On Fri Oct 7, 2022 at 9:23 AM AEST, Segher Boessenkool wrote: > On Fri, Oct 07, 2022 at 07:56:09AM +1000, Nicholas Piggin wrote: > > On Fri Oct 7, 2022 at 5:54 AM AEST, Segher Boessenkool wrote: > > > On Fri, Sep 23, 2022 at 01:30:04PM +1000, Nicholas Piggin wrote: > > > > This adds basic POWER10_CPU option, which builds with -mcpu=power10. > > > > > > > +# No prefix or pcrel > > > > +KBUILD_CFLAGS += $(call cc-option,-mno-prefixed) > > > > +KBUILD_CFLAGS += $(call cc-option,-mno-pcrel) > > > > > > Why do you disable all prefixed insns? What goes wrong if you don't? > > > > Potentially things like kprobes. > > So mention that? "This patch is due to an abundance of caution".
Well it's in next now. I did say *basic*, I'm sure not changing the ABI or adding prefix instructions isn't too mysterious. > > What I meant to ask is if you *saw* something going wrong, not if you > can imagine something going wrong. I can imagine ten gazillion things > going wrong, that is not why I asked :-) > > > > Same question for pcrel. I'm sure you want to optimise it better, but > > > it's not clear to me how it fails now? > > > > For pcrel addressing? Bootstrapping the C environment is one, the > > module dynamic linker is another. > > I don't know what either of those mean. arch/powerpc/kernel/head_64.S and arch/powerpc/kernel/module_64.c Can discuss in the pcrel patch series thread if you would like to know more. > > > Some details in this series. > > > > https://lists.ozlabs.org/pipermail/linuxppc-dev/2022-September/248521.html > > I've watched that series with great interest, but I don't remember > anything like that? Are you refering to the commentary in 7/7? > "Definitely ftrace and probes, possibly BPF and KVM have some breakage. > I haven't looked closely yet."... This doesn't mean much does it :-) > It can be a triviality or two. Or it could be a massive roadblock. > > Just say in a comment where you disable stuff that it is to prevent > possible problems, this is a WIP, that kind of thing? Otherwise other > people (like me :-) ) will read it and think there must be some deeper > reason. Like, changing code to work with pcrel is hard or a lot of > work -- it isn't :-) As you say in 0/7 yourself btw! > I will describe limitations and issues a bit more in changelog of patches to enable prefix and pcrel when I submit as non-RFC candidate. It would probably not be too hard to get things to a workable state that could be merged. > > > > +# No AltiVec or VSX or MMA instructions when building kernel > > > > KBUILD_CFLAGS += $(call cc-option,-mno-altivec) > > > > KBUILD_CFLAGS += $(call cc-option,-mno-vsx) > > > > +KBUILD_CFLAGS += $(call cc-option,-mno-mma) > > > > > > MMA code is never generated unless the code asks for it explicitly. > > > This is fundamental, not just an implementations side effect. > > > > Well, now it double won't be generated :) > > Yeah, but there are many other things you can unnecessarily disable as > well! :-) > > VMX and VSX are disabled here because the compiler *will* use those > registers if it feels like it (that is, if it thinks that will be > faster). MMA is a very different beast: the compiler can never know if > it will be faster, to start with. True, but now I don't have to find the exact clause and have my lawyer confirm that it definitely probably won't change in future and break things. Thanks, Nick