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". 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. > 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! > > > +# 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. Segher
