netdev - Re: [PATCH net-next] gro: relax ID check in inet_gro

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Date:	Thu, 21 Mar 2013 20:11:37 +0200
From:	Dmitry Kravkov <dkravkov@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Dmitry Kravkov <dmitry@...adcom.com>,
	Eilon Greenstein <eilong@...adcom.com>, pshelar@...ira.com,
	hkchu@...gle.com, maze@...gle.com
Subject: Re: [PATCH net-next] gro: relax ID check in inet_gro_receive()

Resending for netdev in plantext.


On Thu, Mar 21, 2013 at 6:08 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>
> On Thu, 2013-03-21 at 11:46 -0400, David Miller wrote:
> > From: Eric Dumazet <eric.dumazet@...il.com>
> > Date: Wed, 20 Mar 2013 21:52:33 -0700
> >
> > > GRE TSO support doesn't increment the ID in the inner IP header.
> >
> > Is this a fundamental limitation of doing TSO over GRO or
> > were the Broadcom folks just being lazy with their firmware
> > implementation?
> >
>
> Well, I suspect this hardware is not capable of doing the proper ID
> manipulation twice. (inner and outer header)

This is correct: ID only for one of the headers can be handled with
current FW/HW, for other DF is set.

> Still TSO support permits a single GRE flow going from 3Gbps to 9Gbps on
> our hosts. So even if the inner IP id is 'broken', we are going to use
> TSO.
>
> Note we are limited by the receiver, as the receiver has to perform the
> tcp checksum in software (bnx2x doesnt support CHECKSUM_COMPLETE yet)
>
> Hopefully next firmware or NIC will do the right thing.
>
> > I really don't want to apply this patch, because ipv4 frames
> > even with DF set should have an incrementing ID field, in
> > order to accomodate various header compression schemes.
> >
> > We go out of our way to do this for normal unencapsulated TCP stream
> > packets, rather than set the ID field to zero (which we did for some
> > time until the compression issue was pointed out to us).
>
> I understand your concern, but this check in GRO brings nothing at all.
>
> Once we receive frames with 'bad IPv4 ID', should we accept them or drop
> them ?
>
> TCP stack doesn't care at receive (obviously as this ID is not a concern
> for the transport layer), so GRO should do the same, as GRO is a best
> effort to reduce cpu load.
>
> I fully understand the 'tos' check because of proper ECN support, but
> the ttl check or id check are totally useless and time consuming.
>
> GRO aggregation should roughly work the same than TCP coalescing, and we
> don't care of IP ID or ttl in TCP stack.
>
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