Annotation of wikisrc/guide/kernel.mdwn, revision 1.2

1.1       jdf         1: # Compiling the kernel
                      3: Most NetBSD users will sooner or later want to recompile their kernel, or
                      4: compile a customized kernel. This might be for several reasons:
                      6:  * you can install bug-fixes, security updates, or new functionality by
                      7:    rebuilding the kernel from updated sources.
                      8:  * by removing unused device drivers and kernel sub-systems from your
                      9:    configuration, you can dramatically reduce kernel size and, therefore, memory
                     10:    usage.
                     11:  * by enabling optimisations more specific to your hardware, or tuning the
                     12:    system to match your specific sizing and workload, you can improve
                     13:    performance.
                     14:  * you can access additional features by enabling kernel options or sub-systems,
                     15:    some of which are experimental or disabled by default.
                     16:  * you can solve problems of detection/conflicts of peripherals.
                     17:  * you can customize some options (for example keyboard layout, BIOS clock
                     18:    offset, ...)
                     19:  * you can get a deeper knowledge of the system.
                     21: ## Requirements and procedure
                     23: To recompile the kernel you must have installed the compiler set (`comp.tgz`).
                     25: The basic steps to an updated or customised kernel then are:
                     27:  1. Install or update the kernel sources
                     28:  2. Create or modify the kernel configuration file
                     29:  3. Building the kernel from the configuration file, either manually or using
                     30:     ``
                     31:  4. Install the kernel
                     34: ## Installing the kernel sources
                     36: You can get the kernel sources from AnonCVS (see [[Obtaining the
                     37: sources|guide/fetch]]), or from the `syssrc.tgz` tarball that is located in the
                     38: `source/sets/` directory of the release that you are using.
                     40: If you chose to use AnonCVS to fetch the entire source tree, be patient, the
                     41: operation can last many minutes, because the repository contains thousands of
                     42: files.
                     44: If you have a source tarball, you can extract it as root:
                     46:     # cd /
                     47:     # tar zxf /path/to/syssrc.tgz
                     49: Even if you used the tarball from the release, you may wish to use AnonCVS to
                     50: update the sources with changes that have been applied since the release. This
                     51: might be especially relevant if you are updating the kernel to include the fix
                     52: for a specific bug, including a vulnerability described in a NetBSD Security
                     53: Advisory. You might want to get the latest sources on the relevant release or
                     54: critical updates branch for your version, or Security Advisories will usually
                     55: contain information on the dates or revisions of the files containing the
                     56: specific fixes concerned. See [[Fetching by CVS|guide/fetch#cvs]] for more
                     57: details on the CVS commands used to update sources from these branches.
                     59: Once you have the sources available, you can create a custom kernel: this is not
                     60: as difficult as you might think. In fact, a new kernel can be created in a few
                     61: steps which will be described in the following sections.
                     63: ## Creating the kernel configuration file
                     65: The directories described in this section are i386 specific. Users of other
                     66: architectures must substitute the appropriate directories, see the
                     67: subdirectories of `src/sys/arch` for a list.
                     69: The kernel configuration file defines the type, the number and the
                     70: characteristics of the devices supported by the kernel as well as several kernel
                     71: configuration options. For the i386 port, kernel configuration files are located
                     72: in the `/usr/src/sys/arch/i386/conf` directory.
                     74: Please note that the names of the kernel configuration files are historically in
                     75: all uppercase, so they are easy to distinguish from other files in that
                     76: directory:
                     78:     $ cd /usr/src/sys/arch/i386/conf/
                     79:     $ ls
                     80:     CARDBUS                 GENERIC_PS2TINY         NET4501
                     81:     CVS                     GENERIC_TINY            SWINGER
                     82:     DELPHI                  GENERIC_VERIEXEC        SWINGER.MP
                     83:     DISKLESS                INSTALL                 VIRTUALPC
                     84:     GENERIC                 INSTALL.MP              files.i386
                     85:     GENERIC.FAST_IPSEC      INSTALL_LAPTOP          kern.ldscript
                     86:     GENERIC.MP              INSTALL_PS2             kern.ldscript.4MB
                     87:     GENERIC.MPDEBUG         INSTALL_SMALL 
                     88:     GENERIC.local           INSTALL_TINY            majors.i386
                     89:     GENERIC_DIAGNOSTIC      IOPENER                 std.i386
                     90:     GENERIC_ISDN            LAMB
                     91:     GENERIC_LAPTOP          Makefile.i386
                     93: The easiest way to create a new file is to copy an existing one and modify it.
                     94: Usually the best choice on most platforms is the GENERIC configuration, as it
                     95: contains most drivers and options. In the configuration file there are comments
                     96: describing the options; a more detailed description is found in the
                     97: [options(4)](
                     98: man page. So, the usual procedure is:
                    100:     $ cp GENERIC MYKERNEL
                    101:     $ vi MYKERNEL
                    103: The modification of a kernel configuration file basically involves three operations:
                    105:  1. support for hardware devices is included/excluded in the kernel (for
                    106:     example, SCSI support can be removed if it is not needed.)
                    107:  2. support for kernel features is enabled/disabled (for example, enable NFS
                    108:     client support, enable Linux compatibility, ...)
                    109:  3. tuning kernel parameters.
                    111: Lines beginning with `#` are comments; lines are disabled by commenting them
                    112: and enabled by removing the comment character. It is better to comment lines
                    113: instead of deleting them; it is always possible uncomment them later.
                    115: The output of the
                    116: [dmesg(8)](
                    117: command can be used to determine which lines can be disabled. For each line of
                    118: the type:
                    120:     XXX at YYY
                    122: both `XXX` and `YYY` must be active in the kernel configuration file. You'll
                    123: probably have to experiment a bit before achieving a minimal configuration but
                    124: on a desktop system without SCSI and PCMCIA you can halve the kernel size.
                    126: You should also examine the options in the configuration file and disable the
                    127: ones that you don't need. Each option has a short comment describing it, which
                    128: is normally sufficient to understand what the option does. Many options have a
                    129: longer and more detailed description in the
                    130: [options(4)](
                    131: man page. While you are at it you should set correctly the options for local
                    132: time on the CMOS clock. For example:
                    134:     options RTC_OFFSET=-60
                    136: ## Building the kernel manually
                    138: Based on your kernel configuration file, either one of the standard
                    139: configurations or your customised configuration, a new kernel must be built.
                    141: These steps can either be performed manually, or using the `` command
                    142: that was introduced in section [Chapter 31, *Crosscompiling NetBSD with
                    143: ``*](chap-build.html "Chapter 31. Crosscompiling NetBSD with").
                    144: This section will give instructions on how to build a native kernel using manual
                    145: steps, the following section
                    146: [[Building the kernel using|guide/]]
                    147: describes how to use **** to do the same.
                    149:  * Configure the kernel
                    150:  * Generate dependencies
                    151:  * Compile the kernel
                    153: ### Configuring the kernel manually
                    155: When you've finished modifying the kernel configuration file (which we'll call
                    156: `MYKERNEL`), you should issue the following command:
                    158:     $ config MYKERNEL
                    160: If `MYKERNEL` contains no errors, the
                    161: [config(1)](
                    162: program will create the necessary files for the compilation of the kernel,
                    163: otherwise it will be necessary to correct the errors before running
                    164: [config(1)](
                    165: again.
                    167: ### Notes for crosscompilings
                    169: As the
                    170: [config(1)](
                    171: program used to create header files and Makefile for a kernel build is platform
                    172: specific, it is necessary to use the `nbconfig` program that's part of a newly
                    173: created toolchain (created for example with
                    175:     /usr/src/ -m sparc64 tools/
                    177: ). That aside, the procedure is just as like compiling a "native" NetBSD kernel.
                    178: The command is for example:
                    180:     % /usr/src/tooldir.NetBSD-4.0-i386/bin/nbconfig MYKERNEL
                    182: This command has created a directory `../compile/MYKERNEL` with a number of
                    183: header files defining information about devices to compile into the kernel, a
                    184: Makefile that is setup to build all the needed files for the kernel, and link
                    185: them together.
                    187: ### Generating dependencies and recompiling manually
                    189: Dependencies generation and kernel compilation is performed by the following
                    190: commands:
                    192:     $ cd ../compile/MYKERNEL
                    193:     $ make depend
                    194:     $ make
                    196: It can happen that the compilation stops with errors; there can be a variety of
                    197: reasons but the most common cause is an error in the configuration file which
                    198: didn't get caught by
                    199: [config(1)](
                    200: Sometimes the failure is caused by a hardware problem (often faulty RAM chips):
                    201: the compilation puts a higher stress on the system than most applications do.
                    202: Another typical error is the following: option B, active, requires option A
                    203: which is not active. A full compilation of the kernel can last from some minutes
                    204: to several hours, depending on the hardware.
                    206: The result of a successful make command is the `netbsd` file in the compile
                    207: directory, ready to be installed.
                    209: ### Notes for crosscompilings
                    211: For crosscompiling a sparc64 kernel, it is necessary to use the crosscompiler
                    212: toolchain's `nbmake-sparc64` shell wrapper, which calls
                    213: [make(1)]( with
                    214: all the necessary settings for crosscompiling for a sparc64 platform:
                    216:     % cd ../compile/MYKERNEL/
                    217:     % /usr/src/tooldir.NetBSD-4.0-i386/bin/nbmake-sparc64 depend
                    218:     % /usr/src/tooldir.NetBSD-4.0-i386/bin/nbmake-sparc64
                    220: This will churn away a bit, then spit out a kernel:
                    222:     ...
                    223:     text    data     bss     dec     hex filename
                    224:     5016899  163728  628752 5809379  58a4e3 netbsd
                    225:     % ls -l netbsd
                    226:     -rwxr-xr-x  1 feyrer  666  5874663 Dec  2 23:17 netbsd
                    227:     % file netbsd
                    228:     netbsd: ELF 64-bit MSB executable, SPARC V9, version 1 (SYSV), statically linked, not stripped
                    230: Now the kernel in the file `netbsd` can either be transferred to an UltraSPARC
                    231: machine (via NFS, FTP, scp, etc.) and booted from a possible harddisk, or
                    232: directly from the cross-development machine using NFS.
1.2     ! jdf       234: ## Building the kernel using
1.1       jdf       235: 
                    236: After creating and possibly editing the kernel config file, the manual steps of
                    237: configuring the kernel, generating dependencies and recompiling can also be done
                    238: using the `src/` script, all in one go:
                    240:     $ cd /usr/src
                    241:     $ ./ kernel=MYKERNEL
                    243: This will perform the same steps as above, with one small difference: before
                    244: compiling, all old object files will be removed, to start with a fresh build.
                    245: This is usually overkill, and it's fine to keep the old file and only rebuild
                    246: the ones whose dependencies have changed. To do this, add the `-u` option to
                    247: ``:
                    249:     $ cd /usr/src
                    250:     $ ./ -u kernel=MYKERNEL
                    252: At the end of its job, `` will print out the location where the new
                    253: compiled kernel can be found. It can then be installed.
                    255: ## Installing the new kernel
                    257: Whichever method was used to produce the new kernel file, it must now be
                    258: installed. The new kernel file should be copied to the root directory, after
                    259: saving the previous version.
                    261:     # mv /netbsd /netbsd.old
                    262:     # mv netbsd /
                    264: Customization can considerably reduce the kernel's size. In the following
                    265: example `netbsd.old` is the install kernel and `netbsd` is the new kernel.
                    267:     -rwxr-xr-x  3 root  wheel  3523098 Dec 10 00:13 /netbsd
                    268:     -rwxr-xr-x  3 root  wheel  7566271 Dec 10 00:13 /netbsd.old
                    270: The new kernel is activated after rebooting:
                    272:     # shutdown -r now
                    274: ## If something went wrong
                    276: When the computer is restarted it can happen that the new kernel doesn't work as
                    277: expected or even doesn't boot at all. Don't worry: if this happens, just reboot
                    278: with the previously saved kernel and remove the new one (it is better to reboot
                    279: `single user`):
                    281:  * Reboot the machine
                    282:  * Press the space bar at the boot prompt during the 5 seconds countdown
                    284:         boot:
                    286:  * Type
                    288:         > boot netbsd.old -s
                    290:  * Now issue the following commands to restore the previous version of the kernel:
                    292:         # fsck /
                    293:         # mount /
                    294:         # mv netbsd.old netbsd
                    295:         # reboot
                    297: This will give you back the working system you started with, and you can revise
                    298: your custom kernel config file to resolve the problem. In general, it's wise to
                    299: start with a GENERIC kernel first, and then make gradual changes.

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