[PATCH] powerpc64/idle: Fix SP offsets when saving GPRs

[Nicholas Piggin]

Excerpts from Michael Ellerman's message of January 30, 2021 9:32 pm:
> "Christopher M. Riedl" <cmr at codefail.de> writes:
>> The idle entry/exit code saves/restores GPRs in the stack "red zone"
>> (Protected Zone according to PowerPC64 ELF ABI v2). However, the offset
>> used for the first GPR is incorrect and overwrites the back chain - the
>> Protected Zone actually starts below the current SP. In practice this is
>> probably not an issue, but it's still incorrect so fix it.
> 
> Nice catch.
> 
> Corrupting the back chain means you can't backtrace from there, which
> could be confusing for debugging one day.

Yeah, we seem to have got away without noticing because the CPU will 
wake up and return out of here before it tries to unwind the stack,
but if you tried to walk it by hand if the CPU got stuck in idle or 
something, then we'd get confused.

> It does make me wonder why we don't just create a stack frame and use
> the normal macros? It would use a bit more stack space, but we shouldn't
> be short of stack space when going idle.
> 
> Nick, was there a particular reason for using the red zone?

I don't recall a particular reason, I think a normal stack frame is 
probably a good idea.

Thanks,
Nick