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
11: * by enabling optimisations more specific to your hardware, or tuning the
12: system to match your specific sizing and workload, you can improve
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
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
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
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 largepages.inc
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
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
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
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 `build.sh` command
142: that was introduced in section [Chapter 31, *Crosscompiling NetBSD with
143: `build.sh`*](chap-build.html "Chapter 31. Crosscompiling NetBSD with build.sh").
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 build.sh|guide/kernel#building_the_kernel_using_build.sh]]
147: describes how to use **build.sh** 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
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
167: ### Notes for crosscompilings
169: As the
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/build.sh -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
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
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)](http://netbsd.gw.com/cgi-bin/man-cgi?make+1+NetBSD-5.0.1+i386) 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:
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.
234: ## Building the kernel using build.sh
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/build.sh` script, all in one go:
240: $ cd /usr/src
241: $ ./build.sh 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
249: $ cd /usr/src
250: $ ./build.sh -u kernel=MYKERNEL
252: At the end of its job, `build.sh` 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
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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