linux-kernel - Re: [CRYPTO] is it really optimized ?

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Date:	Thu, 19 Apr 2007 10:07:53 +0200
From:	"Francis Moreau" <francis.moro@...il.com>
To:	"Roland Dreier" <rdreier@...co.com>
Cc:	"Herbert Xu" <herbert@...dor.apana.org.au>,
	helge.hafting@...el.hist.no, linux-kernel@...r.kernel.org,
	linux-crypto@...r.kernel.org
Subject: Re: [CRYPTO] is it really optimized ?

On 4/17/07, Roland Dreier <rdreier@...co.com> wrote:
>  > > It seems trivial to keep the last key you were given and do a quick
>  > > memcmp in your setkey method to see if it's different from the last
>  > > key you pushed to hardware, and set a flag if it is.  Then only do
>  > > your set_key() if you have a new key to pass to hardware.
>  > >
>  > > I'm assuming the expense is in the aes_write() calls, and you could
>  > > avoid them if you know you're not writing something new.
>
>  > that's a wrong assumption. aes_write()/aes_read() are both used to
>  > access to the controller and are slow (no cache involved).
>
> Sorry, I wasn't clear.  I meant that the hardware access is what is
> slow, and that anything you do on the CPU is relatively cheap compared
> to that.
>
> So my suggestion is just to keep a cache (in CPU memory) of what you
> have already loaded into the HW, and before reloading the HW just
> check the cache and don't do the actual HW access if you're not going
> to change the HW contents.  So you avoid any extra aes_write and
> aes_read calls in the cache hit case.
>
> This would have the advantage of making anything that does lots of
> bulk encryption fast without special casing ecryptfs.
>

I'm not sure how "memcmp(key, cache, KEY_SIZE)" would impact AES
performance. I need to give it a test but can't today. I'll do
tomorrow and give you back the result.

Thanks
-- 
Francis
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