docs/BOOT_LOADER_INTERFACE.md

   1 ---
   2 title: The Boot Loader Interface
   3 ---
   4
   5 # The Boot Loader Interface
   6
   7 systemd can interface with the boot loader to receive performance data and
   8 other information, and pass control information. This is only supported on EFI
   9 systems. Data is transferred between the boot loader and systemd in EFI
  10 variables. All EFI variables use the vendor UUID
  11 `4a67b082-0a4c-41cf-b6c7-440b29bb8c4f`.
  12
  13 * The EFI Variable `LoaderTimeInitUSec` contains the timestamp in microseconds
  14   when the loader was initialized. This value is the time spent in the firmware
  15   for initialization, it is formatted as numeric, NUL-terminated, decimal
  16   string, in UTF-16.
  17
  18 * The EFI Variable `LoaderTimeExecUSec` contains the timestamp in microseconds
  19   when the loader finished its work and is about to execute the kernel. The
  20   time spent in the loader is the difference between `LoaderTimeExecUSec` and
  21   `LoaderTimeInitUSec`. This value is formatted the same way as
  22   `LoaderTimeInitUSec`.
  23
  24 * The EFI variable `LoaderDevicePartUUID` contains the partition GUID of the
  25   ESP the boot loader was run from formatted as NUL-terminated UTF16 string, in
  26   normal GUID syntax.
  27
  28 * The EFI variable `LoaderConfigTimeout` contains the boot menu timeout
  29   currently in use. It may be modified both by the boot loader and by the
  30   host. The value should be formatted as numeric, NUL-terminated, decimal
  31   string, in UTF-16. The time is specified in µs.
  32
  33 * Similarly, the EFI variable `LoaderConfigTimeoutOneShot` contains a boot menu
  34   timeout for a single following boot. It is set by the OS in order to request
  35   display of the boot menu on the following boot. When set overrides
  36   `LoaderConfigTimeout`. It is removed automatically after being read by the
  37   boot loader, to ensure it only takes effect a single time. This value is
  38   formatted the same way as `LoaderConfigTimeout`. If set to `0` the boot menu
  39   timeout is turned off, and the menu is shown indefinitely.
  40
  41 * The EFI variable `LoaderEntries` may contain a series of boot loader entry
  42   identifiers, one after the other, each individually NUL terminated. This may
  43   be used to let the OS know which boot menu entries were discovered by the
  44   boot loader. A boot loader entry identifier should be a short, non-empty
  45   alphanumeric string (possibly containing `-`, too). The list should be in the
  46   order the entries are shown on screen during boot. See below regarding a
  47   recommended vocabulary for boot loader entry identifiers.
  48
  49 * The EFI variable `LoaderEntryDefault` contains the default boot loader entry
  50   to use. It contains a NUL-terminated boot loader entry identifier.
  51
  52 * Similarly, the EFI variable `LoaderEntryOneShot` contains the default boot
  53   loader entry to use for a single following boot. It is set by the OS in order
  54   to request booting into a specific menu entry on the following boot. When set
  55   overrides `LoaderEntryDefault`. It is removed automatically after being read
  56   by the boot loader, to ensure it only takes effect a single time. This value
  57   is formatted the same way as `LoaderEntryDefault`.
  58
  59 * The EFI variable `LoaderEntrySelected` contains the boot loader entry
  60   identifier that was booted. It is set by the boot loader and read by
  61   the OS in order to identify which entry has been used for the current boot.
  62
  63 * The EFI variable `LoaderFeatures` contains a 64bit unsigned integer with a
  64   number of flags bits that are set by the boot loader and passed to the OS and
  65   indicate the features the boot loader supports. Specifically, the following
  66   bits are defined:
  67
  68   * `1 << 0` → The boot loader honours `LoaderConfigTimeout` when set.
  69   * `1 << 1` → The boot loader honours `LoaderConfigTimeoutOneShot` when set.
  70   * `1 << 2` → The boot loader honours `LoaderEntryDefault` when set.
  71   * `1 << 3` → The boot loader honours `LoaderEntryOneShot` when set.
  72   * `1 << 4` → The boot loader supports boot counting as described in [Automatic Boot Assessment](https://systemd.io/AUTOMATIC_BOOT_ASSESSMENT).
  73   * `1 << 5` → The boot loader supports looking for boot menu entries in the Extended Boot Loader Partition.
  74   * `1 << 6` → The boot loader supports passing a random seed to the OS.
  75
  76 * The EFI variable `LoaderRandomSeed` contains a binary random seed if set. It
  77   is set by the boot loader to pass an entropy seed read from the ESP partition
  78   to the OS. The system manager then credits this seed to the kernel's entropy
  79   pool. It is the responsibility of the boot loader to ensure the quality and
  80   integrity of the random seed.
  81
  82 * The EFI variable `LoaderSystemToken` contains binary random data,
  83   persistently set by the OS installer. Boot loaders that support passing
  84   random seeds to the OS should use this data and combine it with the random
  85   seed file read from the ESP. By combining this random data with the random
  86   seed read off the disk before generating a seed to pass to the OS and a new
  87   seed to store in the ESP the boot loader can protect itself from situations
  88   where "golden" OS images that include a random seed are replicated and used
  89   on multiple systems. Since the EFI variable storage is usually independent
  90   (i.e. in physical NVRAM) of the ESP file system storage, and only the latter
  91   is part of "golden" OS images, this ensures that different systems still come
  92   up with different random seeds. Note that the `LoaderSystemToken` is
  93   generally only written once, by the OS installer, and is usually not touched
  94   after that.
  95
  96 If `LoaderTimeInitUSec` and `LoaderTimeExecUSec` are set, `systemd-analyze`
  97 will include them in its boot-time analysis.  If `LoaderDevicePartUUID` is set,
  98 systemd will mount the ESP that was used for the boot to `/boot`, but only if
  99 that directory is empty, and only if no other file systems are mounted
 100 there. The `systemctl reboot --boot-loader-entry=…` and `systemctl reboot
 101 --boot-loader-menu=…` commands rely on the `LoaderFeatures` ,
 102 `LoaderConfigTimeoutOneShot`, `LoaderEntries`, `LoaderEntryOneShot`
 103 variables. `LoaderRandomSeed` is read by PID during early boot and credited to
 104 the kernel's random pool.
 105
 106 ## Boot Loader Entry Identifiers
 107
 108 While boot loader entries may be named relatively freely, it's highly
 109 recommended to follow the following rules when picking identifiers for the
 110 entries, so that programs (and users) can derive basic context and meaning from
 111 the identifiers as passed in `LoaderEntries`, `LoaderEntryDefault`,
 112 `LoaderEntryOneShot`, `LoaderEntrySelected`, and possibly show nicely localized
 113 names for them in UIs.
 114
 115 1. When boot loader entries are defined through [Boot Loader
 116    Specification](https://systemd.io/BOOT_LOADER_SPECIFICATION) drop-in files
 117    the identifier should be derived directly from the drop-in snippet name, but
 118    with the `.conf` (or `.efi` in case of Type #2 entries) suffix removed.
 119
 120 2. Entries automatically discovered by the boot loader (as opposed to being
 121    configured in configuration files) should generally have an identifier
 122    prefixed with `auto-`.
 123
 124 3. Boot menu entries referring to Microsoft Windows installations should either
 125    use the identifier `windows` or use the `windows-` prefix for the
 126    identifier. If a menu entry is automatically discovered, it should be
 127    prefixed with `auto-`, see above (Example: this means an automatically
 128    discovered Windows installation might have the identifier `auto-windows` or
 129    `auto-windows-10` or so.).
 130
 131 4. Similar, boot menu entries referring to Apple MacOS X installations should
 132    use the identifier `osx` or one that is prefixed with `osx-`. If such an
 133    entry is automatically discovered by the boot loader use `auto-osx` as
 134    identifier, or `auto-osx-` as prefix for the identifier, see above.
 135
 136 5. If a boot menu entry encapsulates the EFI shell program, it should use the
 137    identifier `efi-shell` (or when automatically discovered: `auto-efi-shell`,
 138    see above).
 139
 140 6. If a boot menu entry encapsulates a reboot into EFI firmware setup feature,
 141    it should use the identifier `reboot-to-firmware-setup` (or
 142    `auto-reboot-to-firmware-setup` in case it is automatically discovered).