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To: linux-kernel@vger.kernel.org
From: "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: Using %cr2 to reference "current"
Date: 6 Nov 2001 11:09:50 -0800
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In-Reply-To: <20011106121313.B16245@redhat.com> <Pine.LNX.4.33.0111060918380.2194-100000@penguin.transmeta.com> <20011106134234.A27718@redhat.com>
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Followup to:  <20011106134234.A27718@redhat.com>
By author:    Benjamin LaHaise <bcrl@redhat.com>
In newsgroup: linux.dev.kernel
>
> On Tue, Nov 06, 2001 at 09:49:15AM -0800, Linus Torvalds wrote:
> > That said, how expensive is loading %cr2 anyway? We can do all the same
> > tricks with a 16kB stack and just playing games with using the higher bits
> > as the "offset", ie things like
> 
> Here are some numbers:
> 
> read cr2 best: 11  av: 11.12
> write cr2 cr2 best: 61  av: 64.42
> read cr2 best: 11  av: 11.12
> write cr2 cr2 best: 61  av: 65.01
> read stk best: 10  av: 11.03
> write cr2 stk best: 61  av: 64.95
> read stk best: 10  av: 11.03
> write cr2 stk best: 61  av: 65.23
> 
> Which come from insmod of the below two modules.  I didn't test writing to 
> the stack register, but I expect it's similarly expensive as it affects the 
> call return stack and other behind the scenes dependancies.  Suffice it to 
> say that reading %cr2 is essentially free on my box (athlon mp).  Maybe 
> we should use it as a pointer into a per-cpu area to avoid writing it?
> 

You still have to write it every time you take a page fault.  You're
adding 60-odd cycles to the page fault path at least.

Not to mention any system which does microcoded reads of %cr2, which
apparently the Athlon XP doesn't.

	-hpa
-- 
<hpa@transmeta.com> at work, <hpa@zytor.com> in private!
"Unix gives you enough rope to shoot yourself in the foot."
http://www.zytor.com/~hpa/puzzle.txt	<amsp@zytor.com>
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