Linux 7.4 EFI Brings Improvement For Helping Systemd When Kernel Booted Directly

The EFI updates have been merged for the Linux 7.4 kernel and include an improvement for when the system was booted directly into the kernel on EFI-based systems without using a bootloader like GRUB or systemd-boot. Now when booting directly into the kernel, the LoaderDevicePartUUID EFI variable will be populated from the stub to help systemd in turn discover the file-system layout. Vincent Mailhol who worked on this Boot Loader Interface support for the kernel's EFI libstub explained in the patch series: "The Boot Loader Interface (BLI) defines EFI variables that expose boot loader state to the running OS. LoaderInfo identifies the boot loader, while LoaderDevicePartUUID records the GPT partition UUID of the partition containing it. LoaderDevicePartUUID is used, for example, by systemd-gpt-auto-generator to identify the disk the boot loader was launched from and automatically detect and mount partitions on it. GRUB and systemd-boot populate these variables, but when the kernel is started directly by EFI firmware, there is no conventional external boot loader to provide them. In that case, because the EFI stub performs the boot loader role, it should provide the variables itself. Use LoaderInfo as a sentinel: if it is already set by an earlier boot stage or cannot be set, bail out. Otherwise, populate the other BLI variables. Parse the loaded image device path, extract the GUID signature from its GPT HD() node and publish it under the Linux loader entry vendor GUID as the volatile LoaderDevicePartUUID EFI variable." This change for those booting their Linux kernel directly from EFI and relying on systemd-gpt-auto-generator or related tooling can be found in Linux 7.4 via this merge. The Boot Loader Interface standard is defined by systemd.