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7. Configuring the Kernel


7.a. Timezone

You first need to select your timezone so that your system knows where it is located. Look for your timezone in /usr/share/zoneinfo, then copy it to /etc/localtime. Please avoid the /usr/share/zoneinfo/Etc/GMT* timezones as their names do not indicate the expected zones. For instance, GMT-8 is in fact GMT+8.

Code Listing 1.1: Setting the timezone information

# ls /usr/share/zoneinfo
(Suppose you want to use GMT)
# cp /usr/share/zoneinfo/GMT /etc/localtime

7.b. Installing the Sources

Choosing a Kernel

The core around which all distributions are built is the Linux kernel. It is the layer between the user programs and your system hardware. Gentoo provides its users several possible kernel sources. A full listing with description is available at the Gentoo Kernel Guide.

For AMD64-based systems we have gentoo-sources (kernel v2.6 source patched with amd64 specific fixes for stability, performance and hardware support).

Choose your kernel source and install it using emerge. The USE="-doc" is necessary to avoid installing xorg-x11 or other dependencies at this point. USE="symlink" is not necessary for a new install, but ensures proper creation of the /usr/src/linux symlink.

Code Listing 2.1: Installing a kernel source

# USE="-doc symlink" emerge gentoo-sources

When you take a look in /usr/src you should see a symlink called linux pointing to your kernel source. In this case, the installed kernel source points to gentoo-sources-2.6.12-r10. Your version may be different, so keep this in mind.

Code Listing 2.2: Viewing the kernel source symlink

# ls -l /usr/src/linux
lrwxrwxrwx    1 root    root       12 Jul 28 11:04 /usr/src/linux -> linux-2.6.12-gentoo-r10

Now it is time to configure and compile your kernel source. You can use genkernel for this, which will build a generic kernel as used by the Installation CD. We explain the "manual" configuration first though, as it is the best way to optimize your environment.

If you want to manually configure your kernel, continue now with Default: Manual Configuration. If you want to use genkernel you should read Alternative: Using genkernel instead.

7.c. Default: Manual Configuration


Manually configuring a kernel is often seen as the most difficult procedure a Linux user ever has to perform. Nothing is less true -- after configuring a couple of kernels you don't even remember that it was difficult ;)

However, one thing is true: you must know your system when you start configuring a kernel manually. Most information can be gathered by emerging pciutils (emerge pciutils) which contains lspci. You will now be able to use lspci within the chrooted environment. You may safely ignore any pcilib warnings (like pcilib: cannot open /sys/bus/pci/devices) that lspci throws out. Alternatively, you can run lspci from a non-chrooted environment. The results are the same. You can also run lsmod to see what kernel modules the Installation CD uses (it might provide you with a nice hint on what to enable).

Now go to your kernel source directory and execute make menuconfig. This will fire up an ncurses-based configuration menu.

Code Listing 3.1: Invoking menuconfig

# cd /usr/src/linux
# make menuconfig

You will be greeted with several configuration sections. We'll first list some options you must activate (otherwise Gentoo will not function, or not function properly without additional tweaks).

Activating Required Options

First of all, activate the use of development and experimental code/drivers. You need this, otherwise some very important code/drivers won't show up:

Code Listing 3.2: Selecting experimental code/drivers, General setup

Code maturity level options --->
  [*] Prompt for development and/or incomplete code/drivers
General setup --->
  [*] Support for hot-pluggable devices

Then we select the exact processor type. The x86_64 kernel maintainer strongly recommends users enable MCE features so that they are able to be notified of any hardware problems. On x86_64, these errors are not printed to dmesg like on other architectures, but to /dev/mcelog. This requires the app-admin/mcelog package.

Code Listing 3.3: Selecting processor type and features

(Choose appropriate)
Processor type and features  --->
   [ ] Intel MCE Features
   [ ] AMD MCE Features
  Processor family (AMD-Opteron/Athlon64)  --->
    ( ) AMD-Opteron/Athlon64
    ( ) Intel EM64T
    ( ) Generic-x86-64"

Now go to File Systems and select support for the filesystems you use. Don't compile them as modules, otherwise your Gentoo system will not be able to mount your partitions. Also select Virtual memory and /proc file system.

Code Listing 3.4: Selecting necessary file systems

File systems --->
  Pseudo Filesystems --->
    <*> /proc file system support
    <*> Virtual memory file system support (former shm fs)

(Select one or more of the following options as needed by your system)
  <*> Reiserfs support
  <*> Ext3 journalling file system support
  <*> JFS filesystem support
  <*> Second extended fs support
  <*> XFS filesystem support

If you are using PPPoE to connect to the Internet or you are using a dial-up modem, you will need the following options in the kernel:

Code Listing 3.5: Selecting PPPoE necessary drivers

Device Drivers --->
  Networking Support --->
    <*> PPP (point-to-point protocol) support
    <*>   PPP support for async serial ports
    <*>   PPP support for sync tty ports

The two compression options won't harm but are not definitely needed, neither does the PPP over Ethernet option, that might only be used by rp-pppoe when configured to do kernel mode PPPoE.

If you require it, don't forget to include support in the kernel for your ethernet card.

If you have a multi-CPU Opteron or a multi-core (e.g. AMD64 X2) system, you should activate "Symmetric multi-processing support":

Code Listing 3.6: Activating SMP support

Processor type and features --->
  [*] Symmetric multi-processing support

Note: In multi-core systems, each core counts as one processor.

If you use USB Input Devices (like Keyboard or Mouse) don't forget to enable those as well:

Code Listing 3.7: Activating USB Support for Input Devices

Device Drivers --->
  USB Support --->
  <*>   USB Human Interface Device (full HID) support
  [*]   HID input layer support 

If you want to run a multilib-enabled system (which is the default), you should make sure that support for 32bit executables is activated:

Code Listing 3.8: Activating 32-Bit Emulation Support

Executable file formats / Emulations --->
  [*] IA32 Emulation
  [*]   IA32 a.out support

When you've finished configuring the kernel, continue with Compiling and Installing.

Compiling and Installing

Now that your kernel is configured, it is time to compile and install it. Exit the configuration and start the compilation process:

Code Listing 3.9: Compiling the kernel

# make && make modules_install

When the kernel has finished compiling, copy the kernel image to /boot. Use whatever name you feel is appropriate for your kernel choice and remember it as you will need it later on when you configure your bootloader. Remember to replace <kernel-version> with the name and version of your kernel.

Code Listing 3.10: Installing the kernel

# cp arch/x86_64/boot/bzImage /boot/<kernel-version>

Now continue with Configuring Kernel Modules.

7.d. Alternative: Using genkernel

If you are reading this section, you have chosen to use our genkernel script to configure your kernel for you.

Now that your kernel source tree is installed, it's now time to compile your kernel by using our genkernel script to automatically build a kernel for you. genkernel works by configuring a kernel nearly identically to the way our Installation CD kernel is configured. This means that when you use genkernel to build your kernel, your system will generally detect all your hardware at boot-time, just like our Installation CD does. Because genkernel doesn't require any manual kernel configuration, it is an ideal solution for those users who may not be comfortable compiling their own kernels.

Now, let's see how to use genkernel. First, emerge the genkernel ebuild:

Code Listing 4.1: Emerging genkernel

# emerge genkernel

Now, compile your kernel sources by running genkernel all. Be aware though, as genkernel compiles a kernel that supports almost all hardware, this compilation will take quite a while to finish!

Note that, if your boot partition doesn't use ext2 or ext3 as filesystem you need to manually configure your kernel using genkernel --menuconfig all and add support for your filesystem in the kernel (i.e. not as a module).

Code Listing 4.2: Running genkernel

# genkernel all

Once genkernel completes, a kernel, full set of modules and initial root disk (initrd) will be created. We will use the kernel and initrd when configuring a boot loader later in this document. Write down the names of the kernel and initrd as you will need it when writing the bootloader configuration file. The initrd will be started immediately after booting to perform hardware autodetection (just like on the Installation CD) before your "real" system starts up.

Code Listing 4.3: Checking the created kernel image name and initrd

# ls /boot/kernel* /boot/initramfs*

Now, let's perform one more step to get our system to be more like the Installation CD -- let's emerge coldplug. While the initrd autodetects hardware that is needed to boot your system, coldplug autodetects everything else. To emerge and enable coldplug, type the following:

Code Listing 4.4: Emerging and enabling coldplug

# emerge coldplug
# rc-update add coldplug boot

7.e. Configuring Kernel Modules

Configuring the Modules

You should list the modules you want automatically loaded in /etc/modules.autoload.d/kernel-2.4 (or kernel-2.6). You can add extra options to the modules too if you want.

To view all available modules, run the following find command. Don't forget to substitute "<kernel version>" with the version of the kernel you just compiled:

Code Listing 5.1: Viewing all available modules

# find /lib/modules/<kernel version>/ -type f -iname '*.o' -or -iname '*.ko'

For instance, to automatically load the 3c59x.o module, edit the kernel-2.6 file and enter the module name in it.

Code Listing 5.2: Editing /etc/modules.autoload.d/kernel-2.6

# nano -w /etc/modules.autoload.d/kernel-2.6

Code Listing 5.3: /etc/modules.autoload.d/kernel-2.6


Continue the installation with Configuring your System.

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Page updated August 9, 2006

Summary: The Linux kernel is the core of every distribution. This chapter explains how to configure your kernel.

Sven Vermeulen

Roy Marples

Daniel Robbins

Chris Houser

Jerry Alexandratos

Seemant Kulleen
Gentoo x86 Developer

Tavis Ormandy
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Jason Huebel
Gentoo AMD64 Developer

Guy Martin
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Joe Kallar
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