Michael Ellerman writes:

There's a small period early in boot where we don't know which cpu we're
running on. That's ok, except that it means we have no paca, or more
correctly that our paca pointer points somewhere random.

So that we can safely call things like smp_processor_id(), we need a paca,
so just assume we're on cpu 0. No code should _write_ to the paca before
we've set the correct one up.

OK, but why not just do a setup_paca(0) at the start of early_setup(),
in C code?  You can also remove the redundant paca setting in
start_here_common() in head_64.S.

Paul.