linux-kernel - Re: [PATCH v2] powerpc/32: Add support for out-of-line static calls

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Message-ID: <c0b8cb0e-cd96-d660-db87-51ded45435c2@csgroup.eu>
Date:   Fri, 11 Mar 2022 15:56:57 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jason Baron <jbaron@...mai.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH v2] powerpc/32: Add support for out-of-line static calls

Hi Peter,

Le 31/08/2021 à 16:00, Peter Zijlstra a écrit :
> On Tue, Aug 31, 2021 at 01:12:26PM +0000, Christophe Leroy wrote:
> 
> Yes, this should work nicely!
> 
> Since you have the two nop's at the end, you could frob in an
> optimization for __static_call_return0 without too much issue.
> 
> Replace the two nops with (excuse my ppc asm):
> 
> 	li r3, 0
> 	blr
> 
> and augment arch_static_call_transform() with something like:
> 
> 	if (func == &__static_call_return0)
> 		err = patch_branch(tramp, tramp+24, 0);

I just discovered that we likely have an issue with the implementation 
of that RET0 static call.

Looking at System.map I have:

	c0004fc0 t __static_call_return0
	c0011518 t __static_call_return0
	c00d8160 t __static_call_return0


So when we do:

	if (func == &__static_call_return0)

It is unlikely that we'll get the expected one.


I see __static_call_return0 is defined as 'static inline' in 
include/linux/static_call.h

Any reason for not having it as a single global symbol instead ?

Thanks
Christophe