In linux-1.0 there used to be an asm macro move_to_user_mode()
which kernel called in main() before doing a fork() and init().
This may be an ancient question to many kernel hackers
with 2.1.x going on. It is not clear to me in 2.0 where do
we switch modes( cpl0 -> cpl3). Is answer lying in kernel_thread
doing a trap with __NR_clone.