docs/_booting/BOOT_LOADER_INTERFACE.md

   1 ---
   2 title: Boot Loader Interface
   3 category: Booting
   4 layout: default
   5 SPDX-License-Identifier: LGPL-2.1-or-later
   6 ---
   7
   8 # The Boot Loader Interface
   9
  10 systemd can interface with the boot loader to receive performance data and
  11 other information, and pass control information. This is only supported on EFI
  12 systems. Data is transferred between the boot loader and systemd in EFI
  13 variables. All EFI variables use the vendor UUID
  14 `4a67b082-0a4c-41cf-b6c7-440b29bb8c4f`.
  15
  16 * The EFI Variable `LoaderTimeInitUSec` contains the timestamp in microseconds
  17   when the loader was initialized. This value is the time spent in the firmware
  18   for initialization, it is formatted as numeric, NUL-terminated, decimal
  19   string, in UTF-16.
  20
  21 * The EFI Variable `LoaderTimeExecUSec` contains the timestamp in microseconds
  22   when the loader finished its work and is about to execute the kernel. The
  23   time spent in the loader is the difference between `LoaderTimeExecUSec` and
  24   `LoaderTimeInitUSec`. This value is formatted the same way as
  25   `LoaderTimeInitUSec`.
  26
  27 * The EFI variable `LoaderDevicePartUUID` contains the partition GUID of the
  28   ESP the boot loader was run from formatted as NUL-terminated UTF16 string, in
  29   normal GUID syntax.
  30
  31 * The EFI variable `LoaderConfigTimeout` contains the boot menu timeout
  32   currently in use. It may be modified both by the boot loader and by the
  33   host. The value should be formatted as numeric, NUL-terminated, decimal
  34   string, in UTF-16. The time is specified in seconds. In addition some
  35   non-numeric string values are also accepted. A value of `menu-force`
  36   will disable the timeout and show the menu indefinitely. If set to `0` or
  37   `menu-hidden` the default entry is booted immediately without showing a menu.
  38   Unless a value of `menu-disabled` is set, the boot loader should provide a
  39   way to interrupt this by for example listening for key presses for a brief
  40   moment before booting.
  41
  42 * Similarly, the EFI variable `LoaderConfigTimeoutOneShot` contains a boot menu
  43   timeout for a single following boot. It is set by the OS in order to request
  44   display of the boot menu on the following boot. When set overrides
  45   `LoaderConfigTimeout`. It is removed automatically after being read by the
  46   boot loader, to ensure it only takes effect a single time. This value is
  47   formatted the same way as `LoaderConfigTimeout`. If set to `0` the boot menu
  48   timeout is turned off, and the menu is shown indefinitely.
  49
  50 * The EFI variable `LoaderEntries` may contain a series of boot loader entry
  51   identifiers, one after the other, each individually NUL terminated. This may
  52   be used to let the OS know which boot menu entries were discovered by the
  53   boot loader. A boot loader entry identifier should be a short, non-empty
  54   alphanumeric string (possibly containing `-`, too). The list should be in the
  55   order the entries are shown on screen during boot. See below regarding a
  56   recommended vocabulary for boot loader entry identifiers.
  57
  58 * The EFI variable `LoaderEntryDefault` contains the default boot loader entry
  59   to use. It contains a NUL-terminated boot loader entry identifier.
  60
  61 * Similarly, the EFI variable `LoaderEntryOneShot` contains the default boot
  62   loader entry to use for a single following boot. It is set by the OS in order
  63   to request booting into a specific menu entry on the following boot. When set
  64   overrides `LoaderEntryDefault`. It is removed automatically after being read
  65   by the boot loader, to ensure it only takes effect a single time. This value
  66   is formatted the same way as `LoaderEntryDefault`.
  67
  68 * The EFI variable `LoaderEntrySelected` contains the boot loader entry
  69   identifier that was booted. It is set by the boot loader and read by
  70   the OS in order to identify which entry has been used for the current boot.
  71
  72 * The EFI variable `LoaderFeatures` contains a 64-bit unsigned integer with a
  73   number of flags bits that are set by the boot loader and passed to the OS and
  74   indicate the features the boot loader supports. Specifically, the following
  75   bits are defined:
  76
  77   * `1 << 0` → The boot loader honours `LoaderConfigTimeout` when set.
  78   * `1 << 1` → The boot loader honours `LoaderConfigTimeoutOneShot` when set.
  79   * `1 << 2` → The boot loader honours `LoaderEntryDefault` when set.
  80   * `1 << 3` → The boot loader honours `LoaderEntryOneShot` when set.
  81   * `1 << 4` → The boot loader supports boot counting as described in [Automatic Boot Assessment](AUTOMATIC_BOOT_ASSESSMENT).
  82   * `1 << 5` → The boot loader supports looking for boot menu entries in the Extended Boot Loader Partition.
  83   * `1 << 6` → The boot loader supports passing a random seed to the OS.
  84   * `1 << 13` → The boot loader honours `menu-disabled` option when set.
  85
  86 * The EFI variable `LoaderSystemToken` contains binary random data,
  87   persistently set by the OS installer. Boot loaders that support passing
  88   random seeds to the OS should use this data and combine it with the random
  89   seed file read from the ESP. By combining this random data with the random
  90   seed read off the disk before generating a seed to pass to the OS and a new
  91   seed to store in the ESP the boot loader can protect itself from situations
  92   where "golden" OS images that include a random seed are replicated and used
  93   on multiple systems. Since the EFI variable storage is usually independent
  94   (i.e. in physical NVRAM) of the ESP file system storage, and only the latter
  95   is part of "golden" OS images, this ensures that different systems still come
  96   up with different random seeds. Note that the `LoaderSystemToken` is
  97   generally only written once, by the OS installer, and is usually not touched
  98   after that.
  99
 100 If `LoaderTimeInitUSec` and `LoaderTimeExecUSec` are set, `systemd-analyze`
 101 will include them in its boot-time analysis.  If `LoaderDevicePartUUID` is set,
 102 systemd will mount the ESP that was used for the boot to `/boot`, but only if
 103 that directory is empty, and only if no other file systems are mounted
 104 there. The `systemctl reboot --boot-loader-entry=…` and `systemctl reboot
 105 --boot-loader-menu=…` commands rely on the `LoaderFeatures` ,
 106 `LoaderConfigTimeoutOneShot`, `LoaderEntries`, `LoaderEntryOneShot`
 107 variables.
 108
 109 ## Boot Loader Entry Identifiers
 110
 111 While boot loader entries may be named relatively freely, it's highly
 112 recommended to follow the following rules when picking identifiers for the
 113 entries, so that programs (and users) can derive basic context and meaning from
 114 the identifiers as passed in `LoaderEntries`, `LoaderEntryDefault`,
 115 `LoaderEntryOneShot`, `LoaderEntrySelected`, and possibly show nicely localized
 116 names for them in UIs.
 117
 118 1. When boot loader entries are defined through the
 119    [Boot Loader Specification](https://uapi-group.org/specifications/specs/boot_loader_specification/)
 120    files, the identifier should be derived directly from the file name,
 121    but with the `.conf` (Type #1 snippets) or `.efi` (Type #2 images)
 122    suffix removed.
 123
 124 2. Entries automatically discovered by the boot loader (as opposed to being
 125    configured in configuration files) should generally have an identifier
 126    prefixed with `auto-`.
 127
 128 3. Boot menu entries referring to Microsoft Windows installations should either
 129    use the identifier `windows` or use the `windows-` prefix for the
 130    identifier. If a menu entry is automatically discovered, it should be
 131    prefixed with `auto-`, see above (Example: this means an automatically
 132    discovered Windows installation might have the identifier `auto-windows` or
 133    `auto-windows-10` or so.).
 134
 135 4. Similarly, boot menu entries referring to Apple macOS installations should
 136    use the identifier `osx` or one that is prefixed with `osx-`. If such an
 137    entry is automatically discovered by the boot loader use `auto-osx` as
 138    identifier, or `auto-osx-` as prefix for the identifier, see above.
 139
 140 5. If a boot menu entry encapsulates the EFI shell program, it should use the
 141    identifier `efi-shell` (or when automatically discovered: `auto-efi-shell`,
 142    see above).
 143
 144 6. If a boot menu entry encapsulates a reboot into EFI firmware setup feature,
 145    it should use the identifier `reboot-to-firmware-setup` (or
 146    `auto-reboot-to-firmware-setup` in case it is automatically discovered).
 147
 148 ## Links
 149
 150 [Boot Loader Specification](https://uapi-group.org/specifications/specs/boot_loader_specification)<br>
 151 [Discoverable Partitions Specification](https://uapi-group.org/specifications/specs/discoverable_partitions_specification)<br>
 152 [`systemd-boot(7)`](https://www.freedesktop.org/software/systemd/man/systemd-boot.html)<br>
 153 [`bootctl(1)`](https://www.freedesktop.org/software/systemd/man/bootctl.html)<br>
 154 [`systemd-gpt-auto-generator(8)`](https://www.freedesktop.org/software/systemd/man/systemd-gpt-auto-generator.html)