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systemd/man/systemd-boot.xml
Gerd Hoffmann 57ea8012d6 sd-boot: add support for support enrolling dbx 
usage:
  (1) get latest revocation list for your architecture
      from https://uefi.org/revocationlistfile
  (2) copy the file to $ESP/loader/keys/$name/dbx.auth
2024-03-21 23:03:42 +00:00

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<?xml version='1.0'?> <!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
<refentry id="systemd-boot" conditional='ENABLE_BOOTLOADER'
xmlns:xi="http://www.w3.org/2001/XInclude">
<refentryinfo>
<title>systemd-boot</title>
<productname>systemd</productname>
</refentryinfo>
<refmeta>
<refentrytitle>systemd-boot</refentrytitle>
<manvolnum>7</manvolnum>
</refmeta>
<refnamediv>
<refname>systemd-boot</refname>
<refname>sd-boot</refname>
<refpurpose>A simple UEFI boot manager</refpurpose>
</refnamediv>
<refsect1>
<title>Description</title>
<para><command>systemd-boot</command> (short: <command>sd-boot</command>) is a simple UEFI boot
manager. It provides a textual menu to select the entry to boot and an editor for the kernel command
line. <command>systemd-boot</command> supports systems with UEFI firmware only.</para>
<para><command>systemd-boot</command> loads boot entry information from the EFI system partition (ESP),
usually mounted at <filename>/efi/</filename>, <filename>/boot/</filename>, or
<filename>/boot/efi/</filename> during OS runtime, as well as from the Extended Boot Loader partition
(XBOOTLDR) if it exists (usually mounted to <filename>/boot/</filename>). Configuration file fragments,
kernels, initrds and other EFI images to boot generally need to reside on the ESP or the Extended Boot
Loader partition. Linux kernels must be built with <option>CONFIG_EFI_STUB</option> to be able to be
directly executed as an EFI image. During boot <command>systemd-boot</command> automatically assembles a
list of boot entries from the following sources:</para>
<itemizedlist>
<listitem><para>Boot entries defined with <ulink
url="https://uapi-group.org/specifications/specs/boot_loader_specification">Boot Loader Specification</ulink> Type #1
description files located in <filename>/loader/entries/</filename> on the ESP and the Extended Boot
Loader Partition. These usually describe Linux kernel images with associated initrd images, but
alternatively may also describe other arbitrary EFI executables.</para></listitem>
<listitem><para>Unified kernel images, <ulink url="https://uapi-group.org/specifications/specs/boot_loader_specification">Boot
Loader Specification</ulink> Type #2, which are executable EFI binaries in
<filename>/EFI/Linux/</filename> on the ESP and the Extended Boot Loader Partition.</para></listitem>
<listitem><para>The Microsoft Windows EFI boot manager, if installed.</para></listitem>
<listitem><para>The Apple macOS boot manager, if installed.</para></listitem>
<listitem><para>The EFI Shell binary, if installed.</para></listitem>
<listitem><para>A <literal>Reboot Into Firmware Interface option</literal>, if supported by the UEFI
firmware.</para></listitem>
<listitem><para>Secure Boot variables enrollment if the UEFI firmware is in setup-mode and files are provided
on the ESP.</para></listitem>
</itemizedlist>
<para><command>systemd-boot</command> supports the following features:</para>
<itemizedlist>
<listitem><para>Basic boot manager configuration changes (such as timeout
configuration, default boot entry selection, …) may be made directly from the boot loader UI at
boot-time, as well as during system runtime with EFI variables.</para></listitem>
<listitem><para>The boot manager integrates with the <command>systemctl</command> command to implement
features such as <command>systemctl reboot --boot-loader-entry=…</command> (for rebooting into a
specific boot menu entry, i.e. "reboot into Windows") and <command>systemctl reboot
--boot-loader-menu=…</command> (for rebooting into the boot loader menu), by implementing the <ulink
url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>. See
<citerefentry><refentrytitle>systemctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> for
details.</para></listitem>
<listitem><para>An EFI variable set by the boot loader informs the OS about the EFI System Partition used
during boot. This is then used to automatically mount the correct EFI System Partition to
<filename>/efi/</filename> or <filename>/boot/</filename> during OS runtime. See
<citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
for details.</para></listitem>
<listitem><para>The boot manager provides information about the boot time spent in UEFI firmware using
the <ulink url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>. This
information can be displayed using
<citerefentry><refentrytitle>systemd-analyze</refentrytitle><manvolnum>1</manvolnum></citerefentry>.
</para></listitem>
<listitem><para>The boot manager implements boot counting and automatic fallback to older, working boot
entries on failure. See <ulink url="https://systemd.io/AUTOMATIC_BOOT_ASSESSMENT">Automatic Boot
Assessment</ulink>.</para></listitem>
<listitem><para>The boot manager optionally reads a random seed from the ESP partition, combines it
with a 'system token' stored in a persistent EFI variable and derives a random seed to use by the OS as
entropy pool initialization, providing a full entropy pool during early boot.</para></listitem>
<listitem><para>The boot manager allows for Secure Boot variables to be enrolled if the UEFI firmware is
in setup-mode. Additionally, variables can be automatically enrolled if configured.</para></listitem>
</itemizedlist>
<para><citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
may be used from a running system to locate the ESP and the Extended Boot Loader Partition, list
available entries, and install <command>systemd-boot</command> itself.</para>
<para><citerefentry><refentrytitle>kernel-install</refentrytitle><manvolnum>8</manvolnum></citerefentry>
may be used to copy kernel images onto the ESP or the Extended Boot Loader Partition and to generate
description files compliant with the Boot Loader
Specification.</para>
<para><citerefentry><refentrytitle>systemd-stub</refentrytitle><manvolnum>7</manvolnum></citerefentry>
may be used as UEFI boot stub for executed kernels, which is useful to show graphical boot splashes
before transitioning into the Linux world. It is also capable of automatically picking up auxiliary
credential files (for boot parameterization) and system extension images, as companion files to the
booted kernel images.</para>
</refsect1>
<refsect1>
<title>Key bindings</title>
<para>The following keys may be used in the boot menu:</para>
<!-- Developer commands Q/v/Ctrl+l deliberately not advertised. -->
<variablelist>
<varlistentry>
<term><keycap>↑</keycap> (Up)</term>
<term><keycap>↓</keycap> (Down)</term>
<term><keycap>j</keycap></term>
<term><keycap>k</keycap></term>
<term><keycap>PageUp</keycap></term>
<term><keycap>PageDown</keycap></term>
<term><keycap>Home</keycap></term>
<term><keycap>End</keycap></term>
<listitem><para>Navigate up/down in the entry list</para>
<xi:include href="version-info.xml" xpointer="v239"/></listitem>
</varlistentry>
<varlistentry>
<term><keycap>↵</keycap> (Enter)</term>
<term><keycap>→</keycap> (Right)</term>
<listitem><para>Boot selected entry</para>
<xi:include href="version-info.xml" xpointer="v239"/></listitem>
</varlistentry>
<varlistentry>
<term><keycap>d</keycap></term>
<listitem><para>Make selected entry the default</para>
<xi:include href="version-info.xml" xpointer="v239"/></listitem>
</varlistentry>
<varlistentry>
<term><keycap>e</keycap></term>
<listitem><para>Edit the kernel command line for selected entry</para>
<xi:include href="version-info.xml" xpointer="v239"/></listitem>
</varlistentry>
<varlistentry>
<term><keycap>+</keycap></term>
<term><keycap>t</keycap></term>
<listitem><para>Increase the timeout before default entry is booted</para>
<xi:include href="version-info.xml" xpointer="v239"/></listitem>
</varlistentry>
<varlistentry>
<term><keycap>-</keycap></term>
<term><keycap>T</keycap></term>
<listitem><para>Decrease the timeout</para>
<xi:include href="version-info.xml" xpointer="v239"/></listitem>
</varlistentry>
<varlistentry>
<term><keycap>r</keycap></term>
<listitem><para>Change screen resolution, skipping any unsupported modes.</para>
<xi:include href="version-info.xml" xpointer="v250"/></listitem>
</varlistentry>
<varlistentry>
<term><keycap>R</keycap></term>
<listitem><para>Reset screen resolution to firmware or configuration file default.</para>
<xi:include href="version-info.xml" xpointer="v250"/></listitem>
</varlistentry>
<varlistentry>
<term><keycap>p</keycap></term>
<listitem><para>Print status</para>
<xi:include href="version-info.xml" xpointer="v250"/></listitem>
</varlistentry>
<varlistentry>
<term><keycap>h</keycap></term>
<term><keycap>?</keycap></term>
<term><keycap>F1</keycap></term>
<listitem><para>Show a help screen</para>
<xi:include href="version-info.xml" xpointer="v239"/></listitem>
</varlistentry>
<varlistentry>
<term><keycap>f</keycap></term>
<listitem><para>Reboot into firmware interface.</para>
<para>For compatibility with the keybindings of several firmware implementations this operation
may also be reached with <keycap>F2</keycap>, <keycap>F10</keycap>, <keycap>Del</keycap> and
<keycap>Esc</keycap>.</para>
<xi:include href="version-info.xml" xpointer="v250"/></listitem>
</varlistentry>
<varlistentry>
<term><keycombo><keycap>Shift</keycap><keycap>o</keycap></keycombo></term>
<listitem><para>Power off the system.</para>
<xi:include href="version-info.xml" xpointer="v255"/></listitem>
</varlistentry>
<varlistentry>
<term><keycombo><keycap>Shift</keycap><keycap>b</keycap></keycombo></term>
<listitem><para>Reboot the system.</para>
<xi:include href="version-info.xml" xpointer="v255"/></listitem>
</varlistentry>
</variablelist>
<para>The following keys may be pressed during bootup or in the boot menu to directly boot a specific
entry:</para>
<variablelist>
<varlistentry>
<term><keycap>l</keycap></term>
<listitem><para>Linux</para>
<xi:include href="version-info.xml" xpointer="v239"/></listitem>
</varlistentry>
<varlistentry>
<term><keycap>w</keycap></term>
<listitem><para>Windows</para>
<xi:include href="version-info.xml" xpointer="v239"/></listitem>
</varlistentry>
<varlistentry>
<term><keycap>a</keycap></term>
<listitem><para>macOS</para>
<xi:include href="version-info.xml" xpointer="v239"/></listitem>
</varlistentry>
<varlistentry>
<term><keycap>s</keycap></term>
<listitem><para>EFI shell</para>
<xi:include href="version-info.xml" xpointer="v239"/></listitem>
</varlistentry>
<varlistentry>
<term><keycap>1</keycap></term>
<term><keycap>2</keycap></term>
<term><keycap>3</keycap></term>
<term><keycap>4</keycap></term>
<term><keycap>5</keycap></term>
<term><keycap>6</keycap></term>
<term><keycap>7</keycap></term>
<term><keycap>8</keycap></term>
<term><keycap>9</keycap></term>
<listitem><para>Boot entry number 1 … 9</para>
<xi:include href="version-info.xml" xpointer="v239"/></listitem>
</varlistentry>
</variablelist>
<para>The boot menu is shown when a non-zero menu timeout has been configured. If the menu timeout has
been set to zero, it is sufficient to press any key — before the boot loader initializes — to bring up
the boot menu, except for the keys listed immediately above as they directly boot into the selected boot
menu item. Note that depending on the firmware implementation the time window where key presses are
accepted before the boot loader initializes might be short. If the window is missed, reboot and try
again, possibly pressing a suitable key (e.g. the space bar) continuously; on most systems it should be
possible to hit the time window after a few attempts. To avoid this problem, consider setting a non-zero
timeout, thus showing the boot menu unconditionally. Some desktop environments might offer an option to
directly boot into the boot menu, to avoid the problem altogether. Alternatively, use the command line
<command>systemctl reboot --boot-loader-menu=0</command> from the shell.</para>
<para>In the editor, most keys simply insert themselves, but the following keys
may be used to perform additional actions:</para>
<variablelist>
<varlistentry>
<term><keycap>←</keycap> (Left)</term>
<term><keycap>→</keycap> (Right)</term>
<term><keycap>Home</keycap></term>
<term><keycap>End</keycap></term>
<listitem><para>Navigate left/right</para>
<xi:include href="version-info.xml" xpointer="v239"/></listitem>
</varlistentry>
<varlistentry>
<term><keycap>Esc</keycap></term>
<term><keycombo><keycap>Ctrl</keycap><keycap>c</keycap></keycombo></term>
<listitem><para>Abort the edit and quit the editor</para>
<xi:include href="version-info.xml" xpointer="v239"/></listitem>
</varlistentry>
<varlistentry>
<term><keycombo><keycap>Ctrl</keycap><keycap>k</keycap></keycombo></term>
<listitem><para>Clear the command line forwards</para>
<xi:include href="version-info.xml" xpointer="v239"/></listitem>
</varlistentry>
<varlistentry>
<term><keycombo><keycap>Ctrl</keycap><keycap>w</keycap></keycombo></term>
<term><keycombo><keycap>Alt</keycap><keycap>Backspace</keycap></keycombo></term>
<listitem><para>Delete word backwards</para>
<xi:include href="version-info.xml" xpointer="v239"/></listitem>
</varlistentry>
<varlistentry>
<term><keycombo><keycap>Ctrl</keycap><keycap>Del</keycap></keycombo></term>
<term><keycombo><keycap>Alt</keycap><keycap>d</keycap></keycombo></term>
<listitem><para>Delete word forwards</para>
<xi:include href="version-info.xml" xpointer="v239"/></listitem>
</varlistentry>
<varlistentry>
<term><keycap>↵</keycap> (Enter)</term>
<listitem><para>Boot entry with the edited command line</para>
<xi:include href="version-info.xml" xpointer="v239"/></listitem>
</varlistentry>
</variablelist>
<para>Note that unless configured otherwise in the UEFI firmware, systemd-boot will
use the US keyboard layout, so key labels might not match for keys like +/-.
</para>
</refsect1>
<refsect1>
<title>Files</title>
<para>The files <command>systemd-boot</command> processes generally reside on the UEFI ESP which is
usually mounted to <filename>/efi/</filename>, <filename>/boot/</filename> or
<filename>/boot/efi/</filename> during OS runtime. It also processes files on the Extended Boot Loader
partition which is typically mounted to <filename>/boot/</filename>, if it
exists.</para>
<para><command>systemd-boot</command> reads runtime configuration such as the boot timeout and default
entry from <filename>/loader/loader.conf</filename> on the ESP (in combination with data read from EFI
variables). See
<citerefentry><refentrytitle>loader.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
<para>Boot entry description files following the <ulink
url="https://uapi-group.org/specifications/specs/boot_loader_specification">Boot Loader Specification</ulink> are read from
<filename>/loader/entries/</filename> on the ESP and the Extended Boot Loader partition.</para>
<para>Unified kernel boot entries following the <ulink
url="https://uapi-group.org/specifications/specs/boot_loader_specification">Boot Loader Specification</ulink> are read from
<filename>/EFI/Linux/</filename> on the ESP and the Extended Boot Loader partition.</para>
<para>Optionally, a random seed for early boot entropy pool provisioning is stored in
<filename>/loader/random-seed</filename> in the ESP.</para>
<para>During initialization, <command>sd-boot</command> automatically loads all driver files placed in
the <filename>/EFI/systemd/drivers/</filename> directory of the ESP. The files placed there must have an
extension of the EFI architecture ID followed by <filename>.efi</filename> (e.g. for x86-64 this means a
suffix of <filename>x64.efi</filename>). This may be used to automatically load file system drivers and
similar, to extend the native firmware support.</para>
<para>Enrollment of Secure Boot variables can be performed manually or automatically if files are available
under <filename>/loader/keys/<replaceable>NAME</replaceable>/{db,dbx,KEK,PK}.auth</filename>, <replaceable>NAME</replaceable>
being the display name for the set of variables in the menu. If one of the sets is named <filename>auto</filename>
then it might be enrolled automatically depending on whether <literal>secure-boot-enroll</literal> is set
to force or not.</para>
</refsect1>
<refsect1>
<title>EFI Variables</title>
<para>The following EFI variables are defined, set and read by <command>systemd-boot</command>, under the
vendor UUID <literal>4a67b082-0a4c-41cf-b6c7-440b29bb8c4f</literal>, for communication between the boot
loader and the OS:</para>
<variablelist class='efi-variables'>
<varlistentry>
<term><varname>LoaderBootCountPath</varname></term>
<listitem><para>If boot counting is enabled, contains the path to the file in whose name the boot counters are
encoded. Set by the boot
loader. <citerefentry><refentrytitle>systemd-bless-boot.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
uses this information to mark a boot as successful as determined by the successful activation of the
<filename>boot-complete.target</filename> target unit.</para>
<xi:include href="version-info.xml" xpointer="v240"/></listitem>
</varlistentry>
<varlistentry>
<term><varname>LoaderConfigTimeout</varname></term>
<term><varname>LoaderConfigTimeoutOneShot</varname></term>
<listitem><para>The menu timeout in seconds. Read by the boot loader. <varname>LoaderConfigTimeout</varname>
is maintained persistently, while <varname>LoaderConfigTimeoutOneShot</varname> is a one-time override which is
read once (in which case it takes precedence over <varname>LoaderConfigTimeout</varname>) and then
removed. <varname>LoaderConfigTimeout</varname> may be manipulated with the
<keycap>t</keycap>/<keycap>T</keycap> keys, see above.</para>
<xi:include href="version-info.xml" xpointer="v240"/></listitem>
</varlistentry>
<varlistentry>
<term><varname>LoaderConfigConsoleMode</varname></term>
<listitem><para>The numerical menu console mode. Read by the boot loader. <varname>LoaderConfigConsoleMode</varname>
is maintained persistently. <varname>LoaderConfigConsoleMode</varname> may be manipulated with the
<keycap>r</keycap>/<keycap>R</keycap> keys, see above.</para>
<xi:include href="version-info.xml" xpointer="v250"/></listitem>
</varlistentry>
<varlistentry>
<term><varname>LoaderDevicePartUUID</varname></term>
<listitem><para>Contains the partition UUID of the EFI System Partition the boot loader was run from. Set by
the boot
loader. <citerefentry><refentrytitle>systemd-gpt-auto-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
uses this information to automatically find the disk booted from, in order to discover various other partitions
on the same disk automatically.</para>
<xi:include href="version-info.xml" xpointer="v240"/></listitem>
</varlistentry>
<varlistentry>
<term><varname>LoaderEntries</varname></term>
<listitem><para>A list of the identifiers of all discovered boot loader entries. Set by the boot
loader.</para>
<xi:include href="version-info.xml" xpointer="v240"/></listitem>
</varlistentry>
<varlistentry>
<term><varname>LoaderEntryDefault</varname></term>
<term><varname>LoaderEntryOneShot</varname></term>
<listitem><para>The identifier of the default boot loader entry. Set primarily by the OS and read by the boot
loader. <varname>LoaderEntryOneShot</varname> sets the default entry for the next boot only, while
<varname>LoaderEntryDefault</varname> sets it persistently for all future
boots. <citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
<option>set-default</option> and <option>set-oneshot</option> commands make use of these variables. The boot
loader modifies <varname>LoaderEntryDefault</varname> on request, when the <keycap>d</keycap> key is used, see
above.</para>
<xi:include href="version-info.xml" xpointer="v240"/></listitem>
</varlistentry>
<varlistentry>
<term><varname>LoaderEntryLastBooted</varname></term>
<listitem><para>The identifier of the boot loader entry last attempted. Set and read by the boot loader,
only when <filename>/loader/loader.conf</filename> has default set to <literal>@saved</literal>. See
<citerefentry><refentrytitle>loader.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>.</para>
<para> The boot loader will ensure <varname>LoaderEntryLastBooted</varname> is up-to date for every boot,
updating it as needed and will omit changing it all together when <varname>LoaderEntryOneShot</varname>
is set.</para>
<para>The boot loader reads the variable, which takes higher priority than
<varname>LoaderEntryDefault</varname>. The variable is ignored when <varname>LoaderEntryOneShot</varname>
is set.</para>
<para><varname>LoaderEntryLastBooted</varname> cannot be used as indication that the last boot was
successful or not.</para>
<xi:include href="version-info.xml" xpointer="v250"/></listitem>
</varlistentry>
<varlistentry>
<term><varname>LoaderEntrySelected</varname></term>
<listitem><para>The identifier of the boot loader entry currently being booted. Set by the boot
loader.</para>
<xi:include href="version-info.xml" xpointer="v240"/></listitem>
</varlistentry>
<varlistentry>
<term><varname>LoaderFeatures</varname></term>
<listitem><para>A set of flags indicating the features the boot loader supports. Set by the boot loader. Use
<citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view this
data.</para>
<xi:include href="version-info.xml" xpointer="v240"/></listitem>
</varlistentry>
<varlistentry>
<term><varname>LoaderFirmwareInfo</varname></term>
<term><varname>LoaderFirmwareType</varname></term>
<listitem><para>Brief firmware information. Set by the boot loader. Use
<citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view this
data.</para>
<xi:include href="version-info.xml" xpointer="v240"/></listitem>
</varlistentry>
<varlistentry>
<term><varname>LoaderImageIdentifier</varname></term>
<listitem><para>The path of executable of the boot loader used for the current boot, relative to the EFI System
Partition's root directory. Set by the boot loader. Use
<citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view this
data.</para>
<xi:include href="version-info.xml" xpointer="v240"/></listitem>
</varlistentry>
<varlistentry>
<term><varname>LoaderInfo</varname></term>
<listitem><para>Brief information about the boot loader. Set by the boot loader. Use
<citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry> to view this
data.</para>
<xi:include href="version-info.xml" xpointer="v240"/></listitem>
</varlistentry>
<varlistentry>
<term><varname>LoaderTimeExecUSec</varname></term>
<term><varname>LoaderTimeInitUSec</varname></term>
<term><varname>LoaderTimeMenuUsec</varname></term>
<listitem><para>Information about the time spent in various parts of the boot loader. Set by the boot
loader. Use <citerefentry><refentrytitle>systemd-analyze</refentrytitle><manvolnum>1</manvolnum></citerefentry>
to view this data. </para>
<xi:include href="version-info.xml" xpointer="v240"/></listitem>
</varlistentry>
<varlistentry>
<term><varname>LoaderSystemToken</varname></term>
<listitem><para>A binary random data field, that is used for generating the random seed to pass to
the OS (see above). Note that this random data is generally only generated once, during OS
installation, and is then never updated again.</para>
<xi:include href="version-info.xml" xpointer="v243"/></listitem>
</varlistentry>
</variablelist>
<para>Many of these variables are defined by the <ulink
url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink>.</para>
</refsect1>
<refsect1>
<title>SMBIOS Type 11 Strings</title>
<para><command>systemd-boot</command> can be configured using SMBIOS Type 11 strings. Applicable strings
consist of a name, followed by <literal>=</literal>, followed by the value. Unless
<command>systemd-boot</command> detects it is running inside a confidential computing environment,
<command>systemd-boot</command> will search the table for a string with a specific name, and if found,
use its value. The following strings are read:</para>
<variablelist>
<varlistentry>
<term><varname>io.systemd.boot.kernel-cmdline-extra</varname></term>
<listitem><para>If set, the value of this string is added to the list of kernel command line
arguments for Boot Loader Specification Type 1 entries that are measured in PCR12 and passed to the
kernel.</para>
<xi:include href="version-info.xml" xpointer="v256"/></listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1>
<title>Boot Counting</title>
<para><command>systemd-boot</command> implements a simple boot counting mechanism on top of the <ulink
url="https://uapi-group.org/specifications/specs/boot_loader_specification">Boot Loader Specification</ulink>, for automatic and unattended
fallback to older kernel versions/boot loader entries when a specific entry continuously fails. Any boot loader
entry file and unified kernel image file that contains a <literal>+</literal> followed by one or two numbers (if
two they need to be separated by a <literal>-</literal>), before the <filename>.conf</filename> or
<filename>.efi</filename> suffix is subject to boot counting: the first of the two numbers ('tries left') is
decreased by one on every boot attempt, the second of the two numbers ('tries done') is increased by one (if 'tries
done' is absent it is considered equivalent to 0). Depending on the current value of these two counters the boot
entry is considered to be in one of three states:</para>
<orderedlist>
<listitem><para>If the 'tries left' counter of an entry is greater than zero the entry is considered to be in
'indeterminate' state. This means the entry has not completed booting successfully yet, but also hasn't been
determined not to work.</para></listitem>
<listitem><para>If the 'tries left' counter of an entry is zero it is considered to be in 'bad' state. This means
no further attempts to boot this item will be made (that is, unless all other boot entries are also in 'bad'
state), as all attempts to boot this entry have not completed successfully.</para></listitem>
<listitem><para>If the 'tries left' and 'tries done' counters of an entry are absent it is considered to be in
'good' state. This means further boot counting for the entry is turned off, as it successfully booted at least
once. The
<citerefentry><refentrytitle>systemd-bless-boot.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>
service moves the currently booted entry from 'indeterminate' into 'good' state when a boot attempt completed
successfully.</para></listitem>
</orderedlist>
<para>Generally, when new entries are added to the boot loader, they first start out in 'indeterminate' state,
i.e. with a 'tries left' counter greater than zero. The boot entry remains in this state until either it managed to
complete a full boot successfully at least once (in which case it will be in 'good' state) — or the 'tries left'
counter reaches zero (in which case it will be in 'bad' state).</para>
<para>Example: let's say a boot loader entry file <filename>foo.conf</filename> is set up for 3 boot tries. The
installer will hence create it under the name <filename>foo+3.conf</filename>. On first boot, the boot loader will
rename it to <filename>foo+2-1.conf</filename>. If that boot does not complete successfully, the boot loader will
rename it to <filename>foo+1-2.conf</filename> on the following boot. If that fails too, it will finally be renamed
<filename>foo+0-3.conf</filename> by the boot loader on next boot, after which it will be considered 'bad'. If the
boot succeeds however the entry file will be renamed to <filename>foo.conf</filename> by the OS, so that it is
considered 'good' from then on.</para>
<para>The boot menu takes the 'tries left' counter into account when sorting the menu entries: entries in 'bad'
state are ordered at the beginning of the list, and entries in 'good' or 'indeterminate' at the end. The user can
freely choose to boot any entry of the menu, including those already marked 'bad'. If the menu entry to boot is
automatically determined, this means that 'good' or 'indeterminate' entries are generally preferred (as the bottom
item of the menu is the one booted by default), and 'bad' entries will only be considered if there are no 'good' or
'indeterminate' entries left.</para>
<para>The <citerefentry><refentrytitle>kernel-install</refentrytitle><manvolnum>8</manvolnum></citerefentry> kernel
install framework optionally sets the initial 'tries left' counter to the value specified in
<filename>/etc/kernel/tries</filename> when a boot loader entry is first created.</para>
</refsect1>
<refsect1>
<title>Using <command>systemd-boot</command> in virtual machines</title>
<para>When using qemu with OVMF (UEFI Firmware for virtual machines) the <option>-kernel</option> switch
works not only for linux kernels, but for any EFI binary, including sd-boot and unified linux
kernels. Example command line for loading <command>systemd-boot</command> on x64:</para>
<para>
<command>qemu-system-x86_64 <replaceable>[ ... ]</replaceable>
-kernel /usr/lib/systemd/boot/efi/systemd-bootx64.efi</command>
</para>
<para>systemd-boot will detect that it was started directly instead of being loaded from ESP and will
search for the ESP in that case, taking into account boot order information from the hypervisor (if
available).</para>
</refsect1>
<refsect1>
<title>See Also</title>
<para><simplelist type="inline">
<member><citerefentry><refentrytitle>bootctl</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>loader.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>systemd-bless-boot.service</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>systemd-boot-random-seed.service</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>kernel-install</refentrytitle><manvolnum>8</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>systemd-stub</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
<member><ulink url="https://uapi-group.org/specifications/specs/boot_loader_specification">Boot Loader Specification</ulink></member>
<member><ulink url="https://systemd.io/BOOT_LOADER_INTERFACE">Boot Loader Interface</ulink></member>
<member><ulink url="https://systemd.io/TPM2_PCR_MEASUREMENTS">TPM2 PCR Measurements Made by systemd</ulink></member>
</simplelist></para>
</refsect1>
</refentry>
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