netdev - Re: Where exactly will arch_fast

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Message-ID: <20141208161959.8852.qmail@ns.horizon.com>
Date:	8 Dec 2014 11:19:59 -0500
From:	"George Spelvin" <linux@...izon.com>
To:	hannes@...essinduktion.org, linux@...izon.com
Cc:	davem@...emloft.net, dborkman@...hat.com,
	herbert@...dor.apana.org.au, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, tgraf@...g.ch, tytso@....edu
Subject: Re: Where exactly will arch_fast_hash be used

>>> In case of openvswitch it shows a performance improvment. The seed
>>> parameter could be used as an initial biasing of the crc32 function, but
>>> in case of openvswitch it is only set to 0.
 
>> NACK. [...]

> Sorry for being unclear, I understood that and didn't bother patching
> that '0' with a random seed exactly because of this.

And I'm sorry for delivering a long lecture on a subject you already
understood perfectly well.

I'd just been thinking about it because of Herbert's comments, so it was
conveniently at hand. :-)

Out of curiousity, what *were* you referring to when you talked
about biasing the crc32 function?  "Biasing" is a good term becuase
it just applies an offset, but what do you gain from doing that?


There are nifty things one can do with the CRC32 instruction, however.
A lot of ciphers these days use an ARX (add, rotate, XOR) kernel.
A crc32 instruction, although linear, does some very powerful rotate &
xor operations, and could replace the XOR and rotate.
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