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

["Nicholas Piggin" <npiggin@gmail.com>]

On Fri Oct 7, 2022 at 5:54 AM AEST, Segher Boessenkool wrote:
> Hi!
>
> 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.

> 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.

Some details in this series.

https://lists.ozlabs.org/pipermail/linuxppc-dev/2022-September/248521.html

>
> Please say in the comment what is wrong, don't spread fear :-)
>
> > +# 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 :)

Thanks,
Nick