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

[Christopher M. Riedl]

On Sat Jan 30, 2021 at 7:44 AM CST, Nicholas Piggin wrote:
> 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.

I'll send a version using STACKFRAMESIZE - I assume that's the "normal"
stack frame :)

I admit I am a bit confused when I saw the similar but much smaller
STACK_FRAME_OVERHEAD which is also used in _some_ cases to save/restore
a few registers.

>
> Thanks,
> Nick