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

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Message-ID: <1337243968.3403.3.camel@edumazet-glaptop>
Date:	Thu, 17 May 2012 10:39:28 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	pablo@...filter.org, 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, 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;



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