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Subject: Re: [PATCH v3 5/6] powerpc/64: Add support for out-of-line static
 calls
From: Benjamin Gray <bgray@linux.ibm.com>
To: Michael Ellerman <mpe@ellerman.id.au>,
        Christophe Leroy
	 <christophe.leroy@csgroup.eu>,
        "linuxppc-dev@lists.ozlabs.org"
	 <linuxppc-dev@lists.ozlabs.org>
Date: Thu, 06 Oct 2022 16:01:44 +1100
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On Thu, 2022-10-06 at 11:39 +1100, Michael Ellerman wrote:
> Christophe Leroy <christophe.leroy@csgroup.eu> writes:
> > However, thinking out loudly, I'm wondering, could we make things
> > any=20
> > simpler when CONFIG_MODULES is not selected, or is that a too much=20
> > corner case on PPC64 ?
>=20
> I'd say it's mostly a corner case.
>=20
> Obviously no distros ship with modules disabled.=20
>=20
> AFAIK even the stripped down kernels we use in CPU bringup have
> modules
> enabled.
>=20
> So I think it's probably not worth worrying about, unless there's an
> obvious and fairly simple optimisation.
>=20
> cheers

Yeah, I think supporting this case would amount to a whole new
trampoline implementation. Something like the original RFC
implementation would be best here as there would only be one TOC to
worry about.

Instead, I expect this can all be optimised better once we can apply
patches to call-sites. If so, we can patch the call-site NOPs ourselves
without marking the trampoline as caller-saved TOC, which would remove
the suboptimal save-r2 trampolines. Then we could use separate local &
global entry points like the RFC. This would unify the kernel/module
trampolines again.

The benefit of this series is that it works with just what the ABI
offers, without extra kernel tools / linker magic. But I would
definitely revisit it once call-site patching is possible, especially
when working on inline static calls.