Re: [PATCH] powerpc/64s: POWER10 CPU Kconfig build option

[Segher Boessenkool <segher@kernel.crashing.org>]

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