* Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote: > On Mon, Jul 20, 2015 at 09:56:11AM +0200, Ingo Molnar wrote: > > > > > > The reason I suggested to put FRAME in the macro name is to try to prevent it > > > from being accidentally used for leaf functions, where it isn't needed. > > > > Well, we could use LEAF_FUNCTION to mark that fact. > > > > Wether a function written in assembly is a leaf function or not is a higher level > > (and thus more valuable) piece of information whether we generate frame pointer > > debuginfo or not. > > > > > Also the naming of FUNCTION_ENTRY and FUNCTION_RETURN doesn't do anything to > > > distinguish them from the already ubiquitous ENTRY and ENDPROC. So as a kernel > > > developer it seems confusing to me, e.g. how do I remember when to use > > > FUNCTION_ENTRY vs ENTRY? > > > > 'ENDPROC' is really leftover from older debuginfo cruft, it's not a valuable > > construct IMHO, even if it's (sadly) ubiquitious. > > > > We want to create new, clean, as minimal as possible and as clearly named as > > possible debuginfo constructs from first principles. > > Ok. So if I understand right, the proposal is: > > Replace *all* x86 usage of ENTRY/ENDPROC with either: > > FUNCTION_ENTRY(func) > FUNCTION_RETURN(func) > > or > > LEAF_FUNCTION_ENTRY(func) > LEAF_FUNCTION_RETURN(func) > > Those sound fine to me. Yeah - but keep the old constructs as well and don't necessarily do the full migration straight away, only once the dust has settled - to reduce churn. > I should point out that there are still a few cases where the more granular > FRAME/ENDFRAME and ENTRY/ENDPROC macros would still be needed. > > For example, if the function ends with a jump instead of a ret. If the > jump is a sibling call, the code would look like: > > FUNCTION_ENTRY(func) > ... > ENDFRAME > jmp another_func > ENDPROC(func) > > > Or if it's a jump within the function to an internal ret: > > FUNCTION_ENTRY(func) > ... > 1: ... > ENDFRAME > ret > 2: ... > jmp 1b > ENDPROC(func) > > > Or if it jumps to some shared code before returning: > > FUNCTION_ENTRY(func_1) > ... > jmp common_return > ENDPROC(func_1) > > FUNCTION_ENTRY(func_2) > ... > jmp common_return > ENDPROC(func_2) > > common_return: > ... > ENDFRAME > ret > > > So in some cases we'd still need the more granular macros, unless we > decided to make special macros for these cases as well. Ok, I see how the naming scheme I proposed won't work with all that very well, but I'd still suggest using consistently named patterns. Let me suggest yet another approach. How about open-coding something like this: FUNCTION_START(func) push_bp mov_sp_bp ... pop_bp ret FUNCTION_END(func) This is just two easy things: - a redefine of the FUNCTION_ENTRY and ENDPROC names - the introduction of three quasi-mnemonics: push_bp, mov_sp_bp, pop_bp - which all look very similar to a real frame setup sequence, except that we can easily make them go away in the !CONFIG_FRAME_POINTERS case. The advantage of this approach would be: - it looks pretty 'natural' and very close to how the real disassembly looks like in CONFIG_FRAME_POINTERS=y kernels. So while it's not as compact as some of the other variants, it's close to what the real instruction sequence looks like and that is a positive quality in itself. - it also makes it apparent 'on sight' that it's probably a bug to have unbalanced push/pop sequences in a regular function, to any reasonably alert assembly coder. - if we ever unsupport framepointer kernels in the (far far) future, we can get rid of all lines with those 3 mnemonics and be done with it. - it's finegrained enough so that we can express all the special function/tail variants you listed above. What do you think? I'd still keep existing frame setup functionality and names and only use these in fixes, new code and new annotations - and do a full rename and cleanup once the dust has settled. Thanks, Ingo -- To unsubscribe from this list: send the line "unsubscribe live-patching" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
