From: Segher Boessenkool <segher@kernel.crashing.org>
To: Nicholas Piggin <npiggin@gmail.com>
Cc: linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH] powerpc/64s: POWER10 CPU Kconfig build option
Date: Thu, 6 Oct 2022 18:23:45 -0500 [thread overview]
Message-ID: <20221006232345.GW25951@gate.crashing.org> (raw)
In-Reply-To: <CNF6C5XSIE75.3R12NULNLHEN2@bobo>
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
next prev parent reply other threads:[~2022-10-06 23:26 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-23 3:30 [PATCH] powerpc/64s: POWER10 CPU Kconfig build option Nicholas Piggin
2022-09-23 5:46 ` Christophe Leroy
2022-09-23 6:23 ` Nicholas Piggin
2022-10-06 18:07 ` Christophe Leroy
2022-10-06 20:15 ` Segher Boessenkool
2022-10-06 22:03 ` Nicholas Piggin
2022-10-10 3:41 ` Nicholas Piggin
2022-10-04 13:25 ` Michael Ellerman
2022-10-06 19:54 ` Segher Boessenkool
2022-10-06 21:56 ` Nicholas Piggin
2022-10-06 23:23 ` Segher Boessenkool [this message]
2022-10-07 0:03 ` Nicholas Piggin
2022-10-07 5:31 ` Michael Ellerman
2022-10-07 14:38 ` Segher Boessenkool
2022-10-07 14:57 ` Segher Boessenkool
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