UEFI Fallback

UEFI provides a mechanism for booting from an EFI file without a corresponding boot variable existing in NVRAM. This is known as the UEFI fallback mode, and it uses a specific file path (/EFI/BOOT) to locate the fallback bootloader.

Trident leverages this UEFI feature with UEFI fallback modes by copying boot files into the UEFI fallback path during OS servicing. These boot files determine what OS gets booted when the system is started in UEFI fallback mode.

There are 3 UEFI fallback modes that determine which OS boot files are used for the UEFI fallback path during OS servicing: disabled, optimistic, and conservative.

Specifying disabled as the UEFI fallback mode means that Trident will not populate the UEFI fallback boot files during OS servicing. This is useful for multiboot scenarios or when the UEFI fallback is managed outside of Trident. Note that if there are no UEFI fallback files present, the system may not boot successfully if the UEFI variables are not found.

The remaining modes, conservative and optimistic, are designed to complement Trident's UEFI variable management during OS servicing.

The default mode is conservative which aligns to how Trident manages the UEFI boot variables during OS servicing.

• trident install
  • finalize: Trident updates the UEFI boot order and UEFI fallback path so that the target OS is booted.
  • commit: No changes to UEFI boot variables or UEFI fallback path are needed as the target OS is already configured to be booted.
• trident update
  • finalize: If the UEFI Fallback mode is conservative, the UEFI fallback path is updated to boot the servicing OS. This aligns to the UEFI variables.
  • commit: If the target OS boots successfully and the UEFI Fallback mode is conservative, the UEFI fallback path is updated to boot the target OS.

The following table summarizes how Trident manages UEFI fallback with conservative during OS servicing:

Trident Stage	trident install	trident update
stage	UEFI fallback unchanged.	UEFI fallback unchanged.
finalize	UEFI fallback updated to boot target OS.	UEFI fallback updated to boot the servicing OS, meaning that any failures will cause the machine to boot into the servicing OS.
commit	No changes needed.	UEFI fallback updated to boot the target OS.

optimistic is very similar to conservative but updates the UEFI fallback path contents to the target OS earlier (during finalize instead of commit):

• trident install
  • finalize: Trident updates the UEFI boot order and UEFI fallback path so that the target OS is booted.
  • commit: No changes to UEFI boot variables or UEFI fallback path are needed as the target OS is already configured to be booted.
• trident update
  • finalize: If the UEFI Fallback mode is optimistic, the UEFI fallback path is updated to boot the target OS. This differs from the UEFI variables.
  • commit: If the target OS boots successfully and the UEFI Fallback mode is optimistic, no update to the UEFI fallback path is needed as it is already updated to boot the target OS.

The following table summarizes how Trident manages UEFI fallback with optimistic during OS servicing:

Trident Stage	trident install	trident update
stage	UEFI fallback unchanged.	UEFI fallback unchanged.
finalize	UEFI fallback updated to boot target OS.	UEFI fallback updated to boot the target OS.
commit	No changes needed.	No changes needed.

To summarize, the available UEFI fallback modes are:

• conservative: This is the default mode. install will configure the target OS as the UEFI fallback OS in finalize. update will configure the servicing OS as the UEFI fallback OS in finalize and the target OS in commit after verifying the boot.
• optimistic: install and update will configure the target OS as the UEFI fallback OS in finalize.
• disabled: No updates are made for the UEFI fallback path.

The UEFI fallback mode can be specified in the Host Configuration file under the os section using the uefiFallback key. For example:

os:
  uefiFallback: "optimistic"