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
  74 If `LoaderTimeInitUSec` and `LoaderTimeExecUSec` are set, `systemd-analyze`
  75 will include them in its boot-time analysis.  If `LoaderDevicePartUUID` is set,
  76 systemd will mount the ESP that was used for the boot to `/boot`, but only if
  77 that directory is empty, and only if no other file systems are mounted
  78 there. The `systemctl reboot --boot-loader-entry=…` and `systemctl reboot
  79 --boot-loader-menu=…` commands rely on the `LoaderFeatures` ,
  80 `LoaderConfigTimeoutOneShot`, `LoaderEntries`, `LoaderEntryOneShot` variables.
  81
  82 ## Boot Loader Entry Identifiers
  83
  84 While boot loader entries may be named relatively freely, it's highly
  85 recommended to follow the following rules when picking identifiers for the
  86 entries, so that programs (and users) can derive basic context and meaning from
  87 the identifiers as passed in `LoaderEntries`, `LoaderEntryDefault`,
  88 `LoaderEntryOneShot`, `LoaderEntrySelected`, and possibly show nicely localized
  89 names for them in UIs.
  90
  91 1. When boot loader entries are defined through [Boot Loader
  92    Specification](https://systemd.io/BOOT_LOADER_SPECIFICATION) drop-in files
  93    the identifier should be derived directly from the drop-in snippet name, but
  94    with the `.conf` (or `.efi` in case of Type #2 entries) suffix removed.
  95
  96 2. Entries automatically discovered by the boot loader (as opposed to being
  97    configured in configuration files) should generally have an identifier
  98    prefixed with `auto-`.
  99
 100 3. Boot menu entries referring to Microsoft Windows installations should either
 101    use the identifier `windows` or use the `windows-` prefix for the
 102    identifier. If a menu entry is automatically discovered, it should be
 103    prefixed with `auto-`, see above (Example: this means an automatically
 104    discovered Windows installation might have the identifier `auto-windows` or
 105    `auto-windows-10` or so.).
 106
 107 4. Similar, boot menu entries referring to Apple MacOS X installations should
 108    use the identifier `osx` or one that is prefixed with `osx-`. If such an
 109    entry is automatically discovered by the boot loader use `auto-osx` as
 110    identifier, or `auto-osx-` as prefix for the identifier, see above.
 111
 112 5. If a boot menu entry encapsulates the EFI shell program, it should use the
 113    identifier `efi-shell` (or when automatically discovered: `auto-efi-shell`,
 114    see above).
 115
 116 6. If a boot menu entry encapsulates a reboot into EFI firmware setup feature,
 117    it should use the identifier `reboot-to-firmware-setup` (or
 118    `auto-reboot-to-firmware-setup` in case it is automatically discovered).