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 20:59:16 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andi Kleen <andi@...stfloor.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

* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> > of code. i386 is 87847 lines of code, x86_64 is 40978 lines of code, 
> > a total of 128825. That means we move about 10% of the code. Not 
> > insignificant but not earth-shattering either. With alot more effort 
> > (and testing) we could realistically go up to maybe 20% - but that's 
> > still a bit low to spread out all the files, isnt it?
> 
> Well, I'd like it to be 100% _eventually_, and just unify the 
> architectures.

ok, having a single bi-arch final tree is indeed intriquing and i didnt 
realize that you were suggesting that. (I had the impression that 
arch/x86/ was more of a 'common library' thing, not a target 
architecture. What felt weird to me was having 3 separate hierarchies - 
but as long as it's just a temporary state it's OK.)

> We've now done that both for S/390 and POWER, and I think in both 
> cases it's been a clear win. So it's not like this is even a radical 
> idea.

the x86_64 and i386 trees have diverged quite a bit though, so this will 
be a major logistical undertaking. And with Andi opposed to 
fundamentally it it also lacks a bit of manpower i guess :-/

> There really is almost nothing in i386 that shouldn't be supported on 
> x86-64 too, unless it literally is the actual low-level asm files and 
> vm86 mode support (which in turn is best left as just a config option 
> that would just *depend* on 32-bit, so even that could sanely be 
> represented in a shared tree without any real downside at all).

yeah. But this really scares the sh*t out of me. I already tried to 
unify some of the most fragile lowlevel bits recently: for example the 
SMP bootup, TSC sync and APIC initialization sequences were totally 
different on x86 and x86_64. And those kind of random deviations have 
spread all around the tree. But ... my experience has been pretty 
positive: touching both codebases at once tends to dust off old code and 
tends to fix more bugs. And in the process of doing that we broke 
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.

Another practical complication is that even for modular stuff, sometimes 
x86_64 has the better code, sometimes i386. But ... the more i think 
about it the more i like it. -m32 certainly works fine and does the 
right thing.

	Ingo
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