The EFI app on x86 has supported booting for a while. With a recent 19-part series, the EFI app can now also boot an OS on ARM. This uses standard boot and the standard extlinux boot flow.
The Core Fixes
To successfully hand off control from the EFI environment to a new operating system, a few key steps must happen in the right order.
- Exit Boot Services: The most important change is ensuring that U-Boot calls
ExitBootServices()before it jumps to the OS kernel. This is a fundamental requirement of the EFI specification. It tells the underlying firmware that its boot-time services (like memory allocation, device I/O, etc.) are no longer needed, effectively handing exclusive control of the hardware over to the operating system. - Pass the Memory Map: An operating system needs to know the physical layout of the memory. This series adds the logic to obtain the EFI memory map and write the physical memory information into the Flattened Device Tree (FDT). This allows the Linux kernel to understand the memory layout and manage it correctly.
- Improved Keyboard Compatibility: On some real-world hardware, such as certain Qualcomm laptops with USB keyboards, simply waiting for a keypress isn’t enough. The underlying system needs time to process other events for the keyboard input to register. The EFI serial driver is updated to use a periodic timer, making the console much more reliable on these devices.
- General Tidying Up: Alongside these major functional updates, the series includes a lot of valuable code maintenance, such as correcting typos, reorganizing Kconfig options, and standardizing the format of EFI GUID declarations.
Tested and Ready
To ensure this new capability remains robust, the patch series also introduces a new continuous integration (CI) test. We now automatically build the efi-arm_app64 target and use it to boot a full Ubuntu 25.04 image on a QEMU virtual machine in the sjg lab. This provides a strong safeguard against future regressions.
A Note for Testers on QEMU
If you’re planning to test this yourself using QEMU, there’s an important detail to be aware of. The changes do not work correctly with QEMU 8.2 when booting Ubuntu 24.04, as the kernel fails very early in the process with a regime_is_user error.
The good news is that this has been fixed in QEMU 9.2 and newer. So, make sure you’re using an up-to-date version of QEMU for your testing! You can read more about the QEMU issue in this bug report.
Next up?
Work on the app continues, with the goal of having it boot the OS using EFI as well.
