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77 lines
4.4 KiB
Markdown
77 lines
4.4 KiB
Markdown
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# The Boot Loader Interface
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systemd can interface with the boot loader to receive performance data and
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other information, and pass control information. This is only supported on EFI
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systems. Data is transferred between the boot loader and systemd in EFI
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variables. All EFI variables use the vendor UUID
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`4a67b082-0a4c-41cf-b6c7-440b29bb8c4f`.
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* The EFI Variable `LoaderTimeInitUSec` contains the timestamp in microseconds
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when the loader was initialized. This value is the time spent in the firmware
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for initialization, it is formatted as numeric, NUL-terminated, decimal
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string, in UTF-16.
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* The EFI Variable `LoaderTimeExecUSec` contains the timestamp in microseconds
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when the loader finished its work and is about to execute the kernel. The
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time spent in the loader is the difference between `LoaderTimeExecUSec` and
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`LoaderTimeInitUSec`. This value is formatted the same way as
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`LoaderTimeInitUSec`.
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* The EFI variable `LoaderDevicePartUUID` contains the partition GUID of the
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ESP the boot loader was run from formatted as NUL-terminated UTF16 string, in
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normal GUID syntax.
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* The EFI variable `LoaderConfigTimeout` contains the boot menu time-out
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currently in use. It may be modified both by the boot loader and by the
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host. The value should be formatted as numeric, NUL-terminated, decimal
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string, in UTF-16. The time is specified in µs.
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* Similarly, the EFI variable `LoaderConfigTimeoutOneShot` contains a boot menu
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time-out for a single following boot. It is set by the OS in order to request
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display of the boot menu on the following boot. When set overrides
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`LoaderConfigTimeout`. It is removed automatically after being read by the
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boot loader, to ensure it only takes effect a single time. This value is
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formatted the same way as `LoaderConfigTimeout`. If set to `0` the boot menu
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time-out is turned off, and the menu is shown indefinitely.
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* The EFI variable `LoaderEntries` may contain a series of boot loader entry
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identifiers, one after the other, each individually NUL terminated. This may
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be used to let the OS know which boot menu entries were discovered by the
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boot loader. A boot loader entry identifier should be a short, non-empty
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alphanumeric string (possibly containing `-`, too). The list should be in the
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order the entries are shown on screen during boot. See below regarding a
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recommended vocabulary for boot loader entry identifiers.
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* The EFI variable `LoaderEntryDefault` contains the default boot loader entry
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to use. It contains a NUL-terminated boot loader entry identifier.
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* Similarly, the EFI variable `LoaderEntryOneShot` contains the default boot
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loader entry to use for a single following boot. It is set by the OS in order
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to request booting into a specific menu entry on the following boot. When set
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overrides `LoaderEntryDefault`. It is removed automatically after being read
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by the boot loader, to ensure it only takes effect a single time. This value
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is formatted the same way as `LoaderEntryDefault`.
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* The EFI variable `LoaderEntrySelected` contains the boot loader entry
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identifier that was booted. It is set by the boot loader and read by
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the OS in order to identify which entry has been used for the current boot.
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* The EFI variable `LoaderFeatures` contains a 64bit unsigned integer with a
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number of flags bits that are set by the boot loader and passed to the OS and
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indicate the features the boot loader supports. Specifically, the following
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bits are defined:
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* `1 << 0` → The boot loader honours `LoaderConfigTimeout` when set.
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* `1 << 1` → The boot loader honours `LoaderConfigTimeoutOneShot` when set.
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* `1 << 2` → The boot loader honours `LoaderEntryDefault` when set.
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* `1 << 3` → The boot loader honours `LoaderEntryOneShot` when set.
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* `1 << 4` → The boot loader supports boot counting as described in [Automatic Boot Assessment](https://systemd.io/AUTOMATIC_BOOT_ASSESSMENT).
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If `LoaderTimeInitUSec` and `LoaderTimeExecUSec` are set, `systemd-analyze`
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will include them in its boot-time analysis. If `LoaderDevicePartUUID` is set,
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systemd will mount the ESP that was used for the boot to `/boot`, but only if
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that directory is empty, and only if no other file systems are mounted
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there. The `systemctl reboot --boot-loader-entry=…` and `systemctl reboot
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--boot-loader-menu=…` commands rely on the `LoaderFeatures` ,
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`LoaderConfigTimeoutOneShot`, `LoaderEntries`, `LoaderEntryOneShot` variables.
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