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

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Message-ID: <20070907223950.GB22646@one.firstfloor.org>
Date:	Sat, 8 Sep 2007 00:39:50 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Andi Kleen <andi@...stfloor.org>, 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

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.

> 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
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