Table of Contents
Most NetBSD users will sooner or later want to compile a customized kernel. This gives you several benefits:
you can dramatically reduce kernel size and, therefore, memory occupation (for example, from 2.5 MB to 1.2 MB). On NetBSD 2.0, compiling a custom kernel reduced the size from 7.5 MB to 3.5 MB.
you can improve performance.
you can tune the system.
you can solve problems of detection/conflicts of peripherals.
you can customize some options (for example keyboard layout, BIOS clock offset, ...)
you can get a deeper knowledge of the system.
tar zxf /path/to/syssrc.tgz
If you chose to use AnonCVS, be patient, the operation can last many minutes, because the repository contains hundreds of files.
Once you have the sources available, you can create a custom kernel: this is not as difficult as you might think. In fact, a new kernel can be created in a few steps which will be described in the following sections.
To recompile the kernel you must have installed the compiler set
The basic steps for kernel compilation then are:
Create/modify the kernel configuration file
Configure the kernel
Compile the kernel
Install the kernel
These steps can either be performed manually, or using the new
build.sh command that was introduced
in section Chapter 27, Crosscompiling NetBSD with
build.sh. This section will
first give instructions on how to build a native kernel using
manual steps, and then describe how to use
build.sh to do the same.
The directories described in this section are i386
specific. Users of other architectures must substitute the
appropriate directories, see the subdirectories of
src/sys/arch for a list.
The kernel configuration file defines the type, the number and
the characteristics of the devices supported by the kernel as
well as several kernel configuration options. For the i386
port, kernel configuration files are located in the
Please note that the names of the kernel configuration files are historically in all uppercase, so they are easy to distinguish from other files in that directory:
lsCARDBUS GENERIC_PS2TINY NET4501 CVS GENERIC_TINY SWINGER DELPHI GENERIC_VERIEXEC SWINGER.MP DISKLESS INSTALL VIRTUALPC GENERIC INSTALL.MP files.i386 GENERIC.FAST_IPSEC INSTALL_LAPTOP kern.ldscript GENERIC.MP INSTALL_PS2 kern.ldscript.4MB GENERIC.MPDEBUG INSTALL_SMALL largepages.inc GENERIC.local INSTALL_TINY majors.i386 GENERIC_DIAGNOSTIC IOPENER std.i386 GENERIC_ISDN LAMB GENERIC_LAPTOP Makefile.i386
The easiest way to create a new file is to copy an existing one and modify it. Usually the best choice on most platforms is the GENERIC configuration, as it contains most drivers and options. In the configuration file there are comments describing the options; a more detailed description is found in the options(4) man page. So, the usual procedure is:
The modification of a kernel configuration file basically involves three operations:
support for hardware devices is included/excluded in the kernel (for example, SCSI support can be removed if it is not needed.)
support for kernel features is enabled/disabled (for example, enable NFS client support, enable Linux compatibility, ...)
tuning kernel parameters.
Lines beginning with “#” are comments; lines are disabled by commenting them and enabled by removing the comment character. It is better to comment lines instead of deleting them; it is always possible uncomment them later.
The output of the dmesg(8) command can be used to determine which lines can be disabled. For each line of the type:
YYY must be active in the kernel
You'll probably have to experiment a bit before achieving a minimal
configuration but on a desktop system without SCSI and PCMCIA you can
halve the kernel size.
You should also examine the options in the configuration file and disable the ones that you don't need. Each option has a short comment describing it, which is normally sufficient to understand what the option does. Many options have a longer and more detailed description in the options(4) man page. While you are at it you should set correctly the options for local time on the CMOS clock. For example:
The adjustkernel Perl script, which is available through pkgsrc, analyzes the output of dmesg(8) and automatically generates a minimal configuration file. The installation of packages is described extensively in in the Chapter 30, The package collection, but installing adjustkernel basically boils down to:
You can now run the script with:
adjustkernel GENERIC >
This script usually works very well, saving a lot of manual editing. But be aware that the script only configures the available devices: you must still configure the other options manually.
When you've finished modifying the kernel configuration file (which
MYKERNEL), you should issue the
MYKERNEL contains no errors, the
config(8) program will create the necessary files for
the compilation of the kernel, otherwise it will be necessary to correct
the errors before running config(8) again.
Dependencies generation and kernel compilation is performed by the following commands:
It can happen that the compilation stops with errors; there can be a variety of reasons but the most common cause is an error in the configuration file which didn't get caught by config(8). Sometimes the failure is caused by a hardware problem (often faulty RAM chips): the compilation puts a higher stress on the system than most applications do. Another typical error is the following: option B, active, requires option A which is not active. A full compilation of the kernel can last from some minutes to several hours, depending on the hardware.
The output of the make command is the
in the compile directory: this file should be copied in the root
directory, after saving the previous version.
mv /netbsd /netbsd.old
mv netbsd /
Customization can considerably reduce the kernel's size.
In the following example
netbsd.old is the
install kernel and
netbsd is the new kernel.
-rwxr-xr-x 3 root wheel 3523098 Dec 10 00:13 /netbsd -rwxr-xr-x 3 root wheel 7566271 Dec 10 00:13 /netbsd.old
The new kernel is activated after rebooting:
shutdown -r now
After creating and possibly editing the kernel config file, the
manual steps of configuring the kernel, generating dependencies
and recompiling can also be done using the
src/build.sh script, all in one go:
This will perform the same steps as above, with one small
difference: before compiling, all old object files will be
removed, to start with a fresh build. This is usually overkill,
and it's fine to keep the old file and only rebuild the ones
whose dependencies have changed. To do this, add the
-u option to
./build.sh -u kernel=MYKERNEL
At the end of its job,
build.sh will print
out the location where the new compiled kernel can be found. It
can then be installed with the steps given above in Section 28.5, “Generating dependencies and recompiling manually”.
When the computer is restarted it can happen that the new kernel doesn't work as expected or even doesn't boot at all. Don't worry: if this happens, just reboot with the previously saved kernel and remove the new one (it is better to reboot “single user”):
Reboot the machine
Press the space bar at the boot prompt during the 5 seconds countdown
boot netbsd.old -s
Now issue the following commands to restore the previous version of the kernel:
mv netbsd.old netbsd
This will give you back the working system you started with, and you can change the kernel config file next to make it really going. In general, it's wise to start with a GENERIC kernel first, and then make gradual changes.