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Re: [PATCH] Add support for 4KS cpu.

To: "Kevin D. Kissell" <>
Subject: Re: [PATCH] Add support for 4KS cpu.
From: Franck <>
Date: Wed, 5 Oct 2005 15:20:59 +0200
Cc: "Maciej W. Rozycki" <>, Ralf Baechle <>,
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Reply-to: Franck <>
2005/10/5, Kevin D. Kissell <>:
> I'm personally not a big believer in security-through-obscurity,
> but there are those, both inside and outside MIPS, who felt that
> the security of SmartMIPS cores would be enhanced if we didn't
> give away all of the details.  As a consequence, we put off
> publishing the nitty-gritty details of SmartMIPS for quite a while.
> I note that we now have the programmers' manual on-line at,
> so I guess I'm implicitly cleared to discuss it in at least that level
> of detail.

well, I agree with you on "security-through-obscurity" point. From
outside, I feel like MIPS has a lot of things to _hide_ athough that's
not a good feeling for security systems. Anyways...

> A key element of SmartMIPS that allows for a ~2x speedup for
> crypto codes that rely on extended precision math (RSA, ECC)
> is the combination of an extension to the Hi/Lo accumulator
> (called "ACX") with a special extract-and-reduce instruction
> ("MFLHXU").  If one wants to use that in Linux - or at least,
> if one wants to allow more than one thread to be able to use
> it at a time - one needs to save/restore ACX on the kernel
> stackframe, along with Hi and Lo.

Correct. I must at least add this in the patch. I'll do it soon.

Thanks for enlight this.

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