WSL2-Linux-Kernel/Documentation/admin-guide
Linus Torvalds 500a434fc5 Driver core changes for 5.19-rc1
Here is the set of driver core changes for 5.19-rc1.
 
 Note, I'm not really happy with this pull request as-is, see below for
 details, but overall this is all good for everything but a small set of
 systems, which we have a fix for already.
 
 Lots of tiny driver core changes and cleanups happened this cycle,
 but the two major things were:
 
 	- firmware_loader reorganization and additions including the
 	  ability to have XZ compressed firmware images and the ability
 	  for userspace to initiate the firmware load when it needs to,
 	  instead of being always initiated by the kernel. FPGA devices
 	  specifically want this ability to have their firmware changed
 	  over the lifetime of the system boot, and this allows them to
 	  work without having to come up with yet-another-custom-uapi
 	  interface for loading firmware for them.
 	- physical location support added to sysfs so that devices that
 	  know this information, can tell userspace where they are
 	  located in a common way.  Some ACPI devices already support
 	  this today, and more bus types should support this in the
 	  future.
 
 Smaller changes included:
 	- driver_override api cleanups and fixes
 	- error path cleanups and fixes
 	- get_abi script fixes
 	- deferred probe timeout changes.
 
 It's that last change that I'm the most worried about.  It has been
 reported to cause boot problems for a number of systems, and I have a
 tested patch series that resolves this issue.  But I didn't get it
 merged into my tree before 5.18-final came out, so it has not gotten any
 linux-next testing.
 
 I'll send the fixup patches (there are 2) as a follow-on series to this
 pull request if you want to take them directly, _OR_ I can just revert
 the probe timeout changes and they can wait for the next -rc1 merge
 cycle.  Given that the fixes are tested, and pretty simple, I'm leaning
 toward that choice.  Sorry this all came at the end of the merge window,
 I should have resolved this all 2 weeks ago, that's my fault as it was
 in the middle of some travel for me.
 
 All have been tested in linux-next for weeks, with no reported issues
 other than the above-mentioned boot time outs.
 
 Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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Merge tag 'driver-core-5.19-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/driver-core

Pull driver core updates from Greg KH:
 "Here is the set of driver core changes for 5.19-rc1.

  Lots of tiny driver core changes and cleanups happened this cycle, but
  the two major things are:

   - firmware_loader reorganization and additions including the ability
     to have XZ compressed firmware images and the ability for userspace
     to initiate the firmware load when it needs to, instead of being
     always initiated by the kernel. FPGA devices specifically want this
     ability to have their firmware changed over the lifetime of the
     system boot, and this allows them to work without having to come up
     with yet-another-custom-uapi interface for loading firmware for
     them.

   - physical location support added to sysfs so that devices that know
     this information, can tell userspace where they are located in a
     common way. Some ACPI devices already support this today, and more
     bus types should support this in the future.

  Smaller changes include:

   - driver_override api cleanups and fixes

   - error path cleanups and fixes

   - get_abi script fixes

   - deferred probe timeout changes.

  It's that last change that I'm the most worried about. It has been
  reported to cause boot problems for a number of systems, and I have a
  tested patch series that resolves this issue. But I didn't get it
  merged into my tree before 5.18-final came out, so it has not gotten
  any linux-next testing.

  I'll send the fixup patches (there are 2) as a follow-on series to this
  pull request.

  All have been tested in linux-next for weeks, with no reported issues
  other than the above-mentioned boot time-outs"

* tag 'driver-core-5.19-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/driver-core: (55 commits)
  driver core: fix deadlock in __device_attach
  kernfs: Separate kernfs_pr_cont_buf and rename_lock.
  topology: Remove unused cpu_cluster_mask()
  driver core: Extend deferred probe timeout on driver registration
  MAINTAINERS: add Russ Weight as a firmware loader maintainer
  driver: base: fix UAF when driver_attach failed
  test_firmware: fix end of loop test in upload_read_show()
  driver core: location: Add "back" as a possible output for panel
  driver core: location: Free struct acpi_pld_info *pld
  driver core: Add "*" wildcard support to driver_async_probe cmdline param
  driver core: location: Check for allocations failure
  arch_topology: Trace the update thermal pressure
  kernfs: Rename kernfs_put_open_node to kernfs_unlink_open_file.
  export: fix string handling of namespace in EXPORT_SYMBOL_NS
  rpmsg: use local 'dev' variable
  rpmsg: Fix calling device_lock() on non-initialized device
  firmware_loader: describe 'module' parameter of firmware_upload_register()
  firmware_loader: Move definitions from sysfs_upload.h to sysfs.h
  firmware_loader: Fix configs for sysfs split
  selftests: firmware: Add firmware upload selftests
  ...
2022-06-03 11:48:47 -07:00
..
LSM
acpi Documentation/admin-guide/acpi: Add documentation for fine grain control 2022-02-25 20:49:30 +01:00
aoe
auxdisplay
blockdev A handful of late-arriving documentation fixes and the addition of an SVG 2022-06-02 15:36:06 -07:00
cgroup-v1 mm/memcg: disable threshold event handlers on PREEMPT_RT 2022-03-22 15:57:02 -07:00
cifs Documentation, arch: Remove leftovers from CIFS_WEAK_PW_HASH 2021-12-17 14:12:03 +01:00
device-mapper dm ima: update dm documentation for ima measurement support 2021-08-20 16:07:37 -04:00
gpio gpio: sim: add doc file to index file 2022-01-24 09:21:28 +01:00
hw-vuln x86/speculation: Update link to AMD speculation whitepaper 2022-02-28 18:37:12 +01:00
kdump Merge branch 'akpm' (patches from Andrew) 2022-03-24 14:14:07 -07:00
laptops lg-laptop: Move setting of battery charge limit to common location 2022-02-23 11:36:24 +01:00
media media: docs: Fix vimc default pipeline graph 2022-04-24 08:49:35 +01:00
mm Docs/admin-guide/mm/damon/reclaim: document 'commit_inputs' parameter 2022-05-13 07:20:09 -07:00
namespaces
nfs NFS: update documentation for the nfs4_unique_id parameter 2022-05-17 15:30:03 -04:00
perf docs: fix 'make htmldocs' warning in perf 2022-03-04 10:21:57 -07:00
pm Documentation: admin-guide: PM: Add Out of Band mode 2022-05-25 15:48:26 +02:00
sysctl Yang Shi has improved the behaviour of khugepaged collapsing of readonly 2022-05-26 12:32:41 -07:00
README.rst Drop some straggling mentions of gcc-4.9 as being stale 2021-09-13 10:29:44 -07:00
abi-obsolete.rst
abi-removed.rst
abi-stable.rst
abi-testing.rst
abi.rst
bcache.rst
binderfs.rst docs: binderfs: add section about feature files 2021-07-21 13:46:36 +02:00
binfmt-misc.rst
bootconfig.rst docs: bootconfig: Add how to embed the bootconfig into kernel 2022-04-26 17:58:51 -04:00
braille-console.rst
btmrvl.rst
bug-bisect.rst
bug-hunting.rst
cgroup-v2.rst Yang Shi has improved the behaviour of khugepaged collapsing of readonly 2022-05-26 12:32:41 -07:00
clearing-warn-once.rst
cpu-load.rst
cputopology.rst topology/sysfs: get rid of htmldoc warning 2021-12-07 11:20:45 +01:00
dell_rbu.rst
devices.rst
devices.txt net: wan: remove support for COSA and SRP synchronous serial boards 2022-04-27 12:22:56 +01:00
dynamic-debug-howto.rst dyndbg: refine verbosity 1-4 summary-detail 2021-10-21 13:01:25 +02:00
edid.rst
efi-stub.rst
ext4.rst
features.rst
filesystem-monitoring.rst docs: Fix formatting of literal sections in fanotify docs 2021-11-01 12:45:06 +01:00
highuid.rst
hw_random.rst
index.rst docs: add two documents about regression handling 2022-02-24 12:57:25 -07:00
init.rst
initrd.rst
iostats.rst Documentation: block/diskstats: update function names 2022-02-24 12:18:54 -07:00
java.rst
jfs.rst
kernel-parameters.rst Docs/admin: alphabetize some kernel-parameters (part 1) 2022-04-16 02:49:05 -06:00
kernel-parameters.txt Driver core changes for 5.19-rc1 2022-06-03 11:48:47 -07:00
kernel-per-CPU-kthreads.rst doc: Remove obsolete kernel-per-CPU-kthreads RCU_FAST_NO_HZ advice 2021-11-30 17:23:06 -08:00
lcd-panel-cgram.rst
ldm.rst
lockup-watchdogs.rst doc: watchdog: modify the explanation related to watchdog thread 2021-06-29 10:53:46 -07:00
md.rst
module-signing.rst
mono.rst
numastat.rst
parport.rst
perf-security.rst
pnp.rst
pstore-blk.rst
ramoops.rst dt-bindings: reserved-memory: ramoops: update ramoops.yaml references 2021-10-19 11:54:16 -05:00
rapidio.rst
ras.rst
reporting-issues.rst docs: reporting-issues.rst: link new document about regressions 2022-02-24 12:57:25 -07:00
reporting-regressions.rst docs: *-regressions.rst: explain how quickly issues should be handled 2022-02-24 12:57:25 -07:00
rtc.rst
security-bugs.rst
serial-console.rst
spkguide.txt speakup: Fix typo in documentation "boo" -> "boot" 2021-11-01 11:17:21 -06:00
svga.rst
syscall-user-dispatch.rst
sysfs-rules.rst
sysrq.rst Documentation: sysrq: convert to third person 2021-07-15 06:33:44 -06:00
tainted-kernels.rst
thunderbolt.rst
ufs.rst
unicode.rst
vga-softcursor.rst
video-output.rst
xfs.rst

README.rst

.. _readme:

Linux kernel release 5.x <http://kernel.org/>
=============================================

These are the release notes for Linux version 5.  Read them carefully,
as they tell you what this is all about, explain how to install the
kernel, and what to do if something goes wrong.

What is Linux?
--------------

  Linux is a clone of the operating system Unix, written from scratch by
  Linus Torvalds with assistance from a loosely-knit team of hackers across
  the Net. It aims towards POSIX and Single UNIX Specification compliance.

  It has all the features you would expect in a modern fully-fledged Unix,
  including true multitasking, virtual memory, shared libraries, demand
  loading, shared copy-on-write executables, proper memory management,
  and multistack networking including IPv4 and IPv6.

  It is distributed under the GNU General Public License v2 - see the
  accompanying COPYING file for more details.

On what hardware does it run?
-----------------------------

  Although originally developed first for 32-bit x86-based PCs (386 or higher),
  today Linux also runs on (at least) the Compaq Alpha AXP, Sun SPARC and
  UltraSPARC, Motorola 68000, PowerPC, PowerPC64, ARM, Hitachi SuperH, Cell,
  IBM S/390, MIPS, HP PA-RISC, Intel IA-64, DEC VAX, AMD x86-64 Xtensa, and
  ARC architectures.

  Linux is easily portable to most general-purpose 32- or 64-bit architectures
  as long as they have a paged memory management unit (PMMU) and a port of the
  GNU C compiler (gcc) (part of The GNU Compiler Collection, GCC). Linux has
  also been ported to a number of architectures without a PMMU, although
  functionality is then obviously somewhat limited.
  Linux has also been ported to itself. You can now run the kernel as a
  userspace application - this is called UserMode Linux (UML).

Documentation
-------------

 - There is a lot of documentation available both in electronic form on
   the Internet and in books, both Linux-specific and pertaining to
   general UNIX questions.  I'd recommend looking into the documentation
   subdirectories on any Linux FTP site for the LDP (Linux Documentation
   Project) books.  This README is not meant to be documentation on the
   system: there are much better sources available.

 - There are various README files in the Documentation/ subdirectory:
   these typically contain kernel-specific installation notes for some
   drivers for example. Please read the
   :ref:`Documentation/process/changes.rst <changes>` file, as it
   contains information about the problems, which may result by upgrading
   your kernel.

Installing the kernel source
----------------------------

 - If you install the full sources, put the kernel tarball in a
   directory where you have permissions (e.g. your home directory) and
   unpack it::

     xz -cd linux-5.x.tar.xz | tar xvf -

   Replace "X" with the version number of the latest kernel.

   Do NOT use the /usr/src/linux area! This area has a (usually
   incomplete) set of kernel headers that are used by the library header
   files.  They should match the library, and not get messed up by
   whatever the kernel-du-jour happens to be.

 - You can also upgrade between 5.x releases by patching.  Patches are
   distributed in the xz format.  To install by patching, get all the
   newer patch files, enter the top level directory of the kernel source
   (linux-5.x) and execute::

     xz -cd ../patch-5.x.xz | patch -p1

   Replace "x" for all versions bigger than the version "x" of your current
   source tree, **in_order**, and you should be ok.  You may want to remove
   the backup files (some-file-name~ or some-file-name.orig), and make sure
   that there are no failed patches (some-file-name# or some-file-name.rej).
   If there are, either you or I have made a mistake.

   Unlike patches for the 5.x kernels, patches for the 5.x.y kernels
   (also known as the -stable kernels) are not incremental but instead apply
   directly to the base 5.x kernel.  For example, if your base kernel is 5.0
   and you want to apply the 5.0.3 patch, you must not first apply the 5.0.1
   and 5.0.2 patches. Similarly, if you are running kernel version 5.0.2 and
   want to jump to 5.0.3, you must first reverse the 5.0.2 patch (that is,
   patch -R) **before** applying the 5.0.3 patch. You can read more on this in
   :ref:`Documentation/process/applying-patches.rst <applying_patches>`.

   Alternatively, the script patch-kernel can be used to automate this
   process.  It determines the current kernel version and applies any
   patches found::

     linux/scripts/patch-kernel linux

   The first argument in the command above is the location of the
   kernel source.  Patches are applied from the current directory, but
   an alternative directory can be specified as the second argument.

 - Make sure you have no stale .o files and dependencies lying around::

     cd linux
     make mrproper

   You should now have the sources correctly installed.

Software requirements
---------------------

   Compiling and running the 5.x kernels requires up-to-date
   versions of various software packages.  Consult
   :ref:`Documentation/process/changes.rst <changes>` for the minimum version numbers
   required and how to get updates for these packages.  Beware that using
   excessively old versions of these packages can cause indirect
   errors that are very difficult to track down, so don't assume that
   you can just update packages when obvious problems arise during
   build or operation.

Build directory for the kernel
------------------------------

   When compiling the kernel, all output files will per default be
   stored together with the kernel source code.
   Using the option ``make O=output/dir`` allows you to specify an alternate
   place for the output files (including .config).
   Example::

     kernel source code: /usr/src/linux-5.x
     build directory:    /home/name/build/kernel

   To configure and build the kernel, use::

     cd /usr/src/linux-5.x
     make O=/home/name/build/kernel menuconfig
     make O=/home/name/build/kernel
     sudo make O=/home/name/build/kernel modules_install install

   Please note: If the ``O=output/dir`` option is used, then it must be
   used for all invocations of make.

Configuring the kernel
----------------------

   Do not skip this step even if you are only upgrading one minor
   version.  New configuration options are added in each release, and
   odd problems will turn up if the configuration files are not set up
   as expected.  If you want to carry your existing configuration to a
   new version with minimal work, use ``make oldconfig``, which will
   only ask you for the answers to new questions.

 - Alternative configuration commands are::

     "make config"      Plain text interface.

     "make menuconfig"  Text based color menus, radiolists & dialogs.

     "make nconfig"     Enhanced text based color menus.

     "make xconfig"     Qt based configuration tool.

     "make gconfig"     GTK+ based configuration tool.

     "make oldconfig"   Default all questions based on the contents of
                        your existing ./.config file and asking about
                        new config symbols.

     "make olddefconfig"
                        Like above, but sets new symbols to their default
                        values without prompting.

     "make defconfig"   Create a ./.config file by using the default
                        symbol values from either arch/$ARCH/defconfig
                        or arch/$ARCH/configs/${PLATFORM}_defconfig,
                        depending on the architecture.

     "make ${PLATFORM}_defconfig"
                        Create a ./.config file by using the default
                        symbol values from
                        arch/$ARCH/configs/${PLATFORM}_defconfig.
                        Use "make help" to get a list of all available
                        platforms of your architecture.

     "make allyesconfig"
                        Create a ./.config file by setting symbol
                        values to 'y' as much as possible.

     "make allmodconfig"
                        Create a ./.config file by setting symbol
                        values to 'm' as much as possible.

     "make allnoconfig" Create a ./.config file by setting symbol
                        values to 'n' as much as possible.

     "make randconfig"  Create a ./.config file by setting symbol
                        values to random values.

     "make localmodconfig" Create a config based on current config and
                           loaded modules (lsmod). Disables any module
                           option that is not needed for the loaded modules.

                           To create a localmodconfig for another machine,
                           store the lsmod of that machine into a file
                           and pass it in as a LSMOD parameter.

                           Also, you can preserve modules in certain folders
                           or kconfig files by specifying their paths in
                           parameter LMC_KEEP.

                   target$ lsmod > /tmp/mylsmod
                   target$ scp /tmp/mylsmod host:/tmp

                   host$ make LSMOD=/tmp/mylsmod \
                           LMC_KEEP="drivers/usb:drivers/gpu:fs" \
                           localmodconfig

                           The above also works when cross compiling.

     "make localyesconfig" Similar to localmodconfig, except it will convert
                           all module options to built in (=y) options. You can
                           also preserve modules by LMC_KEEP.

     "make kvm_guest.config"   Enable additional options for kvm guest kernel
                               support.

     "make xen.config"   Enable additional options for xen dom0 guest kernel
                         support.

     "make tinyconfig"  Configure the tiniest possible kernel.

   You can find more information on using the Linux kernel config tools
   in Documentation/kbuild/kconfig.rst.

 - NOTES on ``make config``:

    - Having unnecessary drivers will make the kernel bigger, and can
      under some circumstances lead to problems: probing for a
      nonexistent controller card may confuse your other controllers.

    - A kernel with math-emulation compiled in will still use the
      coprocessor if one is present: the math emulation will just
      never get used in that case.  The kernel will be slightly larger,
      but will work on different machines regardless of whether they
      have a math coprocessor or not.

    - The "kernel hacking" configuration details usually result in a
      bigger or slower kernel (or both), and can even make the kernel
      less stable by configuring some routines to actively try to
      break bad code to find kernel problems (kmalloc()).  Thus you
      should probably answer 'n' to the questions for "development",
      "experimental", or "debugging" features.

Compiling the kernel
--------------------

 - Make sure you have at least gcc 5.1 available.
   For more information, refer to :ref:`Documentation/process/changes.rst <changes>`.

   Please note that you can still run a.out user programs with this kernel.

 - Do a ``make`` to create a compressed kernel image. It is also
   possible to do ``make install`` if you have lilo installed to suit the
   kernel makefiles, but you may want to check your particular lilo setup first.

   To do the actual install, you have to be root, but none of the normal
   build should require that. Don't take the name of root in vain.

 - If you configured any of the parts of the kernel as ``modules``, you
   will also have to do ``make modules_install``.

 - Verbose kernel compile/build output:

   Normally, the kernel build system runs in a fairly quiet mode (but not
   totally silent).  However, sometimes you or other kernel developers need
   to see compile, link, or other commands exactly as they are executed.
   For this, use "verbose" build mode.  This is done by passing
   ``V=1`` to the ``make`` command, e.g.::

     make V=1 all

   To have the build system also tell the reason for the rebuild of each
   target, use ``V=2``.  The default is ``V=0``.

 - Keep a backup kernel handy in case something goes wrong.  This is
   especially true for the development releases, since each new release
   contains new code which has not been debugged.  Make sure you keep a
   backup of the modules corresponding to that kernel, as well.  If you
   are installing a new kernel with the same version number as your
   working kernel, make a backup of your modules directory before you
   do a ``make modules_install``.

   Alternatively, before compiling, use the kernel config option
   "LOCALVERSION" to append a unique suffix to the regular kernel version.
   LOCALVERSION can be set in the "General Setup" menu.

 - In order to boot your new kernel, you'll need to copy the kernel
   image (e.g. .../linux/arch/x86/boot/bzImage after compilation)
   to the place where your regular bootable kernel is found.

 - Booting a kernel directly from a floppy without the assistance of a
   bootloader such as LILO, is no longer supported.

   If you boot Linux from the hard drive, chances are you use LILO, which
   uses the kernel image as specified in the file /etc/lilo.conf.  The
   kernel image file is usually /vmlinuz, /boot/vmlinuz, /bzImage or
   /boot/bzImage.  To use the new kernel, save a copy of the old image
   and copy the new image over the old one.  Then, you MUST RERUN LILO
   to update the loading map! If you don't, you won't be able to boot
   the new kernel image.

   Reinstalling LILO is usually a matter of running /sbin/lilo.
   You may wish to edit /etc/lilo.conf to specify an entry for your
   old kernel image (say, /vmlinux.old) in case the new one does not
   work.  See the LILO docs for more information.

   After reinstalling LILO, you should be all set.  Shutdown the system,
   reboot, and enjoy!

   If you ever need to change the default root device, video mode,
   etc. in the kernel image, use your bootloader's boot options
   where appropriate.  No need to recompile the kernel to change
   these parameters.

 - Reboot with the new kernel and enjoy.

If something goes wrong
-----------------------

 - If you have problems that seem to be due to kernel bugs, please check
   the file MAINTAINERS to see if there is a particular person associated
   with the part of the kernel that you are having trouble with. If there
   isn't anyone listed there, then the second best thing is to mail
   them to me (torvalds@linux-foundation.org), and possibly to any other
   relevant mailing-list or to the newsgroup.

 - In all bug-reports, *please* tell what kernel you are talking about,
   how to duplicate the problem, and what your setup is (use your common
   sense).  If the problem is new, tell me so, and if the problem is
   old, please try to tell me when you first noticed it.

 - If the bug results in a message like::

     unable to handle kernel paging request at address C0000010
     Oops: 0002
     EIP:   0010:XXXXXXXX
     eax: xxxxxxxx   ebx: xxxxxxxx   ecx: xxxxxxxx   edx: xxxxxxxx
     esi: xxxxxxxx   edi: xxxxxxxx   ebp: xxxxxxxx
     ds: xxxx  es: xxxx  fs: xxxx  gs: xxxx
     Pid: xx, process nr: xx
     xx xx xx xx xx xx xx xx xx xx

   or similar kernel debugging information on your screen or in your
   system log, please duplicate it *exactly*.  The dump may look
   incomprehensible to you, but it does contain information that may
   help debugging the problem.  The text above the dump is also
   important: it tells something about why the kernel dumped code (in
   the above example, it's due to a bad kernel pointer). More information
   on making sense of the dump is in Documentation/admin-guide/bug-hunting.rst

 - If you compiled the kernel with CONFIG_KALLSYMS you can send the dump
   as is, otherwise you will have to use the ``ksymoops`` program to make
   sense of the dump (but compiling with CONFIG_KALLSYMS is usually preferred).
   This utility can be downloaded from
   https://www.kernel.org/pub/linux/utils/kernel/ksymoops/ .
   Alternatively, you can do the dump lookup by hand:

 - In debugging dumps like the above, it helps enormously if you can
   look up what the EIP value means.  The hex value as such doesn't help
   me or anybody else very much: it will depend on your particular
   kernel setup.  What you should do is take the hex value from the EIP
   line (ignore the ``0010:``), and look it up in the kernel namelist to
   see which kernel function contains the offending address.

   To find out the kernel function name, you'll need to find the system
   binary associated with the kernel that exhibited the symptom.  This is
   the file 'linux/vmlinux'.  To extract the namelist and match it against
   the EIP from the kernel crash, do::

     nm vmlinux | sort | less

   This will give you a list of kernel addresses sorted in ascending
   order, from which it is simple to find the function that contains the
   offending address.  Note that the address given by the kernel
   debugging messages will not necessarily match exactly with the
   function addresses (in fact, that is very unlikely), so you can't
   just 'grep' the list: the list will, however, give you the starting
   point of each kernel function, so by looking for the function that
   has a starting address lower than the one you are searching for but
   is followed by a function with a higher address you will find the one
   you want.  In fact, it may be a good idea to include a bit of
   "context" in your problem report, giving a few lines around the
   interesting one.

   If you for some reason cannot do the above (you have a pre-compiled
   kernel image or similar), telling me as much about your setup as
   possible will help.  Please read
   'Documentation/admin-guide/reporting-issues.rst' for details.

 - Alternatively, you can use gdb on a running kernel. (read-only; i.e. you
   cannot change values or set break points.) To do this, first compile the
   kernel with -g; edit arch/x86/Makefile appropriately, then do a ``make
   clean``. You'll also need to enable CONFIG_PROC_FS (via ``make config``).

   After you've rebooted with the new kernel, do ``gdb vmlinux /proc/kcore``.
   You can now use all the usual gdb commands. The command to look up the
   point where your system crashed is ``l *0xXXXXXXXX``. (Replace the XXXes
   with the EIP value.)

   gdb'ing a non-running kernel currently fails because ``gdb`` (wrongly)
   disregards the starting offset for which the kernel is compiled.