Basic Kernel Configuration (Unix Power Tools, 3rd Edition)

44.3. Basic Kernel Configuration

Generally a Unix kernel is made up of some core, which handles fundamental functionality like virtual memory, and a lot of modules for various devices. A kernel configuration file is used to build a kernel and, on some platforms, a set of loadable kernel modules.

A kernel configuration file has a list of kernel options and then a list of devices and device options. The kernel build process uses this file to determine exactly what to build; this way you can have a kernel that supports exactly the hardware you have in your machine but isn't using any extra resources to support hardware you don't have.

Some example device lines from various kernel configuration files:

#
# FreeBSD samples
#
maxusers        128
options         INCLUDE_CONFIG_FILE
options         INET                    #InterNETworking
device          isa
device          pci
device          ata0    at isa? port IO_WD1 irq 14
device          ata
device          atadisk                 # ATA disk drives
device          atapicd                 # ATAPI CDROM drives
device          atapifd                 # ATAPI floppy drives
device          atapist                 # ATAPI tape drives
options         ATA_STATIC_ID           #Static device numbering

#
# Linux samples
#
# Loadable module support
CONFIG_MODULES=y
CONFIG_MODVERSIONS=y
# CONFIG_KMOD is not set

# General setup
CONFIG_NET=y
CONFIG_PCI=y

# Block devices
CONFIG_BLK_DEV_FD=m
CONFIG_BLK_DEV_IDE=y
# CONFIG_BLK_DEV_HD_IDE is not set
CONFIG_BLK_DEV_IDEDISK=y
CONFIG_BLK_DEV_IDECD=m
CONFIG_BLK_DEV_IDETAPE=m
CONFIG_BLK_DEV_IDEFLOPPY=m
# CONFIG_BLK_DEV_IDESCSI is not set
CONFIG_BLK_DEV_IDEPCI=y
CONFIG_BLK_DEV_IDEDMA=y
CONFIG_IDEDMA_AUTO=y

The kernel build process involves setting up an appropriate configuration file for your platform and then using a tool (generally config(8); check the manpage) to create a kernel build setup from the configuration file. Then you simply run make within the kernel build setup and you have a new kernel. Once the new kernel is installed, you reboot the machine, and poof, you're running on a sleek new customized kernel.

To understand how to configure the kernel on your platform, consult the documentation for that platform. Note that many platforms have tools or even GUIs for helping you configure your kernel. For the free Unixes, search the Web. There are extensive HOWTOs available describing how to configure your kernel in excruciating detail.

Linux has a very detailed HOWTO for kernel configuration at http://www.tldp.org/HOWTO/Kernel-HOWTO.html. The short version is that the configuration file mentioned above is stored in the .config file at the top of the kernel source tree (usually /usr/src/linux). Generally you don't have to edit it directly; instead you'd use make menuconfig or make xconfig, again at the top of the kernel source tree, to use the fancy kernel configuration tools.

-- DJPH