netdev - RE: [PATCH 1/2] net: Toeplitz library functions

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Date:	Tue, 24 Sep 2013 09:32:30 +0100
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Tom Herbert" <therbert@...gle.com>, <davem@...emloft.net>
Cc:	<netdev@...r.kernel.org>, <jesse.brandeburg@...el.com>
Subject: RE: [PATCH 1/2] net: Toeplitz library functions

> +static inline unsigned int
> +toeplitz_hash(const unsigned char *bytes,
> +	      struct toeplitz *toeplitz, int n)
> +{
> +	int i;
> +	unsigned int result = 0;
> +
> +	for (i = 0; i < n; i++)
> +		result ^= toeplitz->key_cache[i][bytes[i]];
> +
> +        return result;
> +};

That is a horrid hash function to be calculating in software.

The code looks very much like a simple 32bit CRC.
It isn't entirely clears exactly where the 'key' gets included,
but I suspect it is just xored with the data bytes.

Using in it hardware is probably fine - the hardware can do
it cheaply (in dedicated logic) as the frame arrives.
The CRC polynomial probably collapses to a few XOR operations
when done byte by byte (the hdlc crc16 collapses to 3 levels
of xor).

IIRC jhash() works on 32bit quantities - so has far fewer
maths operations and well as not having all the random data
accesses (cache misses and displacing other parts of the
working set from the cache).

I also thought the hash was arranged so that tx and rx packets
for a single connection hash to the same value?

	David

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