Linux kernel entry

head.S

for multiple architectures, there is arch/$ARCH/kernel/head.S, where the first instructions reside.

EFI stub

can be as simple as https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/arch/riscv/kernel/head.S?id=cb7d2dd5612a77a2597c00fce770a52c921e2ea5

or designed elegantly to allow for pure DT + EFI coexistence, see arm64 where the EFI stub checks for the EFI system table, calls the method to retrieve the kernel cmdline, injects that information into the FDT and then continues with the kernel as usual. A boot loader can just skip the whole stub and continue with the regular kernel, so loading an EFI kernel works without having the EFI interfaces. Note that the kernel later checks for presence of EFI features, which are tracked by setting bits in the EFI stub. The stub itself is added via Kconfig CONFIG_EFI_STUB, as well as the interface calling, but that comes with a separate CONFIG_EFI (default).

TODO: code references

Start with the architecture-specific arch/$ARCH/kernel/head.S entry points, including the linked riscv head.S change, then compare the arm64 EFI stub behavior described in the issue. Add code references for the kernel entry, EFI stub, CONFIG_EFI_STUB, and CONFIG_EFI paths, with the documentation matching the referenced implementation.