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" <c...@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
