netdev - Re: [Bug 120441] af_packet no longer uses symmetric hashing

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Message-ID: <CAF=yD-Jcji5R6HTjV-ZoS+6-A2xz_Fq8CvhqaUSK-hLRc_Txnw@mail.gmail.com>
Date:	Thu, 16 Jun 2016 11:32:55 -0400
From:	Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:	Daniel Borkmann <daniel@...earbox.net>
Cc:	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Derek Ditch <derek.ditch@...il.com>,
	Network Development <netdev@...r.kernel.org>,
	Tom Herbert <tom@...bertland.com>, jiri@...nulli.us,
	Willem de Bruijn <willemb@...gle.com>, eric@...it.org
Subject: Re: [Bug 120441] af_packet no longer uses symmetric hashing

>> Isn't a symmetric hashing for af-packet something very fundamental?
>> Looks much more like a bug to me.
>
>
> I agree it would be useful resp. fundamental. I'm not quite sure whether
> PACKET_FANOUT_HASH always had this guarantee since the initial
> implementation

It is based on rxhash, which is intended to have this property. See for
instance the commit messages from commit b249dcb82d32 ("rps:
consistent rxhash")

"In case we compute a software skb->rxhash, we can generate a
consistent hash : Its value will be the same in both flow directions."

commit 6862234238e8 ("net: small bug on rxhash calculation") also
explicitly refers to this property.


> f.e. depending on whether the skb got fanout demuxed from RX or TX paths,
> you would either use a hw hash or it would have gone through the kernel's
> (predecessor) flow dissector

It's quite possible that not all hardware hash implementations do the
right thing, indeed.