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|
<?xml version="1.0" encoding="ISO8859-1" standalone="no"?>
<!--
The FreeBSD Documentation Project
$FreeBSD$
-->
<chapter id="kernelconfig">
<chapterinfo>
<authorgroup>
<author>
<firstname>Jim</firstname>
<surname>Mock</surname>
<contrib>Updated and restructured by </contrib>
<!-- Mar 2000 -->
</author>
</authorgroup>
<authorgroup>
<author>
<firstname>Jake</firstname>
<surname>Hamby</surname>
<contrib>Originally contributed by </contrib>
<!-- 6 Oct 1995 -->
</author>
</authorgroup>
</chapterinfo>
<title>Configuring the FreeBSD Kernel</title>
<sect1 id="kernelconfig-synopsis">
<title>Synopsis</title>
<indexterm>
<primary>kernel</primary>
<secondary>building a custom kernel</secondary>
</indexterm>
<para>The kernel is the core of the &os; operating system. It is
responsible for managing memory, enforcing security controls,
networking, disk access, and much more. While more and more of &os;
becomes dynamically configurable it is still occasionally necessary to
reconfigure and recompile your kernel.</para>
<para>After reading this chapter, you will know:</para>
<itemizedlist>
<listitem>
<para>Why you might need to build a custom kernel.</para>
</listitem>
<listitem>
<para>How to write a kernel configuration file, or alter an existing
configuration file.</para>
</listitem>
<listitem>
<para>How to use the kernel configuration file to create and build a
new kernel.</para>
</listitem>
<listitem>
<para>How to install the new kernel.</para>
</listitem>
<listitem>
<para>How to troubleshoot if things go wrong.</para>
</listitem>
</itemizedlist>
<para>All of the commands listed within this chapter by way of example
should be executed as <username>root</username> in order to
succeed.</para>
</sect1>
<sect1 id="kernelconfig-custom-kernel">
<title>Why Build a Custom Kernel?</title>
<para>Traditionally, &os; has had what is called a
<quote>monolithic</quote> kernel. This means that the kernel was one
large program, supported a fixed list of devices, and if you wanted to
change the kernel's behavior then you had to compile a new kernel, and
then reboot your computer with the new kernel.</para>
<para>Today, &os; is rapidly moving to a model where much of the
kernel's functionality is contained in modules which can be
dynamically loaded and unloaded from the kernel as necessary.
This allows the kernel to adapt to new hardware suddenly
becoming available (such as PCMCIA cards in a laptop), or for
new functionality to be brought into the kernel that was not
necessary when the kernel was originally compiled. This is
known as a modular kernel.</para>
<para>Despite this, it is still necessary to carry out some static kernel
configuration. In some cases this is because the functionality is so
tied to the kernel that it can not be made dynamically loadable. In
others it may simply be because no one has yet taken the time to write a
dynamic loadable kernel module for that functionality.</para>
<para>Building a custom kernel is one of the most important rites of
passage for advanced BSD users. This process, while
time consuming, will provide many benefits to your &os; system.
Unlike the <filename>GENERIC</filename> kernel, which must support a
wide range of hardware, a custom kernel only contains support for
<emphasis>your</emphasis> PC's hardware. This has a number of
benefits, such as:</para>
<itemizedlist>
<listitem>
<para>Faster boot time. Since the kernel will only probe the
hardware you have on your system, the time it takes your system to
boot can decrease dramatically.</para>
</listitem>
<listitem>
<para>Lower memory usage. A custom kernel often uses less memory
than the <filename>GENERIC</filename> kernel by omitting unused
features and device drivers. This is important because the kernel
code remains resident in physical memory at all times, preventing
that memory from being used by applications.
For this reason, a custom kernel is especially useful
on a system with a small amount of RAM.</para>
</listitem>
<listitem>
<para>Additional hardware support. A custom kernel allows you to
add in support for devices which are not
present in the <filename>GENERIC</filename> kernel, such as
sound cards.</para>
</listitem>
</itemizedlist>
</sect1>
<sect1 id="kernelconfig-devices">
<sect1info>
<authorgroup>
<author>
<firstname>Tom</firstname>
<surname>Rhodes</surname>
<contrib>Written by </contrib>
</author>
</authorgroup>
</sect1info>
<title>Finding the System Hardware</title>
<para>Before venturing into kernel configuration, it would be wise
to get an inventory of the machine's hardware. In cases where
&os; is not the primary operating system, the inventory list may
easily be created by viewing the current operating system
configuration. For example, µsoft;'s
<application>Device Manager</application> normally contains
important information about installed devices. The
<application>Device Manager</application> is located in the
control panel.</para>
<note>
<para>Some versions of µsoft.windows; have a
<application>System</application> icon which will display a
screen where <application>Device Manager</application> may
be accessed.</para>
</note>
<para>If another operating system does not exist on the machine,
the administrator must find this information out manually. One
method is using the &man.dmesg.8; utility and the &man.man.1;
commands. Most device drivers on &os; have a manual page, listing
supported hardware, and during the boot probe, found hardware
will be listed. For example, the following lines indicate that
the <devicename>psm</devicename> driver found a mouse:</para>
<programlisting>psm0: <PS/2 Mouse> irq 12 on atkbdc0
psm0: [GIANT-LOCKED]
psm0: [ITHREAD]
psm0: model Generic PS/2 mouse, device ID 0</programlisting>
<para>This driver will need to be included in the custom kernel
configuration file or loaded using &man.loader.conf.5;.</para>
<para>On occasion, the data from <command>dmesg</command> will
only show system messages instead of the boot probe output. In
these situations, the output may be obtained by viewing the
<filename>/var/run/dmesg.boot</filename> file.</para>
<para>Another method of finding hardware is by using the
&man.pciconf.8; utility which provides more verbose output.
For example:</para>
<programlisting>ath0@pci0:3:0:0: class=0x020000 card=0x058a1014 chip=0x1014168c rev=0x01 hdr=0x00
vendor = 'Atheros Communications Inc.'
device = 'AR5212 Atheros AR5212 802.11abg wireless'
class = network
subclass = ethernet</programlisting>
<para>This bit of output, obtained using
<command>pciconf <option>-lv</option></command> shows that the
<devicename>ath</devicename> driver located a wireless Ethernet
device. Using
<command>man <replaceable>ath</replaceable></command> will return
the &man.ath.4; manual page.</para>
<para>The <option>-k</option> flag, when passed to &man.man.1;
can also be used to provide useful information. From the
above, one can issue:</para>
<screen>&prompt.root; man -k <replaceable>Atheros</replaceable></screen>
<para>To get a list of manual pages which contain that particular
word:</para>
<programlisting>ath(4) - Atheros IEEE 802.11 wireless network driver
ath_hal(4) - Atheros Hardware Access Layer (HAL)</programlisting>
<para>Armed with a hardware inventory list, the process of building
a custom kernel should appear less daunting.</para>
</sect1>
<sect1 id="kernelconfig-modules">
<title>Kernel Drivers, Subsystems, and Modules</title>
<indexterm>
<primary>kernel</primary>
<secondary>drivers / modules / subsystems</secondary>
</indexterm>
<para>Before building a custom kernel, consider the reasons for
doing so. If there is a need for specific hardware support,
it may already exist as a module.</para>
<para>Kernel modules exist in the
<filename class="directory">/boot/kernel</filename> directory
and may be dynamically loaded into the running kernel using
&man.kldload.8;. Most, if not all kernel drivers have a
specific module and manual page. For example, the last section
noted the <devicename>ath</devicename> wireless Ethernet driver.
This device has the following information in its manual
page:</para>
<programlisting>Alternatively, to load the driver as a module at boot time, place the
following line in &man.loader.conf.5;:
if_ath_load="YES"</programlisting>
<para>As instructed, adding the <literal>if_ath_load="YES"</literal>
line to the <filename>/boot/loader.conf</filename> file will
enable loading this module dynamically at boot time.</para>
<para>In some cases; however, there is no associated module. This
is mostly true for certain subsystems and very important drivers,
for instance, the fast file system (<acronym>FFS</acronym>) is a
required option in the kernel. As is network support (INET).
Unfortunately the only way to tell if a driver is required is to
check for the module itself.</para>
<warning>
<para>It is easy to remove support for a
device or option and end up with a broken kernel. For example, if
the &man.ata.4; driver is removed from the kernel configuration
file, a system using <acronym>ATA</acronym> disk drivers may
not boot without the module added to
<filename>loader.conf</filename>. When in doubt, check for
the module and then just leave support in the kernel.</para>
</warning>
</sect1>
<sect1 id="kernelconfig-building">
<title>Building and Installing a Custom Kernel</title>
<indexterm>
<primary>kernel</primary>
<secondary>building / installing</secondary>
</indexterm>
<note>
<para>It is required to have the full &os; source tree installed
to build the kernel.</para>
</note>
<para>First, let us take a quick tour of the kernel build directory.
All directories mentioned will be relative to the main
<filename>/usr/src/sys</filename> directory, which is also
accessible through the path name <filename>/sys</filename>. There are a
number of subdirectories here representing different parts of the
kernel, but the most important for our purposes are
<filename><replaceable>arch</replaceable>/conf</filename>, where you
will edit your custom kernel configuration, and
<filename>compile</filename>, which is the staging area where your
kernel will be built. <replaceable>arch</replaceable> represents
one of <filename>i386</filename>,
<filename>amd64</filename>, <filename>ia64</filename>,
<filename>powerpc</filename>, <filename>sparc64</filename>, or
<filename>pc98</filename> (an alternative development branch of PC
hardware, popular in Japan). Everything inside a particular
architecture's directory deals with that architecture only; the rest
of the code is machine independent code common to all platforms to which
&os; could potentially be ported. Notice the logical organization of the
directory structure, with each supported device, file system, and
option in its own subdirectory.</para>
<para>The examples in this chapter assume that you are using the i386
architecture. If your system has a different architecture you need
to change the path names accordingly.</para>
<note>
<para>If the directory <filename>/usr/src/</filename> does not
exist on your system (or if it is empty), then the sources have
not been installed. The easiest way to install the full source
is to use &man.csup.1; as described in <xref
linkend="synching"/>. You should also create a symlink to
<filename class="directory">/usr/src/sys/</filename>:</para>
<screen>&prompt.root; <userinput>ln -s /usr/src/sys /sys</userinput></screen>
</note>
<para>Next, change to the
<filename><replaceable>arch</replaceable>/conf</filename> directory
and copy the <filename>GENERIC</filename> configuration file to the
name you want to give your kernel. For example:</para>
<screen>&prompt.root; <userinput>cd /usr/src/sys/<replaceable>i386</replaceable>/conf</userinput>
&prompt.root; <userinput>cp GENERIC <replaceable>MYKERNEL</replaceable></userinput></screen>
<para>Traditionally, this name is in all capital letters and, if you
are maintaining multiple &os; machines with different hardware,
it is a good idea to name it after your machine's hostname. We will
call it <filename><replaceable>MYKERNEL</replaceable></filename> for the
purpose of this example.</para>
<tip>
<para>Storing your kernel configuration file directly under
<filename>/usr/src</filename> can be a bad idea. If you are
experiencing problems it can be tempting to just delete
<filename>/usr/src</filename> and start again. After doing this,
it usually only takes a few seconds for
you to realize that you have deleted your custom kernel
configuration file. Also, do not edit <filename>GENERIC</filename>
directly, as it may get overwritten the next time you
<link linkend="updating-upgrading">update your source tree</link>, and
your kernel modifications will be lost.</para>
<para>You might want to keep your kernel configuration file
elsewhere, and then create a symbolic link to the file in
the <filename><replaceable>i386</replaceable></filename>
directory.</para>
<para>For example:</para>
<screen>&prompt.root; <userinput>cd /usr/src/sys/<replaceable>i386</replaceable>/conf</userinput>
&prompt.root; <userinput>mkdir /root/kernels</userinput>
&prompt.root; <userinput>cp GENERIC /root/kernels/<replaceable>MYKERNEL</replaceable></userinput>
&prompt.root; <userinput>ln -s /root/kernels/<replaceable>MYKERNEL</replaceable></userinput></screen>
</tip>
<para>Now, edit <filename><replaceable>MYKERNEL</replaceable></filename>
with your favorite text editor. If you are just starting out, the only
editor available will probably be <application>vi</application>, which
is too complex to explain here, but is covered well in many books in
the <link linkend="bibliography">bibliography</link>. However, &os; does
offer an easier editor called <application>ee</application> which, if
you are a beginner, should be your editor of choice. Feel free to
change the comment lines at the top to reflect your configuration or
the changes you have made to differentiate it from
<filename>GENERIC</filename>.</para>
<indexterm><primary>SunOS</primary></indexterm>
<para>If you have built a kernel under &sunos; or some other BSD
operating system, much of this file will be very familiar to you.
If you are coming from some other operating system such as DOS, on
the other hand, the <filename>GENERIC</filename> configuration file
might seem overwhelming to you, so follow the descriptions in the
<link linkend="kernelconfig-config">Configuration File</link>
section slowly and carefully.</para>
<note>
<para>If you <link
linkend="updating-upgrading">sync your source tree</link> with the
latest sources of the &os; project,
be sure to always check the file
<filename>/usr/src/UPDATING</filename> before you perform any update
steps. This file describes any important issues or areas
requiring special attention within the updated source code.
<filename>/usr/src/UPDATING</filename> always matches
your version of the &os; source, and is therefore more up to date
with new information than this handbook.</para>
</note>
<para>You must now compile the source code for the kernel.</para>
<procedure>
<title>Building a Kernel</title>
<note>
<para>It is required to have the full &os; source tree installed
to build the kernel.</para>
</note>
<step>
<para>Change to the <filename
class="directory">/usr/src</filename> directory:</para>
<screen>&prompt.root; <userinput>cd /usr/src</userinput></screen>
</step>
<step>
<para>Compile the kernel:</para>
<screen>&prompt.root; <userinput>make buildkernel KERNCONF=<replaceable>MYKERNEL</replaceable></userinput></screen>
</step>
<step>
<para>Install the new kernel:</para>
<screen>&prompt.root; <userinput>make installkernel KERNCONF=<replaceable>MYKERNEL</replaceable></userinput></screen>
</step>
</procedure>
<tip>
<para>By default, when you build a custom kernel,
<emphasis>all</emphasis> kernel modules will be rebuilt as well.
If you want to update a kernel faster or to build only custom
modules, you should edit <filename>/etc/make.conf</filename>
before starting to build the kernel:</para>
<programlisting>MODULES_OVERRIDE = linux acpi sound/sound sound/driver/ds1 ntfs</programlisting>
<para>This variable sets up a list of modules to build instead
of all of them.</para>
<programlisting>WITHOUT_MODULES = linux acpi sound ntfs</programlisting>
<para>This variable sets up a list of top level modules to exclude
from the build process. For other variables which you may find useful
in the process of building kernel, refer to &man.make.conf.5;
manual page.</para>
</tip>
<indexterm>
<primary><filename class="directory">/boot/kernel.old</filename></primary>
</indexterm>
<para>The new kernel will be copied to the <filename
class="directory">/boot/kernel</filename> directory as
<filename>/boot/kernel/kernel</filename> and the old kernel will be moved
to <filename>/boot/kernel.old/kernel</filename>. Now, shutdown the
system and reboot to use your new kernel. If something goes wrong, there
are some <link linkend="kernelconfig-trouble">troubleshooting</link>
instructions at the end of this chapter that you may find useful. Be
sure to read the section which explains how to recover in case your new
kernel <link linkend="kernelconfig-noboot">does not boot</link>.</para>
<note>
<para>Other files relating to the boot process, such as the boot
&man.loader.8; and configuration are stored in
<filename>/boot</filename>. Third party or custom modules
can be placed in <filename class="directory">/boot/kernel</filename>,
although users should be aware that keeping modules in sync with the
compiled kernel is very important. Modules not intended
to run with the compiled kernel may result in instability
or incorrectness.</para>
</note>
</sect1>
<sect1 id="kernelconfig-config">
<sect1info>
<authorgroup>
<author>
<firstname>Joel</firstname>
<surname>Dahl</surname>
<contrib>Updated by </contrib>
</author>
</authorgroup>
</sect1info>
<title>The Configuration File</title>
<indexterm>
<primary>kernel</primary>
<secondary>NOTES</secondary>
</indexterm>
<indexterm><primary>NOTES</primary></indexterm>
<indexterm>
<primary>kernel</primary>
<secondary>configuration file</secondary>
</indexterm>
<para>The general format of a configuration file is quite simple.
Each line contains a keyword and one or more arguments. For
simplicity, most lines only contain one argument. Anything
following a <literal>#</literal> is considered a comment and
ignored. The following sections describe each keyword, in
the order they are listed in <filename>GENERIC</filename>.
<anchor
id="kernelconfig-options"/> For an exhaustive list of architecture
dependent options and devices, see the <filename>NOTES</filename>
file in the same directory as the <filename>GENERIC</filename> file. For
architecture independent options, see
<filename>/usr/src/sys/conf/NOTES</filename>.</para>
<para>An <literal>include</literal> directive is
available for use in configuration files. This allows another
configuration file to be logically included in the current one, making
it easy to maintain small changes relative to an existing file. For
example, if you require a <filename>GENERIC</filename> kernel with
only a small number of additional options or drivers, this allows you
to maintain only a delta with respect to GENERIC:</para>
<programlisting>include GENERIC
ident MYKERNEL
options IPFIREWALL
options DUMMYNET
options IPFIREWALL_DEFAULT_TO_ACCEPT
options IPDIVERT
</programlisting>
<para>Many administrators will find that this model offers significant
benefits over the historic writing of configuration files from scratch:
the local configuration file will express only local differences from
a <filename>GENERIC</filename> kernel and as upgrades are performed,
new features added to <filename>GENERIC</filename> will be added to the
local kernel unless specifically prevented using
<literal>nooptions</literal> or <literal>nodevice</literal>. The
remainder of this chapter addresses the contents of a typical
configuration file and the role various options and devices
play.</para>
<note>
<para>To build a file which contains all available options,
as normally done for testing purposes, run the following
command as <username>root</username>:</para>
<screen>&prompt.root; <userinput>cd /usr/src/sys/<replaceable>i386</replaceable>/conf && make LINT</userinput></screen>
</note>
<indexterm>
<primary>kernel</primary>
<secondary>configuration file</secondary>
</indexterm>
<para>The following is an example of the <filename>GENERIC</filename>
kernel configuration file with various additional comments where needed
for clarity. This example should match your copy in
<filename>/usr/src/sys/<replaceable>i386</replaceable>/conf/GENERIC</filename>
fairly closely.</para>
<indexterm>
<primary>kernel options</primary>
<secondary>machine</secondary>
</indexterm>
<programlisting>machine i386</programlisting>
<para>This is the machine architecture. It must be either
<literal>amd64</literal>,
<literal>i386</literal>, <literal>ia64</literal>,
<literal>pc98</literal>, <literal>powerpc</literal>, or
<literal>sparc64</literal>.</para>
<indexterm>
<primary>kernel options</primary>
<secondary>cpu</secondary>
</indexterm>
<programlisting>cpu I486_CPU
cpu I586_CPU
cpu I686_CPU</programlisting>
<para>The above option specifies the type of CPU you have in your
system. You may have multiple instances of the CPU line (if, for
example, you are not sure whether you should use
<literal>I586_CPU</literal> or <literal>I686_CPU</literal>),
but for a custom kernel it is best to specify only the CPU
you have. If you are unsure of your CPU type, you can check the
<filename>/var/run/dmesg.boot</filename> file to view your boot
messages.</para>
<indexterm>
<primary>kernel options</primary>
<secondary>ident</secondary>
</indexterm>
<programlisting>ident GENERIC</programlisting>
<para>This is the identification of the kernel. You should change
this to whatever you named your kernel,
i.e., <literal><replaceable>MYKERNEL</replaceable></literal> if you have
followed the instructions of the previous examples. The value you put
in the <literal>ident</literal> string will print when you boot up the
kernel, so it is useful to give the new kernel a different name if you
want to keep it separate from your usual kernel (e.g., you want to
build an experimental kernel).</para>
<programlisting>#To statically compile in device wiring instead of /boot/device.hints
#hints "GENERIC.hints" # Default places to look for devices.</programlisting>
<para>The &man.device.hints.5; is
used to configure options of the device drivers. The default
location that &man.loader.8; will check at boot time is
<filename>/boot/device.hints</filename>. Using the
<literal>hints</literal> option you can compile these hints
statically into your kernel. Then there is no need to create a
<filename>device.hints</filename> file in
<filename>/boot</filename>.</para>
<!-- XXX: Add a comment here that explains when compiling hints into
the kernel is a good idea and why. -->
<programlisting>makeoptions DEBUG=-g # Build kernel with gdb(1) debug symbols</programlisting>
<para>The normal build process of &os; includes
debugging information when building the kernel with the
<option>-g</option> option, which enables debugging
information when passed to &man.gcc.1;.</para>
<programlisting>options SCHED_ULE # ULE scheduler</programlisting>
<para>The default system scheduler for &os;. Keep this.</para>
<programlisting>options PREEMPTION # Enable kernel thread preemption</programlisting>
<para>Allows threads that are in the kernel to be preempted
by higher priority threads. It helps with interactivity and
allows interrupt threads to run sooner rather than waiting.</para>
<programlisting>options INET # InterNETworking</programlisting>
<para>Networking support. Leave this in, even if you do not plan to
be connected to a network. Most programs require at least loopback
networking (i.e., making network connections within your PC), so
this is essentially mandatory.</para>
<programlisting>options INET6 # IPv6 communications protocols</programlisting>
<para>This enables the IPv6 communication protocols.</para>
<programlisting>options FFS # Berkeley Fast Filesystem</programlisting>
<para>This is the basic hard drive file system. Leave it in if you
boot from the hard disk.</para>
<programlisting>options SOFTUPDATES # Enable FFS Soft Updates support</programlisting>
<para>This option enables Soft Updates in the kernel, this will
help speed up write access on the disks. Even when this
functionality is provided by the kernel, it must be turned on
for specific disks. Review the output from &man.mount.8; to see
if Soft Updates is enabled for your system disks. If you do not
see the <literal>soft-updates</literal> option then you will
need to activate it using the &man.tunefs.8; (for existing
file systems) or &man.newfs.8; (for new file systems)
commands.</para>
<programlisting>options UFS_ACL # Support for access control lists</programlisting>
<para>This option enables kernel support
for access control lists. This relies on the use of extended
attributes and <acronym>UFS2</acronym>, and the feature is described
in detail in <xref linkend="fs-acl"/>. <acronym>ACL</acronym>s are
enabled by default and should not be
disabled in the kernel if they have been used previously on a file
system, as this will remove the access control lists, changing the
way files are protected in unpredictable ways.</para>
<programlisting>options UFS_DIRHASH # Improve performance on big directories</programlisting>
<para>This option includes functionality to speed up disk
operations on large directories, at the expense of using
additional memory. You would normally keep this for a large
server, or interactive workstation, and remove it if you are
using &os; on a smaller system where memory is at a premium and
disk access speed is less important, such as a firewall.</para>
<programlisting>options MD_ROOT # MD is a potential root device</programlisting>
<para>This option enables support for a memory backed virtual disk
used as a root device.</para>
<indexterm>
<primary>kernel options</primary>
<secondary>NFS</secondary>
</indexterm>
<indexterm>
<primary>kernel options</primary>
<secondary>NFS_ROOT</secondary>
</indexterm>
<programlisting>options NFSCLIENT # Network Filesystem Client
options NFSSERVER # Network Filesystem Server
options NFS_ROOT # NFS usable as /, requires NFSCLIENT</programlisting>
<para>The network file system. Unless you plan to mount partitions
from a &unix; file server over TCP/IP, you can comment these
out.</para>
<indexterm>
<primary>kernel options</primary>
<secondary>MSDOSFS</secondary>
</indexterm>
<programlisting>options MSDOSFS # MSDOS Filesystem</programlisting>
<para>The &ms-dos; file system. Unless you plan to mount a DOS formatted
hard drive partition at boot time, you can safely comment this out.
It will be automatically loaded the first time you mount a DOS
partition, as described above. Also, the excellent
<filename role="package">emulators/mtools</filename> software
allows you to access DOS floppies without having to mount and
unmount them (and does not require <literal>MSDOSFS</literal> at
all).</para>
<programlisting>options CD9660 # ISO 9660 Filesystem</programlisting>
<para>The ISO 9660 file system for CDROMs. Comment it out if you do
not have a CDROM drive or only mount data CDs occasionally (since it
will be dynamically loaded the first time you mount a data CD).
Audio CDs do not need this file system.</para>
<programlisting>options PROCFS # Process filesystem (requires PSEUDOFS)</programlisting>
<para>The process file system. This is a <quote>pretend</quote>
file system mounted on <filename>/proc</filename> which allows
programs like &man.ps.1; to give you more information on what
processes are running. Use of <literal>PROCFS</literal>
is not required under most circumstances, as most
debugging and monitoring tools have been adapted to run without
<literal>PROCFS</literal>: installs will not mount this file
system by default.</para>
<programlisting>options PSEUDOFS # Pseudo-filesystem framework</programlisting>
<para>Kernels making use of <literal>PROCFS</literal> must also
include support for <literal>PSEUDOFS</literal>.</para>
<programlisting>options GEOM_PART_GPT # GUID Partition Tables.</programlisting>
<para>Adds support for <ulink
url="http://en.wikipedia.org/wiki/GUID_Partition_Table">GUID
Partition Tables</ulink>. GPT provides the ability to have a
large number of partitions per disk, 128 in the standard
configuration.</para>
<programlisting>options COMPAT_43 # Compatible with BSD 4.3 [KEEP THIS!]</programlisting>
<para>Compatibility with 4.3BSD. Leave this in; some programs will
act strangely if you comment this out.</para>
<programlisting>options COMPAT_FREEBSD4 # Compatible with &os;4</programlisting>
<para>This option is required
to support applications compiled on older versions of &os;
that use older system call interfaces. It is recommended that
this option be used on all &i386; systems that may
run older applications; platforms that gained support only in
5.X, such as ia64 and &sparc64;, do not require this option.</para>
<programlisting>options COMPAT_FREEBSD5 # Compatible with &os;5</programlisting>
<para>This option is required to
support applications compiled on &os; 5.X versions that use
&os; 5.X system call interfaces.</para>
<programlisting>options COMPAT_FREEBSD6 # Compatible with &os;6</programlisting>
<para>This option is required to
support applications compiled on &os; 6.X versions that use
&os; 6.X system call interfaces.</para>
<programlisting>options COMPAT_FREEBSD7 # Compatible with &os;7</programlisting>
<para>This option is required on &os; 8 and above to
support applications compiled on &os; 7.X versions that use
&os; 7.X system call interfaces.</para>
<programlisting>options SCSI_DELAY=5000 # Delay (in ms) before probing SCSI</programlisting>
<para>This causes the kernel to pause for 5 seconds before probing
each SCSI device in your system. If you only have IDE hard drives,
you can ignore this, otherwise you can try to lower this
number, to speed up booting. Of course, if
you do this and &os; has trouble recognizing your SCSI devices,
you will have to raise it again.</para>
<programlisting>options KTRACE # ktrace(1) support</programlisting>
<para>This enables kernel process tracing, which is useful in
debugging.</para>
<programlisting>options SYSVSHM # SYSV-style shared memory</programlisting>
<para>This option provides for System V shared memory. The most
common use of this is the XSHM extension in X, which many
graphics-intensive programs will automatically take advantage of for
extra speed. If you use X, you will definitely want to include
this.</para>
<programlisting>options SYSVMSG # SYSV-style message queues</programlisting>
<para>Support for System V messages. This option only adds
a few hundred bytes to the kernel.</para>
<programlisting>options SYSVSEM # SYSV-style semaphores</programlisting>
<para>Support for System V semaphores. Less commonly used but only
adds a few hundred bytes to the kernel.</para>
<note>
<para>The <option>-p</option> option of the &man.ipcs.1; command will
list any processes using each of these System V facilities.</para>
</note>
<programlisting>options _KPOSIX_PRIORITY_SCHEDULING # POSIX P1003_1B real-time extensions</programlisting>
<para>Real-time extensions added in the 1993 &posix;. Certain
applications in the Ports Collection use these
(such as <application>&staroffice;</application>).</para>
<programlisting>options KBD_INSTALL_CDEV # install a CDEV entry in /dev</programlisting>
<para>This option is required to allow the creation of keyboard device
nodes in <filename>/dev</filename>.</para>
<programlisting>options ADAPTIVE_GIANT # Giant mutex is adaptive.</programlisting>
<para>Giant is the name of a mutual exclusion mechanism (a sleep mutex)
that protects a large set of kernel resources. Today, this is an
unacceptable performance bottleneck which is actively being replaced
with locks that protect individual resources. The
<literal>ADAPTIVE_GIANT</literal> option causes Giant to be included
in the set of mutexes adaptively spun on. That is, when a thread
wants to lock the Giant mutex, but it is already locked by a thread
on another CPU, the first thread will keep running and wait for the
lock to be released. Normally, the thread would instead go back to
sleep and wait for its next chance to run. If you are not sure,
leave this in.</para>
<note>
<para>Note that on &os; 8.0-RELEASE and later versions, all mutexes are
adaptive by default, unless explicitly set to non-adaptive by
compiling with the <literal>NO_ADAPTIVE_MUTEXES</literal> option. As
a result, Giant is adaptive by default now, and the
<literal>ADAPTIVE_GIANT</literal> option has been removed from the
kernel configuration.</para>
</note>
<indexterm>
<primary>kernel options</primary>
<secondary>SMP</secondary>
</indexterm>
<programlisting>device apic # I/O APIC</programlisting>
<para>The apic device enables the use of the I/O APIC for interrupt
delivery. The apic device can be used in both UP and SMP kernels, but
is required for SMP kernels. Add <literal>options SMP</literal> to
include support for multiple processors.</para>
<note>
<para>The apic device exists only on the i386 architecture, this
configuration line should not be used on other
architectures.</para>
</note>
<programlisting>device eisa</programlisting>
<para>Include this if you have an EISA motherboard. This enables
auto-detection and configuration support for all devices on the EISA
bus.</para>
<programlisting>device pci</programlisting>
<para>Include this if you have a PCI motherboard. This enables
auto-detection of PCI cards and gatewaying from the PCI to ISA
bus.</para>
<programlisting># Floppy drives
device fdc</programlisting>
<para>This is the floppy drive controller.</para>
<programlisting># ATA and ATAPI devices
device ata</programlisting>
<para>This driver supports all ATA and ATAPI devices. You only need
one <literal>device ata</literal> line for the kernel to detect all
PCI ATA/ATAPI devices on modern machines.</para>
<programlisting>device atadisk # ATA disk drives</programlisting>
<para>This is needed along with <literal>device ata</literal> for
ATA disk drives.</para>
<programlisting>device ataraid # ATA RAID drives</programlisting>
<para>This is needed along with <literal>device ata</literal> for ATA
RAID drives.</para>
<programlisting><anchor id="kernelconfig-atapi"/>
device atapicd # ATAPI CDROM drives</programlisting>
<para>This is needed along with <literal>device ata</literal> for
ATAPI CDROM drives.</para>
<programlisting>device atapifd # ATAPI floppy drives</programlisting>
<para>This is needed along with <literal>device ata</literal> for
ATAPI floppy drives.</para>
<programlisting>device atapist # ATAPI tape drives</programlisting>
<para>This is needed along with <literal>device ata</literal> for
ATAPI tape drives.</para>
<programlisting>options ATA_STATIC_ID # Static device numbering</programlisting>
<para>This makes the controller number static; without this,
the device numbers are dynamically allocated.</para>
<programlisting># SCSI Controllers
device ahb # EISA AHA1742 family
device ahc # AHA2940 and onboard AIC7xxx devices
options AHC_REG_PRETTY_PRINT # Print register bitfields in debug
# output. Adds ~128k to driver.
device ahd # AHA39320/29320 and onboard AIC79xx devices
options AHD_REG_PRETTY_PRINT # Print register bitfields in debug
# output. Adds ~215k to driver.
device amd # AMD 53C974 (Teckram DC-390(T))
device isp # Qlogic family
#device ispfw # Firmware for QLogic HBAs- normally a module
device mpt # LSI-Logic MPT-Fusion
#device ncr # NCR/Symbios Logic
device sym # NCR/Symbios Logic (newer chipsets + those of `ncr')
device trm # Tekram DC395U/UW/F DC315U adapters
device adv # Advansys SCSI adapters
device adw # Advansys wide SCSI adapters
device aha # Adaptec 154x SCSI adapters
device aic # Adaptec 15[012]x SCSI adapters, AIC-6[23]60.
device bt # Buslogic/Mylex MultiMaster SCSI adapters
device ncv # NCR 53C500
device nsp # Workbit Ninja SCSI-3
device stg # TMC 18C30/18C50</programlisting>
<para>SCSI controllers. Comment out any you do not have in your
system. If you have an IDE only system, you can remove these
altogether. The <literal>*_REG_PRETTY_PRINT</literal> lines are
debugging options for their respective drivers.</para>
<programlisting># SCSI peripherals
device scbus # SCSI bus (required for SCSI)
device ch # SCSI media changers
device da # Direct Access (disks)
device sa # Sequential Access (tape etc)
device cd # CD
device pass # Passthrough device (direct SCSI access)
device ses # SCSI Environmental Services (and SAF-TE)</programlisting>
<para>SCSI peripherals. Again, comment out any you do not have, or if
you have only IDE hardware, you can remove them completely.</para>
<note>
<para>The USB &man.umass.4; driver and a few other drivers use
the SCSI subsystem even though they are not real SCSI devices.
Therefore make sure not to remove SCSI support, if any such
drivers are included in the kernel configuration.</para>
</note>
<programlisting># RAID controllers interfaced to the SCSI subsystem
device amr # AMI MegaRAID
device arcmsr # Areca SATA II RAID
device asr # DPT SmartRAID V, VI and Adaptec SCSI RAID
device ciss # Compaq Smart RAID 5*
device dpt # DPT Smartcache III, IV - See NOTES for options
device hptmv # Highpoint RocketRAID 182x
device hptrr # Highpoint RocketRAID 17xx, 22xx, 23xx, 25xx
device iir # Intel Integrated RAID
device ips # IBM (Adaptec) ServeRAID
device mly # Mylex AcceleRAID/eXtremeRAID
device twa # 3ware 9000 series PATA/SATA RAID
# RAID controllers
device aac # Adaptec FSA RAID
device aacp # SCSI passthrough for aac (requires CAM)
device ida # Compaq Smart RAID
device mfi # LSI MegaRAID SAS
device mlx # Mylex DAC960 family
device pst # Promise Supertrak SX6000
device twe # 3ware ATA RAID</programlisting>
<para>Supported RAID controllers. If you do not have any of these,
you can comment them out or remove them.</para>
<programlisting># atkbdc0 controls both the keyboard and the PS/2 mouse
device atkbdc # AT keyboard controller</programlisting>
<para>The keyboard controller (<literal>atkbdc</literal>) provides I/O
services for the AT keyboard and PS/2 style pointing devices. This
controller is required by the keyboard driver
(<literal>atkbd</literal>) and the PS/2 pointing device driver
(<literal>psm</literal>).</para>
<programlisting>device atkbd # AT keyboard</programlisting>
<para>The <literal>atkbd</literal> driver, together with
<literal>atkbdc</literal> controller, provides access to the AT 84
keyboard or the AT enhanced keyboard which is connected to the AT
keyboard controller.</para>
<programlisting>device psm # PS/2 mouse</programlisting>
<para>Use this device if your mouse plugs into the PS/2 mouse
port.</para>
<programlisting>device kbdmux # keyboard multiplexer</programlisting>
<para>Basic support for keyboard multiplexing. If you do not plan
to use more than one keyboard on the system, you can safely
remove that line.</para>
<programlisting>device vga # VGA video card driver</programlisting>
<para>The video card driver.</para>
<programlisting>
device splash # Splash screen and screen saver support</programlisting>
<para>Splash screen at start up! Screen savers require this
too.</para>
<programlisting># syscons is the default console driver, resembling an SCO console
device sc</programlisting>
<para><literal>sc</literal> is the default console driver and
resembles a SCO console. Since most full-screen programs access the
console through a terminal database library like
<filename>termcap</filename>, it should not matter whether you use
this or <literal>vt</literal>, the <literal>VT220</literal>
compatible console driver. When you log in, set your
<envar>TERM</envar> variable to <literal>scoansi</literal> if
full-screen programs have trouble running under this console.</para>
<programlisting># Enable this for the pcvt (VT220 compatible) console driver
#device vt
#options XSERVER # support for X server on a vt console
#options FAT_CURSOR # start with block cursor</programlisting>
<para>This is a VT220-compatible console driver, backward compatible to
VT100/102. It works well on some laptops which have hardware
incompatibilities with <literal>sc</literal>. Also set your
<envar>TERM</envar> variable to <literal>vt100</literal> or
<literal>vt220</literal> when you log in. This driver might also
prove useful when connecting to a large number of different machines
over the network, where <filename>termcap</filename> or
<filename>terminfo</filename> entries for the <literal>sc</literal>
device are often not available — <literal>vt100</literal>
should be available on virtually any platform.</para>
<programlisting>device agp</programlisting>
<para>Include this if you have an AGP card in the system. This
will enable support for AGP, and AGP GART for boards which
have these features.</para>
<indexterm>
<primary>APM</primary>
</indexterm>
<programlisting># Power management support (see NOTES for more options)
#device apm</programlisting>
<para>Advanced Power Management support. Useful for laptops,
although this is disabled in
<filename>GENERIC</filename> by default.</para>
<programlisting># Add suspend/resume support for the i8254.
device pmtimer</programlisting>
<para>Timer device driver for power management events, such as APM and
ACPI.</para>
<programlisting># PCCARD (PCMCIA) support
# PCMCIA and cardbus bridge support
device cbb # cardbus (yenta) bridge
device pccard # PC Card (16-bit) bus
device cardbus # CardBus (32-bit) bus</programlisting>
<para>PCMCIA support. You want this if you are using a
laptop.</para>
<programlisting># Serial (COM) ports
device sio # 8250, 16[45]50 based serial ports</programlisting>
<para>These are the serial ports referred to as
<devicename>COM</devicename> ports in the &ms-dos;/&windows;
world.</para>
<note>
<para>If you have an internal modem on <devicename>COM4</devicename>
and a serial port at <devicename>COM2</devicename>, you will have
to change the IRQ of the modem to 2 (for obscure technical reasons,
IRQ2 = IRQ 9) in order to access it
from &os;. If you have a multiport serial card, check the
manual page for &man.sio.4; for more information on the proper
values to add to your <filename>/boot/device.hints</filename>.
Some video cards (notably those based on
S3 chips) use IO addresses in the form of
<literal>0x*2e8</literal>, and since many cheap serial cards do
not fully decode the 16-bit IO address space, they clash with
these cards making the <devicename>COM4</devicename> port
practically unavailable.</para>
<para>Each serial port is required to have a unique IRQ (unless you
are using one of the multiport cards where shared interrupts are
supported), so the default IRQs for <devicename>COM3</devicename>
and <devicename>COM4</devicename> cannot be used.</para>
</note>
<programlisting># Parallel port
device ppc</programlisting>
<para>This is the ISA-bus parallel port interface.</para>
<programlisting>device ppbus # Parallel port bus (required)</programlisting>
<para>Provides support for the parallel port bus.</para>
<programlisting>device lpt # Printer</programlisting>
<para>Support for parallel port printers.</para>
<note>
<para>All three of the above are required to enable parallel printer
support.</para>
</note>
<programlisting>device plip # TCP/IP over parallel</programlisting>
<para>This is the driver for the parallel network interface.</para>
<programlisting>device ppi # Parallel port interface device</programlisting>
<para>The general-purpose I/O (<quote>geek port</quote>) + IEEE1284
I/O.</para>
<programlisting>#device vpo # Requires scbus and da</programlisting>
<indexterm><primary>zip drive</primary></indexterm>
<para>This is for an Iomega Zip drive. It requires
<literal>scbus</literal> and <literal>da</literal> support. Best
performance is achieved with ports in EPP 1.9 mode.</para>
<programlisting>#device puc</programlisting>
<para>Uncomment this device if you have a <quote>dumb</quote> serial
or parallel PCI card that is supported by the &man.puc.4; glue
driver.</para>
<programlisting># PCI Ethernet NICs.
device de # DEC/Intel DC21x4x (<quote>Tulip</quote>)
device em # Intel PRO/1000 adapter Gigabit Ethernet Card
device ixgb # Intel PRO/10GbE Ethernet Card
device txp # 3Com 3cR990 (<quote>Typhoon</quote>)
device vx # 3Com 3c590, 3c595 (<quote>Vortex</quote>)</programlisting>
<para>Various PCI network card drivers. Comment out or remove any of
these not present in your system.</para>
<programlisting># PCI Ethernet NICs that use the common MII bus controller code.
# NOTE: Be sure to keep the 'device miibus' line in order to use these NICs!
device miibus # MII bus support</programlisting>
<para>MII bus support is required for some PCI 10/100 Ethernet NICs,
namely those which use MII-compliant transceivers or implement
transceiver control interfaces that operate like an MII. Adding
<literal>device miibus</literal> to the kernel config pulls in
support for the generic miibus API and all of the PHY drivers,
including a generic one for PHYs that are not specifically handled
by an individual driver.</para>
<programlisting>device bce # Broadcom BCM5706/BCM5708 Gigabit Ethernet
device bfe # Broadcom BCM440x 10/100 Ethernet
device bge # Broadcom BCM570xx Gigabit Ethernet
device dc # DEC/Intel 21143 and various workalikes
device fxp # Intel EtherExpress PRO/100B (82557, 82558)
device lge # Level 1 LXT1001 gigabit ethernet
device msk # Marvell/SysKonnect Yukon II Gigabit Ethernet
device nge # NatSemi DP83820 gigabit ethernet
device nve # nVidia nForce MCP on-board Ethernet Networking
device pcn # AMD Am79C97x PCI 10/100 (precedence over 'lnc')
device re # RealTek 8139C+/8169/8169S/8110S
device rl # RealTek 8129/8139
device sf # Adaptec AIC-6915 (<quote>Starfire</quote>)
device sis # Silicon Integrated Systems SiS 900/SiS 7016
device sk # SysKonnect SK-984x & SK-982x gigabit Ethernet
device ste # Sundance ST201 (D-Link DFE-550TX)
device stge # Sundance/Tamarack TC9021 gigabit Ethernet
device ti # Alteon Networks Tigon I/II gigabit Ethernet
device tl # Texas Instruments ThunderLAN
device tx # SMC EtherPower II (83c170 <quote>EPIC</quote>)
device vge # VIA VT612x gigabit ethernet
device vr # VIA Rhine, Rhine II
device wb # Winbond W89C840F
device xl # 3Com 3c90x (<quote>Boomerang</quote>, <quote>Cyclone</quote>)</programlisting>
<para>Drivers that use the MII bus controller code.</para>
<programlisting># ISA Ethernet NICs. pccard NICs included.
device cs # Crystal Semiconductor CS89x0 NIC
# 'device ed' requires 'device miibus'
device ed # NE[12]000, SMC Ultra, 3c503, DS8390 cards
device ex # Intel EtherExpress Pro/10 and Pro/10+
device ep # Etherlink III based cards
device fe # Fujitsu MB8696x based cards
device ie # EtherExpress 8/16, 3C507, StarLAN 10 etc.
device lnc # NE2100, NE32-VL Lance Ethernet cards
device sn # SMC's 9000 series of Ethernet chips
device xe # Xircom pccard Ethernet
# ISA devices that use the old ISA shims
#device le</programlisting>
<para>ISA Ethernet drivers. See
<filename>/usr/src/sys/<replaceable>i386</replaceable>/conf/NOTES</filename>
for details of which cards are supported by which driver.</para>
<programlisting># Wireless NIC cards
device wlan # 802.11 support</programlisting>
<para>Generic 802.11 support. This line is required for wireless
networking.</para>
<programlisting>device wlan_wep # 802.11 WEP support
device wlan_ccmp # 802.11 CCMP support
device wlan_tkip # 802.11 TKIP support</programlisting>
<para>Crypto support for 802.11 devices. These lines are needed
if you intend to use encryption and 802.11i security
protocols.</para>
<programlisting>device an # Aironet 4500/4800 802.11 wireless NICs.
device ath # Atheros pci/cardbus NIC's
device ath_hal # Atheros HAL (Hardware Access Layer)
device ath_rate_sample # SampleRate tx rate control for ath
device awi # BayStack 660 and others
device ral # Ralink Technology RT2500 wireless NICs.
device wi # WaveLAN/Intersil/Symbol 802.11 wireless NICs.
#device wl # Older non 802.11 Wavelan wireless NIC.</programlisting>
<para>Support for various wireless cards.</para>
<programlisting># Pseudo devices
device loop # Network loopback</programlisting>
<para>This is the generic loopback device for TCP/IP. If you telnet
or FTP to <hostid>localhost</hostid> (aka <hostid
role="ipaddr">127.0.0.1</hostid>) it will come back at you through
this device. This is <emphasis>mandatory</emphasis>.</para>
<programlisting>device random # Entropy device</programlisting>
<para>Cryptographically secure random number generator.</para>
<programlisting>device ether # Ethernet support</programlisting>
<para><literal>ether</literal> is only needed if you have an Ethernet
card. It includes generic Ethernet protocol code.</para>
<programlisting>device sl # Kernel SLIP</programlisting>
<para><literal>sl</literal> is for SLIP support. This has been almost
entirely supplanted by PPP, which is easier to set up, better suited
for modem-to-modem connection, and more powerful.</para>
<programlisting>device ppp # Kernel PPP</programlisting>
<para>This is for kernel PPP support for dial-up connections. There
is also a version of PPP implemented as a userland application that
uses <literal>tun</literal> and offers more flexibility and features
such as demand dialing.</para>
<programlisting>device tun # Packet tunnel.</programlisting>
<para>This is used by the userland PPP software.
See
the <link linkend="userppp">PPP</link> section of this book for more
information.</para>
<programlisting><anchor id="kernelconfig-ptys"/>
device pty # Pseudo-ttys (telnet etc)</programlisting>
<para>This is a <quote>pseudo-terminal</quote> or simulated login port.
It is used by incoming <command>telnet</command> and
<command>rlogin</command> sessions,
<application>xterm</application>, and some other applications such
as <application>Emacs</application>.</para>
<programlisting>device md # Memory <quote>disks</quote></programlisting>
<para>Memory disk pseudo-devices.</para>
<programlisting>device gif # IPv6 and IPv4 tunneling</programlisting>
<para>This implements IPv6 over IPv4 tunneling, IPv4 over IPv6 tunneling,
IPv4 over IPv4 tunneling, and IPv6 over IPv6 tunneling. The
<literal>gif</literal> device is
<quote>auto-cloning</quote>, and will create device nodes as
needed.</para>
<programlisting>device faith # IPv6-to-IPv4 relaying (translation)</programlisting>
<para>This pseudo-device captures packets that are sent to it and
diverts them to the IPv4/IPv6 translation daemon.</para>
<programlisting># The `bpf' device enables the Berkeley Packet Filter.
# Be aware of the administrative consequences of enabling this!
# Note that 'bpf' is required for DHCP.
device bpf # Berkeley packet filter</programlisting>
<para>This is the Berkeley Packet Filter. This pseudo-device allows
network interfaces to be placed in promiscuous mode, capturing every
packet on a broadcast network (e.g., an Ethernet). These packets
can be captured to disk and or examined with the &man.tcpdump.1;
program.</para>
<note>
<para>The &man.bpf.4; device is also used by
&man.dhclient.8; to obtain the IP address of the default router
(gateway) and so on. If you use DHCP, leave this
uncommented.</para>
</note>
<programlisting># USB support
device uhci # UHCI PCI->USB interface
device ohci # OHCI PCI->USB interface
device ehci # EHCI PCI->USB interface (USB 2.0)
device usb # USB Bus (required)
#device udbp # USB Double Bulk Pipe devices
device ugen # Generic
device uhid # <quote>Human Interface Devices</quote>
device ukbd # Keyboard
device ulpt # Printer
device umass # Disks/Mass storage - Requires scbus and da
device ums # Mouse
device ural # Ralink Technology RT2500USB wireless NICs
device urio # Diamond Rio 500 MP3 player
device uscanner # Scanners
# USB Ethernet, requires mii
device aue # ADMtek USB Ethernet
device axe # ASIX Electronics USB Ethernet
device cdce # Generic USB over Ethernet
device cue # CATC USB Ethernet
device kue # Kawasaki LSI USB Ethernet
device rue # RealTek RTL8150 USB Ethernet</programlisting>
<para>Support for various USB devices.</para>
<programlisting># FireWire support
device firewire # FireWire bus code
device sbp # SCSI over FireWire (Requires scbus and da)
device fwe # Ethernet over FireWire (non-standard!)</programlisting>
<para>Support for various Firewire devices.</para>
<para>For more information and additional devices supported by
&os;, see
<filename>/usr/src/sys/<replaceable>i386</replaceable>/conf/NOTES</filename>.</para>
<sect2>
<title>Large Memory Configurations (<acronym>PAE</acronym>)</title>
<indexterm>
<primary>Physical Address Extensions
(<acronym>PAE</acronym>)</primary>
<secondary>large memory</secondary>
</indexterm>
<para>Large memory configuration machines require access to
more than the 4 gigabyte limit on User+Kernel Virtual
Address (<acronym>KVA</acronym>) space. Due to this
limitation, Intel added support for 36-bit physical address
space access in the &pentium; Pro and later line of CPUs.</para>
<para>The Physical Address Extension (<acronym>PAE</acronym>)
capability of the &intel; &pentium; Pro and later CPUs
allows memory configurations of up to 64 gigabytes.
&os; provides support for this capability via the
<option>PAE</option> kernel configuration option, available
in all current release versions of &os;. Due to
the limitations of the Intel memory architecture, no distinction
is made for memory above or below 4 gigabytes. Memory allocated
above 4 gigabytes is simply added to the pool of available
memory.</para>
<para>To enable <acronym>PAE</acronym> support in the kernel,
simply add the following line to your kernel configuration
file:</para>
<programlisting>options PAE</programlisting>
<note>
<para>The <acronym>PAE</acronym> support in &os; is only
available for &intel; IA-32 processors. It should also be
noted, that the <acronym>PAE</acronym> support in &os; has
not received wide testing, and should be considered beta
quality compared to other stable features of &os;.</para>
</note>
<para><acronym>PAE</acronym> support in &os; has a few limitations:</para>
<itemizedlist>
<listitem>
<para>A process is not able to access more than 4
gigabytes of VM space.</para>
</listitem>
<listitem>
<para>Device drivers that do not use the &man.bus.dma.9;
interface will cause data corruption in a
<acronym>PAE</acronym> enabled kernel and are not
recommended for use. For this reason, a
<filename>PAE</filename> kernel
configuration file is provided in &os; which
excludes all drivers not known to work in a
<acronym>PAE</acronym> enabled kernel.</para>
</listitem>
<listitem>
<para>Some system tunables determine memory resource usage
by the amount of available physical memory. Such
tunables can unnecessarily over-allocate due to the
large memory nature of a <acronym>PAE</acronym> system.
One such example is the <option>kern.maxvnodes</option>
sysctl, which controls the maximum number of vnodes allowed
in the kernel. It is advised to adjust this and other
such tunables to a reasonable value.</para>
</listitem>
<listitem>
<para>It might be necessary to increase the kernel virtual
address (<acronym>KVA</acronym>) space or to reduce the
amount of specific kernel resource that is heavily used
(see above) in order to avoid <acronym>KVA</acronym>
exhaustion. The <option>KVA_PAGES</option> kernel option
can be used for increasing the
<acronym>KVA</acronym> space.</para>
</listitem>
</itemizedlist>
<para>For performance and stability concerns, it is advised to
consult the &man.tuning.7; manual page. The &man.pae.4;
manual page contains up-to-date information on &os;'s
<acronym>PAE</acronym> support.</para>
</sect2>
</sect1>
<sect1 id="kernelconfig-trouble">
<title>If Something Goes Wrong</title>
<para>There are four categories of trouble that can occur when
building a custom kernel. They are:</para>
<variablelist>
<varlistentry>
<term><command>config</command> fails:</term>
<listitem>
<para>If the &man.config.8; command fails when you
give it your kernel description, you have probably made a
simple error somewhere. Fortunately,
&man.config.8; will print the line number that it
had trouble with, so that you can quickly locate the line
containing the error. For example, if you see:</para>
<screen>config: line 17: syntax error</screen>
<para>Make sure the
keyword is typed correctly by comparing it to the
<filename>GENERIC</filename> kernel or another
reference.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><command>make</command> fails:</term>
<listitem>
<para>If the <command>make</command> command fails, it usually
signals an error in your kernel description which is not severe
enough for &man.config.8; to catch. Again, look
over your configuration, and if you still cannot resolve the
problem, send mail to the &a.questions; with your kernel
configuration, and it should be diagnosed quickly.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>The kernel does not boot:<anchor
id="kernelconfig-noboot"/></term>
<listitem>
<para>If your new kernel does not boot, or fails to
recognize your devices, do not panic! Fortunately, &os; has
an excellent mechanism for recovering from incompatible
kernels. Simply choose the kernel you want to boot from at
the &os; boot loader. You can access this when the system
boot menu appears. Select the <quote>Escape to a loader
prompt</quote> option, number six. At the prompt, type
<command>boot <replaceable>kernel.old</replaceable></command>,
or the name of any other kernel that will boot properly.
When reconfiguring a kernel, it is always a good idea to keep
a kernel that is known to work on hand.</para>
<para>After booting with a good kernel you can check over your
configuration file and try to build it again. One helpful
resource is the <filename>/var/log/messages</filename> file
which records, among other things, all of the kernel messages
from every successful boot. Also, the &man.dmesg.8; command
will print the kernel messages from the current boot.</para>
<note>
<para>If you are having trouble building a kernel, make sure
to keep a <filename>GENERIC</filename>, or some other kernel
that is known to work on hand as a different name that will
not get erased on the next build. You cannot rely on
<filename>kernel.old</filename> because when installing a
new kernel, <filename>kernel.old</filename> is overwritten
with the last installed kernel which may be non-functional.
Also, as soon as possible, move the working kernel to the
proper <filename class="directory">/boot/kernel</filename>
location or commands such
as &man.ps.1; may not work properly. To do this, simply
rename the directory containing the good kernel:</para>
<screen>&prompt.root; <userinput>mv /boot/kernel <replaceable>/boot/kernel.bad</replaceable></userinput>
&prompt.root; <userinput>mv /boot/<replaceable>kernel.good</replaceable> /boot/kernel</userinput></screen>
</note>
</listitem>
</varlistentry>
<varlistentry>
<term>The kernel works, but &man.ps.1; does not work
any more:</term>
<listitem>
<para>If you have installed a different version of the kernel
from the one that the system utilities have been built with,
for example, a -CURRENT kernel on a -RELEASE, many system-status
commands like &man.ps.1; and &man.vmstat.8; will not work any
more. You should <link linkend="makeworld">recompile and install
a world</link> built with the same version of the source tree as
your kernel. This is one reason it is
not normally a good idea to use a different version of the
kernel from the rest of the operating system.</para>
</listitem>
</varlistentry>
</variablelist>
</sect1>
</chapter>
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