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+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
+	"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
+
+<book id="kgdbOnLinux">
+ <bookinfo>
+  <title>Using kgdb, kdb and the kernel debugger internals</title>
+
+  <authorgroup>
+   <author>
+    <firstname>Jason</firstname>
+    <surname>Wessel</surname>
+    <affiliation>
+     <address>
+      <email>jason.wessel@windriver.com</email>
+     </address>
+    </affiliation>
+   </author>
+  </authorgroup>
+  <copyright>
+   <year>2008,2010</year>
+   <holder>Wind River Systems, Inc.</holder>
+  </copyright>
+  <copyright>
+   <year>2004-2005</year>
+   <holder>MontaVista Software, Inc.</holder>
+  </copyright>
+  <copyright>
+   <year>2004</year>
+   <holder>Amit S. Kale</holder>
+  </copyright>
+
+  <legalnotice>
+   <para>
+   This file is licensed under the terms of the GNU General Public License
+   version 2. This program is licensed "as is" without any warranty of any
+   kind, whether express or implied.
+   </para>
+
+  </legalnotice>
+ </bookinfo>
+
+<toc></toc>
+  <chapter id="Introduction">
+    <title>Introduction</title>
+    <para>
+    The kernel has two different debugger front ends (kdb and kgdb)
+    which interface to the debug core.  It is possible to use either
+    of the debugger front ends and dynamically transition between them
+    if you configure the kernel properly at compile and runtime.
+    </para>
+    <para>
+    Kdb is simplistic shell-style interface which you can use on a
+    system console with a keyboard or serial console.  You can use it
+    to inspect memory, registers, process lists, dmesg, and even set
+    breakpoints to stop in a certain location.  Kdb is not a source
+    level debugger, although you can set breakpoints and execute some
+    basic kernel run control.  Kdb is mainly aimed at doing some
+    analysis to aid in development or diagnosing kernel problems.  You
+    can access some symbols by name in kernel built-ins or in kernel
+    modules if the code was built
+    with <symbol>CONFIG_KALLSYMS</symbol>.
+    </para>
+    <para>
+    Kgdb is intended to be used as a source level debugger for the
+    Linux kernel. It is used along with gdb to debug a Linux kernel.
+    The expectation is that gdb can be used to "break in" to the
+    kernel to inspect memory, variables and look through call stack
+    information similar to the way an application developer would use
+    gdb to debug an application.  It is possible to place breakpoints
+    in kernel code and perform some limited execution stepping.
+    </para>
+    <para>
+    Two machines are required for using kgdb. One of these machines is
+    a development machine and the other is the target machine.  The
+    kernel to be debugged runs on the target machine. The development
+    machine runs an instance of gdb against the vmlinux file which
+    contains the symbols (not a boot image such as bzImage, zImage,
+    uImage...).  In gdb the developer specifies the connection
+    parameters and connects to kgdb.  The type of connection a
+    developer makes with gdb depends on the availability of kgdb I/O
+    modules compiled as built-ins or loadable kernel modules in the test
+    machine's kernel.
+    </para>
+  </chapter>
+  <chapter id="CompilingAKernel">
+  <title>Compiling a kernel</title>
+  <para>
+  <itemizedlist>
+  <listitem><para>In order to enable compilation of kdb, you must first enable kgdb.</para></listitem>
+  <listitem><para>The kgdb test compile options are described in the kgdb test suite chapter.</para></listitem>
+  </itemizedlist>
+  </para>
+  <sect1 id="CompileKGDB">
+    <title>Kernel config options for kgdb</title>
+    <para>
+    To enable <symbol>CONFIG_KGDB</symbol> you should look under
+    "Kernel hacking" / "Kernel debugging" and select "KGDB: kernel debugger".
+    </para>
+    <para>
+    While it is not a hard requirement that you have symbols in your
+    vmlinux file, gdb tends not to be very useful without the symbolic
+    data, so you will want to turn
+    on <symbol>CONFIG_DEBUG_INFO</symbol> which is called "Compile the
+    kernel with debug info" in the config menu.
+    </para>
+    <para>
+    It is advised, but not required, that you turn on the
+    <symbol>CONFIG_FRAME_POINTER</symbol> kernel option which is called "Compile the
+    kernel with frame pointers" in the config menu.  This option
+    inserts code to into the compiled executable which saves the frame
+    information in registers or on the stack at different points which
+    allows a debugger such as gdb to more accurately construct
+    stack back traces while debugging the kernel.
+    </para>
+    <para>
+    If the architecture that you are using supports the kernel option
+    CONFIG_DEBUG_RODATA, you should consider turning it off.  This
+    option will prevent the use of software breakpoints because it
+    marks certain regions of the kernel's memory space as read-only.
+    If kgdb supports it for the architecture you are using, you can
+    use hardware breakpoints if you desire to run with the
+    CONFIG_DEBUG_RODATA option turned on, else you need to turn off
+    this option.
+    </para>
+    <para>
+    Next you should choose one of more I/O drivers to interconnect
+    debugging host and debugged target.  Early boot debugging requires
+    a KGDB I/O driver that supports early debugging and the driver
+    must be built into the kernel directly. Kgdb I/O driver
+    configuration takes place via kernel or module parameters which
+    you can learn more about in the in the section that describes the
+    parameter "kgdboc".
+    </para>
+    <para>Here is an example set of .config symbols to enable or
+    disable for kgdb:
+    <itemizedlist>
+    <listitem><para># CONFIG_DEBUG_RODATA is not set</para></listitem>
+    <listitem><para>CONFIG_FRAME_POINTER=y</para></listitem>
+    <listitem><para>CONFIG_KGDB=y</para></listitem>
+    <listitem><para>CONFIG_KGDB_SERIAL_CONSOLE=y</para></listitem>
+    </itemizedlist>
+    </para>
+  </sect1>
+  <sect1 id="CompileKDB">
+    <title>Kernel config options for kdb</title>
+    <para>Kdb is quite a bit more complex than the simple gdbstub
+    sitting on top of the kernel's debug core.  Kdb must implement a
+    shell, and also adds some helper functions in other parts of the
+    kernel, responsible for printing out interesting data such as what
+    you would see if you ran "lsmod", or "ps".  In order to build kdb
+    into the kernel you follow the same steps as you would for kgdb.
+    </para>
+    <para>The main config option for kdb
+    is <symbol>CONFIG_KGDB_KDB</symbol> which is called "KGDB_KDB:
+    include kdb frontend for kgdb" in the config menu.  In theory you
+    would have already also selected an I/O driver such as the
+    CONFIG_KGDB_SERIAL_CONSOLE interface if you plan on using kdb on a
+    serial port, when you were configuring kgdb.
+    </para>
+    <para>If you want to use a PS/2-style keyboard with kdb, you would
+    select CONFIG_KDB_KEYBOARD which is called "KGDB_KDB: keyboard as
+    input device" in the config menu.  The CONFIG_KDB_KEYBOARD option
+    is not used for anything in the gdb interface to kgdb.  The
+    CONFIG_KDB_KEYBOARD option only works with kdb.
+    </para>
+    <para>Here is an example set of .config symbols to enable/disable kdb:
+    <itemizedlist>
+    <listitem><para># CONFIG_DEBUG_RODATA is not set</para></listitem>
+    <listitem><para>CONFIG_FRAME_POINTER=y</para></listitem>
+    <listitem><para>CONFIG_KGDB=y</para></listitem>
+    <listitem><para>CONFIG_KGDB_SERIAL_CONSOLE=y</para></listitem>
+    <listitem><para>CONFIG_KGDB_KDB=y</para></listitem>
+    <listitem><para>CONFIG_KDB_KEYBOARD=y</para></listitem>
+    </itemizedlist>
+    </para>
+  </sect1>
+  </chapter>
+  <chapter id="kgdbKernelArgs">
+  <title>Kernel Debugger Boot Arguments</title>
+  <para>This section describes the various runtime kernel
+  parameters that affect the configuration of the kernel debugger.
+  The following chapter covers using kdb and kgdb as well as
+  providing some examples of the configuration parameters.</para>
+   <sect1 id="kgdboc">
+   <title>Kernel parameter: kgdboc</title>
+   <para>The kgdboc driver was originally an abbreviation meant to
+   stand for "kgdb over console".  Today it is the primary mechanism
+   to configure how to communicate from gdb to kgdb as well as the
+   devices you want to use to interact with the kdb shell.
+   </para>
+   <para>For kgdb/gdb, kgdboc is designed to work with a single serial
+   port. It is intended to cover the circumstance where you want to
+   use a serial console as your primary console as well as using it to
+   perform kernel debugging.  It is also possible to use kgdb on a
+   serial port which is not designated as a system console.  Kgdboc
+   may be configured as a kernel built-in or a kernel loadable module.
+   You can only make use of <constant>kgdbwait</constant> and early
+   debugging if you build kgdboc into the kernel as a built-in.
+   </para>
+   <para>Optionally you can elect to activate kms (Kernel Mode
+   Setting) integration.  When you use kms with kgdboc and you have a
+   video driver that has atomic mode setting hooks, it is possible to
+   enter the debugger on the graphics console.  When the kernel
+   execution is resumed, the previous graphics mode will be restored.
+   This integration can serve as a useful tool to aid in diagnosing
+   crashes or doing analysis of memory with kdb while allowing the
+   full graphics console applications to run.
+   </para>
+   <sect2 id="kgdbocArgs">
+   <title>kgdboc arguments</title>
+   <para>Usage: <constant>kgdboc=[kms][[,]kbd][[,]serial_device][,baud]</constant></para>
+   <para>The order listed above must be observed if you use any of the
+   optional configurations together.
+   </para>
+   <para>Abbreviations:
+   <itemizedlist>
+   <listitem><para>kms = Kernel Mode Setting</para></listitem>
+   <listitem><para>kbd = Keyboard</para></listitem>
+   </itemizedlist>
+   </para>
+   <para>You can configure kgdboc to use the keyboard, and/or a serial
+   device depending on if you are using kdb and/or kgdb, in one of the
+   following scenarios.  The order listed above must be observed if
+   you use any of the optional configurations together.  Using kms +
+   only gdb is generally not a useful combination.</para>
+   <sect3 id="kgdbocArgs1">
+   <title>Using loadable module or built-in</title>
+   <para>
+   <orderedlist>
+   <listitem><para>As a kernel built-in:</para>
+   <para>Use the kernel boot argument: <constant>kgdboc=&lt;tty-device&gt;,[baud]</constant></para></listitem>
+   <listitem>
+   <para>As a kernel loadable module:</para>
+   <para>Use the command: <constant>modprobe kgdboc kgdboc=&lt;tty-device&gt;,[baud]</constant></para>
+   <para>Here are two examples of how you might format the kgdboc
+   string. The first is for an x86 target using the first serial port.
+   The second example is for the ARM Versatile AB using the second
+   serial port.
+   <orderedlist>
+   <listitem><para><constant>kgdboc=ttyS0,115200</constant></para></listitem>
+   <listitem><para><constant>kgdboc=ttyAMA1,115200</constant></para></listitem>
+   </orderedlist>
+   </para>
+   </listitem>
+   </orderedlist></para>
+   </sect3>
+   <sect3 id="kgdbocArgs2">
+   <title>Configure kgdboc at runtime with sysfs</title>
+   <para>At run time you can enable or disable kgdboc by echoing a
+   parameters into the sysfs.  Here are two examples:</para>
+   <orderedlist>
+   <listitem><para>Enable kgdboc on ttyS0</para>
+   <para><constant>echo ttyS0 &gt; /sys/module/kgdboc/parameters/kgdboc</constant></para></listitem>
+   <listitem><para>Disable kgdboc</para>
+   <para><constant>echo "" &gt; /sys/module/kgdboc/parameters/kgdboc</constant></para></listitem>
+   </orderedlist>
+   <para>NOTE: You do not need to specify the baud if you are
+   configuring the console on tty which is already configured or
+   open.</para>
+   </sect3>
+   <sect3 id="kgdbocArgs3">
+   <title>More examples</title>
+   <para>You can configure kgdboc to use the keyboard, and/or a serial device
+   depending on if you are using kdb and/or kgdb, in one of the
+   following scenarios.
+   <orderedlist>
+   <listitem><para>kdb and kgdb over only a serial port</para>
+   <para><constant>kgdboc=&lt;serial_device&gt;[,baud]</constant></para>
+   <para>Example: <constant>kgdboc=ttyS0,115200</constant></para>
+   </listitem>
+   <listitem><para>kdb and kgdb with keyboard and a serial port</para>
+   <para><constant>kgdboc=kbd,&lt;serial_device&gt;[,baud]</constant></para>
+   <para>Example: <constant>kgdboc=kbd,ttyS0,115200</constant></para>
+   </listitem>
+   <listitem><para>kdb with a keyboard</para>
+   <para><constant>kgdboc=kbd</constant></para>
+   </listitem>
+   <listitem><para>kdb with kernel mode setting</para>
+   <para><constant>kgdboc=kms,kbd</constant></para>
+   </listitem>
+   <listitem><para>kdb with kernel mode setting and kgdb over a serial port</para>
+   <para><constant>kgdboc=kms,kbd,ttyS0,115200</constant></para>
+   </listitem>
+   </orderedlist>
+   </para>
+   <para>NOTE: Kgdboc does not support interrupting the target via the
+   gdb remote protocol.  You must manually send a sysrq-g unless you
+   have a proxy that splits console output to a terminal program.
+   A console proxy has a separate TCP port for the debugger and a separate
+   TCP port for the "human" console.  The proxy can take care of sending
+   the sysrq-g for you.
+   </para>
+   <para>When using kgdboc with no debugger proxy, you can end up
+    connecting the debugger at one of two entry points.  If an
+    exception occurs after you have loaded kgdboc, a message should
+    print on the console stating it is waiting for the debugger.  In
+    this case you disconnect your terminal program and then connect the
+    debugger in its place.  If you want to interrupt the target system
+    and forcibly enter a debug session you have to issue a Sysrq
+    sequence and then type the letter <constant>g</constant>.  Then
+    you disconnect the terminal session and connect gdb.  Your options
+    if you don't like this are to hack gdb to send the sysrq-g for you
+    as well as on the initial connect, or to use a debugger proxy that
+    allows an unmodified gdb to do the debugging.
+   </para>
+   </sect3>
+   </sect2>
+   </sect1>
+   <sect1 id="kgdbwait">
+   <title>Kernel parameter: kgdbwait</title>
+   <para>
+   The Kernel command line option <constant>kgdbwait</constant> makes
+   kgdb wait for a debugger connection during booting of a kernel.  You
+   can only use this option if you compiled a kgdb I/O driver into the
+   kernel and you specified the I/O driver configuration as a kernel
+   command line option.  The kgdbwait parameter should always follow the
+   configuration parameter for the kgdb I/O driver in the kernel
+   command line else the I/O driver will not be configured prior to
+   asking the kernel to use it to wait.
+   </para>
+   <para>
+   The kernel will stop and wait as early as the I/O driver and
+   architecture allows when you use this option.  If you build the
+   kgdb I/O driver as a loadable kernel module kgdbwait will not do
+   anything.
+   </para>
+   </sect1>
+   <sect1 id="kgdbcon">
+   <title>Kernel parameter: kgdbcon</title>
+   <para> The kgdbcon feature allows you to see printk() messages
+   inside gdb while gdb is connected to the kernel.  Kdb does not make
+    use of the kgdbcon feature.
+   </para>
+   <para>Kgdb supports using the gdb serial protocol to send console
+   messages to the debugger when the debugger is connected and running.
+   There are two ways to activate this feature.
+   <orderedlist>
+   <listitem><para>Activate with the kernel command line option:</para>
+   <para><constant>kgdbcon</constant></para>
+   </listitem>
+   <listitem><para>Use sysfs before configuring an I/O driver</para>
+   <para>
+   <constant>echo 1 &gt; /sys/module/kgdb/parameters/kgdb_use_con</constant>
+   </para>
+   <para>
+   NOTE: If you do this after you configure the kgdb I/O driver, the
+   setting will not take effect until the next point the I/O is
+   reconfigured.
+   </para>
+   </listitem>
+   </orderedlist>
+  </para>
+   <para>IMPORTANT NOTE: You cannot use kgdboc + kgdbcon on a tty that is an
+   active system console.  An example of incorrect usage is <constant>console=ttyS0,115200 kgdboc=ttyS0 kgdbcon</constant>
+   </para>
+   <para>It is possible to use this option with kgdboc on a tty that is not a system console.
+   </para>
+  </sect1>
+   <sect1 id="kgdbreboot">
+   <title>Run time parameter: kgdbreboot</title>
+   <para> The kgdbreboot feature allows you to change how the debugger
+   deals with the reboot notification.  You have 3 choices for the
+   behavior.  The default behavior is always set to 0.</para>
+   <orderedlist>
+   <listitem><para>echo -1 > /sys/module/debug_core/parameters/kgdbreboot</para>
+   <para>Ignore the reboot notification entirely.</para>
+   </listitem>
+   <listitem><para>echo 0 > /sys/module/debug_core/parameters/kgdbreboot</para>
+   <para>Send the detach message to any attached debugger client.</para>
+   </listitem>
+   <listitem><para>echo 1 > /sys/module/debug_core/parameters/kgdbreboot</para>
+   <para>Enter the debugger on reboot notify.</para>
+   </listitem>
+   </orderedlist>
+  </sect1>
+  </chapter>
+  <chapter id="usingKDB">
+  <title>Using kdb</title>
+  <para>
+  </para>
+  <sect1 id="quickKDBserial">
+  <title>Quick start for kdb on a serial port</title>
+  <para>This is a quick example of how to use kdb.</para>
+  <para><orderedlist>
+  <listitem><para>Configure kgdboc at boot using kernel parameters:
+  <itemizedlist>
+  <listitem><para><constant>console=ttyS0,115200 kgdboc=ttyS0,115200</constant></para></listitem>
+  </itemizedlist></para>
+  <para>OR</para>
+  <para>Configure kgdboc after the kernel has booted; assuming you are using a serial port console:
+  <itemizedlist>
+  <listitem><para><constant>echo ttyS0 &gt; /sys/module/kgdboc/parameters/kgdboc</constant></para></listitem>
+  </itemizedlist>
+  </para>
+  </listitem>
+  <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>
+  <itemizedlist>
+  <listitem><para>When logged in as root or with a super user session you can run:</para>
+   <para><constant>echo g &gt; /proc/sysrq-trigger</constant></para></listitem>
+  <listitem><para>Example using minicom 2.2</para>
+  <para>Press: <constant>Control-a</constant></para>
+  <para>Press: <constant>f</constant></para>
+  <para>Press: <constant>g</constant></para>
+  </listitem>
+  <listitem><para>When you have telneted to a terminal server that supports sending a remote break</para>
+  <para>Press: <constant>Control-]</constant></para>
+  <para>Type in:<constant>send break</constant></para>
+  <para>Press: <constant>Enter</constant></para>
+  <para>Press: <constant>g</constant></para>
+  </listitem>
+  </itemizedlist>
+  </listitem>
+  <listitem><para>From the kdb prompt you can run the "help" command to see a complete list of the commands that are available.</para>
+  <para>Some useful commands in kdb include:
+  <itemizedlist>
+  <listitem><para>lsmod  -- Shows where kernel modules are loaded</para></listitem>
+  <listitem><para>ps -- Displays only the active processes</para></listitem>
+  <listitem><para>ps A -- Shows all the processes</para></listitem>
+  <listitem><para>summary -- Shows kernel version info and memory usage</para></listitem>
+  <listitem><para>bt -- Get a backtrace of the current process using dump_stack()</para></listitem>
+  <listitem><para>dmesg -- View the kernel syslog buffer</para></listitem>
+  <listitem><para>go -- Continue the system</para></listitem>
+  </itemizedlist>
+  </para>
+  </listitem>
+  <listitem>
+  <para>When you are done using kdb you need to consider rebooting the
+  system or using the "go" command to resuming normal kernel
+  execution.  If you have paused the kernel for a lengthy period of
+  time, applications that rely on timely networking or anything to do
+  with real wall clock time could be adversely affected, so you
+  should take this into consideration when using the kernel
+  debugger.</para>
+  </listitem>
+  </orderedlist></para>
+  </sect1>
+  <sect1 id="quickKDBkeyboard">
+  <title>Quick start for kdb using a keyboard connected console</title>
+  <para>This is a quick example of how to use kdb with a keyboard.</para>
+  <para><orderedlist>
+  <listitem><para>Configure kgdboc at boot using kernel parameters:
+  <itemizedlist>
+  <listitem><para><constant>kgdboc=kbd</constant></para></listitem>
+  </itemizedlist></para>
+  <para>OR</para>
+  <para>Configure kgdboc after the kernel has booted:
+  <itemizedlist>
+  <listitem><para><constant>echo kbd &gt; /sys/module/kgdboc/parameters/kgdboc</constant></para></listitem>
+  </itemizedlist>
+  </para>
+  </listitem>
+  <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>
+  <itemizedlist>
+  <listitem><para>When logged in as root or with a super user session you can run:</para>
+   <para><constant>echo g &gt; /proc/sysrq-trigger</constant></para></listitem>
+  <listitem><para>Example using a laptop keyboard</para>
+  <para>Press and hold down: <constant>Alt</constant></para>
+  <para>Press and hold down: <constant>Fn</constant></para>
+  <para>Press and release the key with the label: <constant>SysRq</constant></para>
+  <para>Release: <constant>Fn</constant></para>
+  <para>Press and release: <constant>g</constant></para>
+  <para>Release: <constant>Alt</constant></para>
+  </listitem>
+  <listitem><para>Example using a PS/2 101-key keyboard</para>
+  <para>Press and hold down: <constant>Alt</constant></para>
+  <para>Press and release the key with the label: <constant>SysRq</constant></para>
+  <para>Press and release: <constant>g</constant></para>
+  <para>Release: <constant>Alt</constant></para>
+  </listitem>
+  </itemizedlist>
+  </listitem>
+  <listitem>
+  <para>Now type in a kdb command such as "help", "dmesg", "bt" or "go" to continue kernel execution.</para>
+  </listitem>
+  </orderedlist></para>
+  </sect1>
+  </chapter>
+  <chapter id="EnableKGDB">
+   <title>Using kgdb / gdb</title>
+   <para>In order to use kgdb you must activate it by passing
+   configuration information to one of the kgdb I/O drivers.  If you
+   do not pass any configuration information kgdb will not do anything
+   at all.  Kgdb will only actively hook up to the kernel trap hooks
+   if a kgdb I/O driver is loaded and configured.  If you unconfigure
+   a kgdb I/O driver, kgdb will unregister all the kernel hook points.
+   </para>
+   <para> All kgdb I/O drivers can be reconfigured at run time, if
+   <symbol>CONFIG_SYSFS</symbol> and <symbol>CONFIG_MODULES</symbol>
+   are enabled, by echo'ing a new config string to
+   <constant>/sys/module/&lt;driver&gt;/parameter/&lt;option&gt;</constant>.
+   The driver can be unconfigured by passing an empty string.  You cannot
+   change the configuration while the debugger is attached.  Make sure
+   to detach the debugger with the <constant>detach</constant> command
+   prior to trying to unconfigure a kgdb I/O driver.
+   </para>
+  <sect1 id="ConnectingGDB">
+  <title>Connecting with gdb to a serial port</title>
+  <orderedlist>
+  <listitem><para>Configure kgdboc</para>
+   <para>Configure kgdboc at boot using kernel parameters:
+   <itemizedlist>
+    <listitem><para><constant>kgdboc=ttyS0,115200</constant></para></listitem>
+   </itemizedlist></para>
+   <para>OR</para>
+   <para>Configure kgdboc after the kernel has booted:
+   <itemizedlist>
+    <listitem><para><constant>echo ttyS0 &gt; /sys/module/kgdboc/parameters/kgdboc</constant></para></listitem>
+   </itemizedlist></para>
+  </listitem>
+  <listitem>
+  <para>Stop kernel execution (break into the debugger)</para>
+  <para>In order to connect to gdb via kgdboc, the kernel must
+  first be stopped.  There are several ways to stop the kernel which
+  include using kgdbwait as a boot argument, via a sysrq-g, or running
+  the kernel until it takes an exception where it waits for the
+  debugger to attach.
+  <itemizedlist>
+  <listitem><para>When logged in as root or with a super user session you can run:</para>
+   <para><constant>echo g &gt; /proc/sysrq-trigger</constant></para></listitem>
+  <listitem><para>Example using minicom 2.2</para>
+  <para>Press: <constant>Control-a</constant></para>
+  <para>Press: <constant>f</constant></para>
+  <para>Press: <constant>g</constant></para>
+  </listitem>
+  <listitem><para>When you have telneted to a terminal server that supports sending a remote break</para>
+  <para>Press: <constant>Control-]</constant></para>
+  <para>Type in:<constant>send break</constant></para>
+  <para>Press: <constant>Enter</constant></para>
+  <para>Press: <constant>g</constant></para>
+  </listitem>
+  </itemizedlist>
+  </para>
+  </listitem>
+  <listitem>
+    <para>Connect from gdb</para>
+    <para>
+    Example (using a directly connected port):
+    </para>
+    <programlisting>
+    % gdb ./vmlinux
+    (gdb) set remotebaud 115200
+    (gdb) target remote /dev/ttyS0
+    </programlisting>
+    <para>
+    Example (kgdb to a terminal server on TCP port 2012):
+    </para>
+    <programlisting>
+    % gdb ./vmlinux
+    (gdb) target remote 192.168.2.2:2012
+    </programlisting>
+    <para>
+    Once connected, you can debug a kernel the way you would debug an
+    application program.
+    </para>
+    <para>
+    If you are having problems connecting or something is going
+    seriously wrong while debugging, it will most often be the case
+    that you want to enable gdb to be verbose about its target
+    communications.  You do this prior to issuing the <constant>target
+    remote</constant> command by typing in: <constant>set debug remote 1</constant>
+    </para>
+  </listitem>
+  </orderedlist>
+  <para>Remember if you continue in gdb, and need to "break in" again,
+  you need to issue an other sysrq-g.  It is easy to create a simple
+  entry point by putting a breakpoint at <constant>sys_sync</constant>
+  and then you can run "sync" from a shell or script to break into the
+  debugger.</para>
+  </sect1>
+  </chapter>
+  <chapter id="switchKdbKgdb">
+  <title>kgdb and kdb interoperability</title>
+  <para>It is possible to transition between kdb and kgdb dynamically.
+  The debug core will remember which you used the last time and
+  automatically start in the same mode.</para>
+  <sect1>
+  <title>Switching between kdb and kgdb</title>
+  <sect2>
+  <title>Switching from kgdb to kdb</title>
+  <para>
+  There are two ways to switch from kgdb to kdb: you can use gdb to
+  issue a maintenance packet, or you can blindly type the command $3#33.
+  Whenever the kernel debugger stops in kgdb mode it will print the
+  message <constant>KGDB or $3#33 for KDB</constant>.  It is important
+  to note that you have to type the sequence correctly in one pass.
+  You cannot type a backspace or delete because kgdb will interpret
+  that as part of the debug stream.
+  <orderedlist>
+  <listitem><para>Change from kgdb to kdb by blindly typing:</para>
+  <para><constant>$3#33</constant></para></listitem>
+  <listitem><para>Change from kgdb to kdb with gdb</para>
+  <para><constant>maintenance packet 3</constant></para>
+  <para>NOTE: Now you must kill gdb. Typically you press control-z and
+  issue the command: kill -9 %</para></listitem>
+  </orderedlist>
+  </para>
+  </sect2>
+  <sect2>
+  <title>Change from kdb to kgdb</title>
+  <para>There are two ways you can change from kdb to kgdb.  You can
+  manually enter kgdb mode by issuing the kgdb command from the kdb
+  shell prompt, or you can connect gdb while the kdb shell prompt is
+  active.  The kdb shell looks for the typical first commands that gdb
+  would issue with the gdb remote protocol and if it sees one of those
+  commands it automatically changes into kgdb mode.</para>
+  <orderedlist>
+  <listitem><para>From kdb issue the command:</para>
+  <para><constant>kgdb</constant></para>
+  <para>Now disconnect your terminal program and connect gdb in its place</para></listitem>
+  <listitem><para>At the kdb prompt, disconnect the terminal program and connect gdb in its place.</para></listitem>
+  </orderedlist>
+  </sect2>
+  </sect1>
+  <sect1>
+  <title>Running kdb commands from gdb</title>
+  <para>It is possible to run a limited set of kdb commands from gdb,
+  using the gdb monitor command.  You don't want to execute any of the
+  run control or breakpoint operations, because it can disrupt the
+  state of the kernel debugger.  You should be using gdb for
+  breakpoints and run control operations if you have gdb connected.
+  The more useful commands to run are things like lsmod, dmesg, ps or
+  possibly some of the memory information commands.  To see all the kdb
+  commands you can run <constant>monitor help</constant>.</para>
+  <para>Example:
+  <informalexample><programlisting>
+(gdb) monitor ps
+1 idle process (state I) and
+27 sleeping system daemon (state M) processes suppressed,
+use 'ps A' to see all.
+Task Addr       Pid   Parent [*] cpu State Thread     Command
+
+0xc78291d0        1        0  0    0   S  0xc7829404  init
+0xc7954150      942        1  0    0   S  0xc7954384  dropbear
+0xc78789c0      944        1  0    0   S  0xc7878bf4  sh
+(gdb)
+  </programlisting></informalexample>
+  </para>
+  </sect1>
+  </chapter>
+  <chapter id="KGDBTestSuite">
+    <title>kgdb Test Suite</title>
+    <para>
+    When kgdb is enabled in the kernel config you can also elect to
+    enable the config parameter KGDB_TESTS.  Turning this on will
+    enable a special kgdb I/O module which is designed to test the
+    kgdb internal functions.
+    </para>
+    <para>
+    The kgdb tests are mainly intended for developers to test the kgdb
+    internals as well as a tool for developing a new kgdb architecture
+    specific implementation.  These tests are not really for end users
+    of the Linux kernel.  The primary source of documentation would be
+    to look in the drivers/misc/kgdbts.c file.
+    </para>
+    <para>
+    The kgdb test suite can also be configured at compile time to run
+    the core set of tests by setting the kernel config parameter
+    KGDB_TESTS_ON_BOOT.  This particular option is aimed at automated
+    regression testing and does not require modifying the kernel boot
+    config arguments.  If this is turned on, the kgdb test suite can
+    be disabled by specifying "kgdbts=" as a kernel boot argument.
+    </para>
+  </chapter>
+  <chapter id="CommonBackEndReq">
+  <title>Kernel Debugger Internals</title>
+  <sect1 id="kgdbArchitecture">
+    <title>Architecture Specifics</title>
+      <para>
+      The kernel debugger is organized into a number of components:
+      <orderedlist>
+      <listitem><para>The debug core</para>
+      <para>
+      The debug core is found in kernel/debugger/debug_core.c.  It contains:
+      <itemizedlist>
+      <listitem><para>A generic OS exception handler which includes
+      sync'ing the processors into a stopped state on an multi-CPU
+      system.</para></listitem>
+      <listitem><para>The API to talk to the kgdb I/O drivers</para></listitem>
+      <listitem><para>The API to make calls to the arch-specific kgdb implementation</para></listitem>
+      <listitem><para>The logic to perform safe memory reads and writes to memory while using the debugger</para></listitem>
+      <listitem><para>A full implementation for software breakpoints unless overridden by the arch</para></listitem>
+      <listitem><para>The API to invoke either the kdb or kgdb frontend to the debug core.</para></listitem>
+      <listitem><para>The structures and callback API for atomic kernel mode setting.</para>
+      <para>NOTE: kgdboc is where the kms callbacks are invoked.</para></listitem>
+      </itemizedlist>
+      </para>
+      </listitem>
+      <listitem><para>kgdb arch-specific implementation</para>
+      <para>
+      This implementation is generally found in arch/*/kernel/kgdb.c.
+      As an example, arch/x86/kernel/kgdb.c contains the specifics to
+      implement HW breakpoint as well as the initialization to
+      dynamically register and unregister for the trap handlers on
+      this architecture.  The arch-specific portion implements:
+      <itemizedlist>
+      <listitem><para>contains an arch-specific trap catcher which
+      invokes kgdb_handle_exception() to start kgdb about doing its
+      work</para></listitem>
+      <listitem><para>translation to and from gdb specific packet format to pt_regs</para></listitem>
+      <listitem><para>Registration and unregistration of architecture specific trap hooks</para></listitem>
+      <listitem><para>Any special exception handling and cleanup</para></listitem>
+      <listitem><para>NMI exception handling and cleanup</para></listitem>
+      <listitem><para>(optional) HW breakpoints</para></listitem>
+      </itemizedlist>
+      </para>
+      </listitem>
+      <listitem><para>gdbstub frontend (aka kgdb)</para>
+      <para>The gdbstub is located in kernel/debug/gdbstub.c. It contains:</para>
+      <itemizedlist>
+        <listitem><para>All the logic to implement the gdb serial protocol</para></listitem>
+      </itemizedlist>
+      </listitem>
+      <listitem><para>kdb frontend</para>
+      <para>The kdb debugger shell is broken down into a number of
+      components.  The kdb core is located in kernel/debug/kdb.  There
+      are a number of helper functions in some of the other kernel
+      components to make it possible for kdb to examine and report
+      information about the kernel without taking locks that could
+      cause a kernel deadlock.  The kdb core contains implements the following functionality.</para>
+      <itemizedlist>
+        <listitem><para>A simple shell</para></listitem>
+        <listitem><para>The kdb core command set</para></listitem>
+        <listitem><para>A registration API to register additional kdb shell commands.</para>
+	<itemizedlist>
+        <listitem><para>A good example of a self-contained kdb module
+        is the "ftdump" command for dumping the ftrace buffer.  See:
+        kernel/trace/trace_kdb.c</para></listitem>
+        <listitem><para>For an example of how to dynamically register
+        a new kdb command you can build the kdb_hello.ko kernel module
+        from samples/kdb/kdb_hello.c.  To build this example you can
+        set CONFIG_SAMPLES=y and CONFIG_SAMPLE_KDB=m in your kernel
+        config.  Later run "modprobe kdb_hello" and the next time you
+        enter the kdb shell, you can run the "hello"
+        command.</para></listitem>
+	</itemizedlist></listitem>
+        <listitem><para>The implementation for kdb_printf() which
+        emits messages directly to I/O drivers, bypassing the kernel
+        log.</para></listitem>
+        <listitem><para>SW / HW breakpoint management for the kdb shell</para></listitem>
+      </itemizedlist>
+      </listitem>
+      <listitem><para>kgdb I/O driver</para>
+      <para>
+      Each kgdb I/O driver has to provide an implementation for the following:
+      <itemizedlist>
+      <listitem><para>configuration via built-in or module</para></listitem>
+      <listitem><para>dynamic configuration and kgdb hook registration calls</para></listitem>
+      <listitem><para>read and write character interface</para></listitem>
+      <listitem><para>A cleanup handler for unconfiguring from the kgdb core</para></listitem>
+      <listitem><para>(optional) Early debug methodology</para></listitem>
+      </itemizedlist>
+      Any given kgdb I/O driver has to operate very closely with the
+      hardware and must do it in such a way that does not enable
+      interrupts or change other parts of the system context without
+      completely restoring them. The kgdb core will repeatedly "poll"
+      a kgdb I/O driver for characters when it needs input.  The I/O
+      driver is expected to return immediately if there is no data
+      available.  Doing so allows for the future possibility to touch
+      watchdog hardware in such a way as to have a target system not
+      reset when these are enabled.
+      </para>
+      </listitem>
+      </orderedlist>
+      </para>
+      <para>
+      If you are intent on adding kgdb architecture specific support
+      for a new architecture, the architecture should define
+      <constant>HAVE_ARCH_KGDB</constant> in the architecture specific
+      Kconfig file.  This will enable kgdb for the architecture, and
+      at that point you must create an architecture specific kgdb
+      implementation.
+      </para>
+      <para>
+      There are a few flags which must be set on every architecture in
+      their &lt;asm/kgdb.h&gt; file.  These are:
+      <itemizedlist>
+        <listitem>
+          <para>
+          NUMREGBYTES: The size in bytes of all of the registers, so
+          that we can ensure they will all fit into a packet.
+          </para>
+        </listitem>
+        <listitem>
+          <para>
+          BUFMAX: The size in bytes of the buffer GDB will read into.
+          This must be larger than NUMREGBYTES.
+          </para>
+        </listitem>
+        <listitem>
+          <para>
+          CACHE_FLUSH_IS_SAFE: Set to 1 if it is always safe to call
+          flush_cache_range or flush_icache_range.  On some architectures,
+          these functions may not be safe to call on SMP since we keep other
+          CPUs in a holding pattern.
+          </para>
+        </listitem>
+      </itemizedlist>
+      </para>
+      <para>
+      There are also the following functions for the common backend,
+      found in kernel/kgdb.c, that must be supplied by the
+      architecture-specific backend unless marked as (optional), in
+      which case a default function maybe used if the architecture
+      does not need to provide a specific implementation.
+      </para>
+!Iinclude/linux/kgdb.h
+  </sect1>
+  <sect1 id="kgdbocDesign">
+  <title>kgdboc internals</title>
+  <sect2>
+  <title>kgdboc and uarts</title>
+  <para>
+  The kgdboc driver is actually a very thin driver that relies on the
+  underlying low level to the hardware driver having "polling hooks"
+  to which the tty driver is attached.  In the initial
+  implementation of kgdboc the serial_core was changed to expose a
+  low level UART hook for doing polled mode reading and writing of a
+  single character while in an atomic context.  When kgdb makes an I/O
+  request to the debugger, kgdboc invokes a callback in the serial
+  core which in turn uses the callback in the UART driver.</para>
+  <para>
+  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>
+#ifdef CONFIG_CONSOLE_POLL
+	.poll_get_char = serial8250_get_poll_char,
+	.poll_put_char = serial8250_put_poll_char,
+#endif
+  </programlisting>
+  Any implementation specifics around creating a polling driver use the
+  <constant>#ifdef CONFIG_CONSOLE_POLL</constant>, as shown above.
+  Keep in mind that polling hooks have to be implemented in such a way
+  that they can be called from an atomic context and have to restore
+  the state of the UART chip on return such that the system can return
+  to normal when the debugger detaches.  You need to be very careful
+  with any kind of lock you consider, because failing here is most likely
+  going to mean pressing the reset button.
+  </para>
+  </sect2>
+  <sect2 id="kgdbocKbd">
+  <title>kgdboc and keyboards</title>
+  <para>The kgdboc driver contains logic to configure communications
+  with an attached keyboard.  The keyboard infrastructure is only
+  compiled into the kernel when CONFIG_KDB_KEYBOARD=y is set in the
+  kernel configuration.</para>
+  <para>The core polled keyboard driver driver for PS/2 type keyboards
+  is in drivers/char/kdb_keyboard.c.  This driver is hooked into the
+  debug core when kgdboc populates the callback in the array
+  called <constant>kdb_poll_funcs[]</constant>.  The
+  kdb_get_kbd_char() is the top-level function which polls hardware
+  for single character input.
+  </para>
+  </sect2>
+  <sect2 id="kgdbocKms">
+  <title>kgdboc and kms</title>
+  <para>The kgdboc driver contains logic to request the graphics
+  display to switch to a text context when you are using
+  "kgdboc=kms,kbd", provided that you have a video driver which has a
+  frame buffer console and atomic kernel mode setting support.</para>
+  <para>
+  Every time the kernel
+  debugger is entered it calls kgdboc_pre_exp_handler() which in turn
+  calls con_debug_enter() in the virtual console layer.  On resuming kernel
+  execution, the kernel debugger calls kgdboc_post_exp_handler() which
+  in turn calls con_debug_leave().</para>
+  <para>Any video driver that wants to be compatible with the kernel
+  debugger and the atomic kms callbacks must implement the
+  mode_set_base_atomic, fb_debug_enter and fb_debug_leave operations.
+  For the fb_debug_enter and fb_debug_leave the option exists to use
+  the generic drm fb helper functions or implement something custom for
+  the hardware.  The following example shows the initialization of the
+  .mode_set_base_atomic operation in
+  drivers/gpu/drm/i915/intel_display.c:
+  <informalexample>
+  <programlisting>
+static const struct drm_crtc_helper_funcs intel_helper_funcs = {
+[...]
+        .mode_set_base_atomic = intel_pipe_set_base_atomic,
+[...]
+};
+  </programlisting>
+  </informalexample>
+  </para>
+  <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
+  drivers/gpu/drm/i915/intel_fb.c:
+  <informalexample>
+  <programlisting>
+static struct fb_ops intelfb_ops = {
+[...]
+       .fb_debug_enter = drm_fb_helper_debug_enter,
+       .fb_debug_leave = drm_fb_helper_debug_leave,
+[...]
+};
+  </programlisting>
+  </informalexample>
+  </para>
+  </sect2>
+  </sect1>
+  </chapter>
+  <chapter id="credits">
+     <title>Credits</title>
+	<para>
+		The following people have contributed to this document:
+		<orderedlist>
+			<listitem><para>Amit Kale<email>amitkale@linsyssoft.com</email></para></listitem>
+			<listitem><para>Tom Rini<email>trini@kernel.crashing.org</email></para></listitem>
+		</orderedlist>
+                In March 2008 this document was completely rewritten by:
+		<itemizedlist>
+		<listitem><para>Jason Wessel<email>jason.wessel@windriver.com</email></para></listitem>
+		</itemizedlist>
+                In Jan 2010 this document was updated to include kdb.
+		<itemizedlist>
+		<listitem><para>Jason Wessel<email>jason.wessel@windriver.com</email></para></listitem>
+		</itemizedlist>
+	</para>
+  </chapter>
+</book>
+