tl;dr: A developer documents booting mainline Linux on an M4 Mac mini, overcoming hurdles including locked registers (GXF, RVBAR), missing MMU mappings for MMIO, and a virtualization-related CPU register write that crashed the kernel. The biggest challenge was that M4's WFI instruction doesn't preserve CPU state as the ARM64 spec requires, with no chicken bit to disable the behavior; the fix—a new kernel bootarg to NOP out WFI/WFIT instructions—has now been merged upstream in Linux and m1n1, enabling multi-core boot on M4/M4 Pro/Max and M5 chips.