linux-kernel - Re: [patch 3/8] Immediate Values

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Date:	Tue, 11 Sep 2007 16:22:59 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	Adrian Bunk <bunk@...sta.de>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [patch 3/8] Immediate Values - Kconfig menu in EMBEDDED

* Andi Kleen (andi@...stfloor.org) wrote:
> On Fri, Sep 07, 2007 at 08:46:48AM -0400, Mathieu Desnoyers wrote:
> > * Andi Kleen (andi@...stfloor.org) wrote:
> > > > +config IMMEDIATE
> > > > +	default y if !DISABLE_IMMEDIATE
> > > 
> > > It's still unclear to me why DISABLE_IMMEDIATE is needed. It would
> > > be better to make it just the default.
> > > 
> > 
> > It is actually the default on any non embedded configuration. Do you
> > think we should make it default to on on embedded configs too ?
> 
> I would prefer to not have any config options at all and let
> the non converted architectures always use a asm-generic fallback.
> 
> > The idea here is to give embedded system developers incentives to
> > create an optimized immediate value header for their architecture. I
> 
> Sounds like a quite bogus way to do this.
> 
> > fear that if it is not trivial to disable when they need to use ROM to
> > put the kernel code (as kprobes is, meaning, with a single config
> > option), they will refuse to event think about including an optimized
> > immediate value header for their architecture.
> 
> #ifdef CONFIG_ARCH_SPECIFIC_READONLY
> #include <asm-generic/generic-immediate.h> 
> #else
> /* optimized implementation */
> #endif
> 
> That's trivial.
> 

Yeah, but then you would make immediate.o an obj-y (both for
kernel/Makefile and arch/*/kernel/Makefile) and it would be built
even on systems that would be configured not to use immediate values.

Therefore, it would compile-in unused code if we do as you propose,
which I am reluctant to do on embedded systems where code size matters.

Mathieu

> > And yes, having a CONFIG_READ_ONLY_TEXT makes sense, but it implies
> > menu dependencies with not only immediate values but also kprobes,
> > paravirt, alternatives, (am I missing others ?)
> 
> paravirt and alternatives are x86 only.
> 
> I don't think CONFIG_READ_ONLY_TEXT on x86 makes sense.
> 
> On other architectures they have to deal with kprobes, but they
> presumably do this already. Not really your problem I suspect.
> 
> 
> > As long as we find a way for people to disable _all_ code patching in
> > their kernel, I'm happy with that. But since every existing code
> > patching mechanism can currently be disabled one by one, it makes sense
> > to do the same for the immediate values. Having a global
> > CONFIG_READ_ONLY_TEXT should IMHO come in a separate effort.
> 
> You're clearly deep into overdesign territory here.
> 
> -Andi

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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