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7. Configuring the Kernel

• Timezone
• Installing the Sources
• Default: Manual Configuration
• Alternative: Using genkernel
• Configuring Kernel Modules

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 make a symlink to /etc/localtime using ln:

# ls /usr/share/zoneinfo
(Suppose you want to use GMT)
# ln -sf /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.

# echo "=sys-kernel/gentoo-sources-2.6.11-r1 ~amd64" >> /etc/portage/package.keywords
# emerge =gentoo-sources-2.6.11-r1

When you take a look in /usr/src you should see a symlink called linux pointing to your kernel source:

# ls -l /usr/src/linux
lrwxrwxrwx    1 root     root           12 Oct 13 11:04 /usr/src/linux -> linux-2.6.11-gentoo-r1

If this isn't the case (i.e. the symlink points to a different kernel source) change the symlink before you continue:

# rm /usr/src/linux
# cd /usr/src
# ln -s linux-2.6.11-gentoo-r1 linux

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

Introduction

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.

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

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. Do not select /dev file system since 2005.0/AMD64 uses udev by default.

File systems --->
  Pseudo Filesystems --->
    <*> /proc file system support
    < > /dev file system support (OBSOLETE)
    <*> 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:

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 system, you should activate "Symmetric multi-processing support":

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

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

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

# make && make modules_install

When the kernel has finished compiling, copy the kernel image to /boot:

# cp arch/x86_64/boot/bzImage /boot/kernel-2.6.11-gentoo-r1

It is also wise to copy over your kernel configuration file to /boot, just in case :)

# cp .config /boot/config-2.6.11-gentoo-r1

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:

# 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).

# 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.

# ls /boot/kernel* /boot/initrd*

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:

# 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:

# 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.

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

3c59x

Continue the installation with Configuring your System.

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