netdev - Re: [PATCH 3/7] netfilter: xt_HMARK: modulus is expensive for hash calculation

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Message-ID: <20120517145550.GA6501@1984>
Date:	Thu, 17 May 2012 16:55:50 +0200
From:	Pablo Neira Ayuso <pablo@...filter.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Laight <David.Laight@...LAB.COM>,
	netfilter-devel@...r.kernel.org, davem@...emloft.net,
	netdev@...r.kernel.org
Subject: Re: [PATCH 3/7] netfilter: xt_HMARK: modulus is expensive for hash
 calculation

On Thu, May 17, 2012 at 10:39:28AM +0200, Eric Dumazet wrote:
> On Thu, 2012-05-17 at 09:16 +0100, David Laight wrote:
> > > From: Pablo Neira Ayuso <pablo@...filter.org>
> > > 
> > > Use:
> > > 
> > > ((u64)(HASH_VAL * HASH_SIZE)) >> 32
> > > 
> > > as suggested by David S. Miller.
> > 
> > That (u64) cast is very unlikely to have any effect.
> > If you want a 64 bit result from the product of two
> > 32 bit values, you have to cast one of the 32 bit values
> > prior to the multiply - as in the patch below.
> 
> Hey, Changelog is a bit wrong (for several reasons) but code is correct.
> 
> return (((u64)hash * info->hmodulus) >> 32) + info->hoffset;

Sorry, for the mistake in the changelog. I copied & pasted it from the
mailing list discussion.
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