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

[Nicholas Piggin]

Excerpts from Christopher M. Riedl's message of February 4, 2021 4:59 pm:
> 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 think STACKFRAMESIZE is STACK_FRAME_OVERHEAD + NVGPRs. LR and CR can 
be saved in the caller's frame so that should be okay.

> 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.

Yeah if you don't need to save all nvgprs you can use caller's frame 
plus a few bytes in the minimum frame as volatile storage.

STACK_FRAME_OVERHEAD should be 32 on LE, but I think the problem is a 
lot of asm uses it and hasn't necessarily been audited to make sure it's 
not assuming it's bigger. You could actually use STACK_FRAME_MIN_SIZE
for new code, maybe we add a STACK_FRAME_MIN_NVGPR_SIZE to match and
use that.

Thanks,
Nick