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

["Kevin D. Kissell" <kevink@mips.com>]

Franck wrote:
> 2005/10/4, Kevin D. Kissell <kevink@mips.com>:
> 
>>They also have some physical security and cryptography accelleration
>>features, some of which use extended CPU state that would
>>require some kernel context management support if anyone wanted
>>to actually use them in Linux applications. The real point of
>>having a CPU_4KSC config flag would be to enable building-in
>>such support.
>>
> what is extended CPU state that you're talking about ?

That would be telling.  ;o)   Seriously, see below.

>>I'm being a teeny bit vague about this, because I'm not 100%
>>certain that all the details of "SmartMIPS" have been published.
>>
> 
> hmm, does that mean that smart mips extension couldn't be supported in
> Linux in case that this extension have not been published ?

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 www.mips.com,
so I guess I'm implicitly cleared to discuss it in at least that level
of detail.

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.

		Regards,

		Kevin K.