Asahi Linux adds UEFI Runtime Services conduit for PSCI power management on Apple Silicon

Apple Silicon’s architecture omits EL3 firmware, which Linux typically relies on for PSCI power management. This forces Asahi Linux to use a downstream cpuidle driver incompatible with upstream kernel requirements. The lack of EL3 also prevents the use of standard SMC or HVC conduits, creating a hard block for battery efficiency improvements.

The workaround repurposes UEFI Runtime Services as a PSCI conduit, allowing the kernel to call into m1n1’s PSCI implementation without requiring EL3. This approach aligns with the PSCI specification’s conduit-agnostic design while avoiding the pitfalls of running the kernel at EL1. Memory reserved by m1n1 ensures the PSCI implementation remains accessible after boot.

Adopting this solution enables Asahi Linux to upstream its power management code, reducing maintenance overhead. It also preserves architectural features like virtualization, which would break if the kernel ran at EL1. The trade-off is added complexity in the boot chain, as the solution depends on m1n1 and U-Boot’s UEFI implementation.

For engineers deploying Linux on Apple Silicon, this change simplifies power management but introduces a dependency on non-standard firmware interactions. The UEFI-based conduit may require adjustments in bootloaders or distros that assume traditional PSCI conduits. Long-term, this could influence how other ARM-based projects handle similar firmware limitations.