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