docs/BOOT_LOADER_INTERFACE.md

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