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, 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>
 917