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Date: Fri, 08 Jul 2011 11:06:59 -0700 (PDT) From: David Miller <davem@...emloft.net> To: johnwheffner@...il.com Cc: mj@....cz, netdev@...r.kernel.org Subject: Re: ipv4: Simplify ARP hash function. From: John Heffner <johnwheffner@...il.com> Date: Fri, 8 Jul 2011 14:03:45 -0400 > On Fri, Jul 8, 2011 at 1:47 PM, David Miller <davem@...emloft.net> wrote: >> From: Martin Mares <mj@....cz> >> Date: Fri, 8 Jul 2011 19:40:55 +0200 >> >>> The hash function is linear, so it can be reduced to: >>> >>> a = key ^ dev->ifindex >>> return (a >> 8) ^ (a >> 16) ^ (a >> 24) // (1) >>> ^ (hash_rnd >> 8) ^ (hash_rnd >> 16) ^ (hash_rnd >> 24) // (2) >> >> Is this really the same? The inclusion of a full 32-bit xor >> with hash_rnd before folding was intentional, so that the >> final folding occurs on a completely "random" value. > > Martin's reduction looks exactly correct to me. Ok, there was also an unintended bug in my original patch, I lost the bottom 8 bits in the fold, the hash function should instead be: +static inline u32 arp_hashfn(u32 key, const struct net_device *dev, u32 hash_rnd) +{ + u32 val = key ^ dev->ifindex ^ hash_rnd; + + return val ^ (val >> 8) ^ (val >> 16) ^ (val >> 24); +} -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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