxy that allows an
 198   unmodified gdb to do the debugging.
 199   </para>
 200   </sect2>
 201   </sect1>
 202   <sect1 id="kgdbcon">
 203   <title>Kernel parameter: kgdbcon</title>
 204   <para>
 205   Kgdb supports using the gdb serial protocol to send console messages
 206   to the debugger when the debugger is connected and running.  There
 207   are two ways to activate this feature.
 208   <orderedlist>
 209   <listitem><para>Activate with the kernel command line option:</para>
 210   <para><constant>kgdbcon</constant></para>
 211   </listitem>
 212   <listitem><para>Use sysfs before configuring an io driver</para>
 213   <para>
 214   <constant>echo 1 &gt; /sys/module/kgdb/parameters/kgdb_use_con</constant>
 215   </para>
 216   <para>
 217   NOTE: If you do this after you configure the kgdb I/O driver, the
 218   setting will not take effect until the next point the I/O is
 219   reconfigured.
 220   </para>
 221   </listitem>
 222   </orderedlist>
 223   </para>
 224   <para>
 225   IMPORTANT NOTE: Using this option with kgdb over the console
 226   (kgdboc) or kgdb over ethernet (kgdboe) is not supported.
 227   </para>
 228   </sect1>
 229   </chapter>
 230   <chapter id="ConnectingGDB">
 231   <title>Connecting gdb</title>
 232     <para>
 233     If you are using kgdboc, you need to have used kgdbwait as a boot
 234     argument, issued a sysrq-g, or the system you are going to debug
 235     has already taken an exception and is waiting for the debugger to
 236     attach before you can connect gdb.
 237     </para>
 238     <para>
 239     If you are not using different kgdb I/O driver other than kgdboc,
 240     you should be able to connect and the target will automatically
 241     respond.
 242     </para>
 243     <para>
 244     Example (using a serial port):
 245     </para>
 246     <programlisting>
 247     % gdb ./vmlinux
 248     (gdb) set remotebaud 115200
 249     (gdb) target remote /dev/ttyS0
 250     </programlisting>
 251     <para>
 252     Example (kgdb to a terminal server):
 253     </para>
 254     <programlisting>
 255     % gdb ./vmlinux
 256     (gdb) target remote udp:192.168.2.2:6443
 257     </programlisting>
 258     <para>
 259     Example (kgdb over ethernet):
 260     </para>
 261     <programlisting>
 262     % gdb ./vmlinux
 263     (gdb) target remote udp:192.168.2.2:6443
 264     </programlisting>
 265     <para>
 266     Once connected, you can debug a kernel the way you would debug an
 267     application program.
 268     </para>
 269     <para>
 270     If you are having problems connecting or something is going
 271     seriously wrong while debugging, it will most often be the case
 272     that you want to enable gdb to be verbose about its target
 273     communications.  You do this prior to issuing the <constant>target
 274     remote</constant> command by typing in: <constant>set remote debug 1</constant>
 275     </para>
 276   </chapter>
 277   <chapter id="KGDBTestSuite">
 278     <title>kgdb Test Suite</title>
 279     <para>
 280     When kgdb is enabled in the kernel config you can also elect to
 281     enable the config parameter KGDB_TESTS.  Turning this on will
 282     enable a special kgdb I/O module which is designed to test the
 283     kgdb internal functions.
 284     </para>
 285     <para>
 286     The kgdb tests are mainly intended for developers to test the kgdb
 287     internals as well as a tool for developing a new kgdb architecture
 288     specific implementation.  These tests are not really for end users
 289     of the Linux kernel.  The primary source of documentation would be
 290     to look in the drivers/misc/kgdbts.c file.
 291     </para>
 292     <para>
 293     The kgdb test suite can also be configured at compile time to run
 294     the core set of tests by setting the kernel config parameter
 295     KGDB_TESTS_ON_BOOT.  This particular option is aimed at automated
 296     regression testing and does not require modifying the kernel boot
 297     config arguments.  If this is turned on, the kgdb test suite can
 298     be disabled by specifying "kgdbts=" as a kernel boot argument.
 299     </para>
 300   </chapter>
 301   <chapter id="CommonBackEndReq">
 302   <title>KGDB Internals</title>
 303   <sect1 id="kgdbArchitecture">
 304     <title>Architecture Specifics</title>
 305       <para>
 306       Kgdb is organized into three basic components:
 307       <orderedlist>
 308       <listitem><para>kgdb core</para>
 309       <para>
 310       The kgdb core is found in kernel/kgdb.c.  It contains:
 311       <itemizedlist>
 312       <listitem><para>All the logic to implement the gdb serial protocol</para></listitem>
 313       <listitem><para>A generic OS exception handler which includes sync'ing the processors into a stopped state on an multi cpu system.</para></listitem>
 314       <listitem><para>The API to talk to the kgdb I/O drivers</para></listitem>
 315       <listitem><para>The API to make calls to the arch specific kgdb implementation</para></listitem>
 316       <listitem><para>The logic to perform safe memory reads and writes to memory while using the debugger</para></listitem>
 317       <listitem><para>A full implementation for software breakpoints unless overridden by the arch</para></listitem>
 318       </itemizedlist>
 319       </para>
 320       </listitem>
 321       <listitem><para>kgdb arch specific implementation</para>
 322       <para>
 323       This implementation is generally found in arch/*/kernel/kgdb.c.
 324       As an example, arch/x86/kernel/kgdb.c contains the specifics to
 325       implement HW breakpoint as well as the initialization to
 326       dynamically register and unregister for the trap handlers on
 327       this architecture.  The arch specific portion implements:
 328       <itemizedlist>
 329       <listitem><para>contains an arch specific trap catcher which
 330       invokes kgdb_handle_exception() to start kgdb about doing its
 331       work</para></listitem>
 332       <listitem><para>translation to and from gdb specific packet format to pt_regs</para></listitem>
 333       <listitem><para>Registration and unregistration of architecture specific trap hooks</para></listitem>
 334       <listitem><para>Any special exception handling and cleanup</para></listitem>
 335       <listitem><para>NMI exception handling and cleanup</para></listitem>
 336       <listitem><para>(optional)HW breakpoints</para></listitem>
 337       </itemizedlist>
 338       </para>
 339       </listitem>
 340       <listitem><para>kgdb I/O driver</para>
 341       <para>
 342       Each kgdb I/O driver has to provide an implemenation for the following:
 343       <itemizedlist>
 344       <listitem><para>configuration via builtin or module</para></listitem>
 345       <listitem><para>dynamic configuration and kgdb hook registration calls</para></listitem>
 346       <listitem><para>read and write character interface</para></listitem>
 347       <listitem><para>A cleanup handler for unconfiguring from the kgdb core</para></listitem>
 348       <listitem><para>(optional) Early debug methodology</para></listitem>
 349       </itemizedlist>
 350       Any given kgdb I/O driver has to operate very closely with the
 351       hardware and must do it in such a way that does not enable
 352       interrupts or change other parts of the system context without
 353       completely restoring them. The kgdb core will repeatedly "poll"
 354       a kgdb I/O driver for characters when it needs input.  The I/O
 355       driver is expected to return immediately if there is no data
 356       available.  Doing so allows for the future possibility to touch
 357       watch dog hardware in such a way as to have a target system not
 358       reset when these are enabled.
 359       </para>
 360       </listitem>
 361       </orderedlist>
 362       </para>
 363       <para>
 364       If you are intent on adding kgdb architecture specific support
 365       for a new architecture, the architecture should define
 366       <constant>HAVE_ARCH_KGDB</constant> in the architecture specific
 367       Kconfig file.  This will enable kgdb for the architecture, and
 368       at that point you must create an architecture specific kgdb
 369       implementation.
 370       </para>
 371       <para>
 372       There are a few flags which must be set on every architecture in
 373       their &lt;asm/kgdb.h&gt; file.  These are:
 374       <itemizedlist>
 375         <listitem>
 376           <para>
 377           NUMREGBYTES: The size in bytes of all of the registers, so
 378           that we can ensure they will all fit into a packet.
 379           </para>
 380           <para>
 381           BUFMAX: The size in bytes of the buffer GDB will read into.
 382           This must be larger than NUMREGBYTES.
 383           </para>
 384           <para>
 385           CACHE_FLUSH_IS_SAFE: Set to 1 if it is always safe to call
 386           flush_cache_range or flush_icache_range.  On some architectures,
 387           these functions may not be safe to call on SMP since we keep other
 388           CPUs in a holding pattern.
 389           </para>
 390         </listitem>
 391       </itemizedlist>
 392       </para>
 393       <para>
 394       There are also the following functions for the common backend,
 395       found in kernel/kgdb.c, that must be supplied by the
 396       architecture-specific backend unless marked as (optional), in
 397       which case a default function maybe used if the architecture
 398       does not need to provide a specific implementation.
 399       </para>
 400 !Iinclude/linux/kgdb.h
 401   </sect1>
 402   <sect1 id="kgdbocDesign">
 403   <title>kgdboc internals</title>
 404   <para>
 405   The kgdboc driver is actually a very thin driver that relies on the
 406   underlying low level to the hardware driver having "polling hooks"
 407   which the to which the tty driver is attached.  In the initial
 408   implementation of kgdboc it the serial_core was changed to expose a
 409   low level uart hook for doing polled mode reading and writing of a
 410   single character while in an atomic context.  When kgdb makes an I/O
 411   request to the debugger, kgdboc invokes a call back in the serial
 412   core which in turn uses the call back in the uart driver.  It is
 413   certainly possible to extend kgdboc to work with non-uart based
 414   consoles in the future.
 415   </para>
 416   <para>
 417   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>
 418 #ifdef CONFIG_CONSOLE_POLL
 419         .poll_get_char = serial8250_get_poll_char,
 420         .poll_put_char = serial8250_put_poll_char,
 421 #endif
 422   </programlisting>
 423   Any implementation specifics around creating a polling driver use the
 424   <constant>#ifdef CONFIG_CONSOLE_POLL</constant>, as shown above.
 425   Keep in mind that polling hooks have to be implemented in such a way
 426   that they can be called from an atomic context and have to restore
 427   the state of the uart chip on return such that the system can return
 428   to normal when the debugger detaches.  You need to be very careful
 429   with any kind of lock you consider, because failing here is most
 430   going to mean pressing the reset button.
 431   </para>
 432   </sect1>
 433   </chapter>
 434   <chapter id="credits">
 435      <title>Credits</title>
 436         <para>
 437                 The following people have contributed to this document:
 438                 <orderedlist>
 439                         <listitem><para>Amit Kale<email>amitkale@linsyssoft.com</email></para></listitem>
 440                         <listitem><para>Tom Rini<email>trini@kernel.crashing.org</email></para></listitem>
 441                 </orderedlist>
 442                 In March 2008 this document was completely rewritten by:
 443                 <itemizedlist>
 444                 <listitem><para>Jason Wessel<email>jason.wessel@windriver.com</email></para></listitem>
 445                 </itemizedlist>
 446         </para>
 447   </chapter>
 448 </book>
 449