linux-kernel - Re: [PATCH 00/18] Make common x86 arch area for i386 and x86_64

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Date:	Wed, 14 Mar 2007 21:19:44 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Wright <chrisw@...s-sol.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Glauber de Oliveira Costa <glommer@...il.com>
Subject: Re: [PATCH 00/18] Make common x86 arch area for i386 and x86_64 - Take 2

* Andi Kleen <andi@...stfloor.org> wrote:

> > Andrew's laptop only half a dozen times! ;) But .. in the long run, 
> > it's alot easier to think about unified code. 32-bit x86 will 
> > certainly stay with us for at least 10-20 years, and the best model 
> > for maintainance is having one codebase.
> 
> Not sure -- i'm often glad I don't have to care about all the old 
> 32bit systems on x86-64. [...]

the basic dynamics of legacies does not change if we have only 50% of 
them: right now x86_64 is just growing its own set of legacies, at the 
same rate as i386 did it 10 years ago. That makes little difference in 
practice: those legacies will quickly necessiate the _same_ kinds of 
abstractions that allow the flexible injection of hardware-dependent 
quirks. In another 5 years the x86_64 tree will end up looking and 
behaving _just like the i386_ tree, the only difference will be less 
compatibility. (In fact, it will likely look worse because currently our 
efforts are 50% split between i386 and x86_64, and the random 
differences between the two arches are wasting developer resources.)

so we might as well unify the two trees and /learn/ from i386's 
legacies, while integrating them. Those legacies, by the rule of large 
numbers, will revisit x86_64 too (or have already visited it). We 
already have per-APIC-version quirks, per CPU model quirks, etc., etc. 
The main cost of a quirk is the abstraction it necessiates, not the 
quirk handler itself. (which, once the framework is there, is modular)

Also, 90% of our users are still running 32bit kernels _even on 64-bit 
capable hardware_, so we might as well prepare ourselves for a really 
long march towards a pure 64-bit world. (Which will likely never come.)

	Ingo
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