netdev - Re: [net-next PATCH 0/2] GENEVE/VXLAN: Enable outer Tx checksum by default

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Date:	Tue, 23 Feb 2016 09:31:09 -0800
From:	Jesse Gross <jesse@...nel.org>
To:	Tom Herbert <tom@...bertland.com>
Cc:	Edward Cree <ecree@...arflare.com>,
	Alex Duyck <aduyck@...antis.com>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Alexander Duyck <alexander.duyck@...il.com>
Subject: Re: [net-next PATCH 0/2] GENEVE/VXLAN: Enable outer Tx checksum by default

On Tue, Feb 23, 2016 at 8:47 AM, Tom Herbert <tom@...bertland.com> wrote:
> On Tue, Feb 23, 2016 at 7:18 AM, Edward Cree <ecree@...arflare.com> wrote:
>> On 23/02/16 03:31, Jesse Gross wrote:
>>> The only issue that I see is that making TSO completely unaware of
>>> outer headers will likely cause performance regressions in some cases.
>>> Imagine if we have an incoming TCP stream with incrementing IP IDs
>>> that we aggregate through GRO and forward. Today's TSO would be able
>>> to recreate the stream by incrementing the ID as new segments are
>>> created. However, if the outgoing NIC is truly only dealing with the
>>> L4 header then it wouldn't be able to do this.
>> Perhaps TSO should force setting the DF bit, so that the IP ID can be
>> ignored.  After all, if your network is going to cause fragmentation and
>> reassembly, your performance will probably be bad enough that TSO won't
>> help you much.  (And TCP usually wants DF anyway so it can do PMTUD.)
>> Arguably, as soon as we perform GRO on traffic to be forwarded, we've
>> already violated the end-to-end principle (there are always imaginable
>> situations in which a different packet stream comes out than went in),
>> so it doesn't really matter if we go on to change the network layer
>> parameters in this way - it's not really the same IP datagram any more
>> so it's OK for its identification to change.
>> And of course this problem doesn't present itself for IPv6 :)
>
> Right, GRO should probably not coalesce packets with non-zero IP
> identifiers due to the loss of information. Besides that, RFC6848 says
> the IP identifier should only be set for fragmentation anyway so there
> shouldn't be any issue and really no need for HW TSO (or LRO) to
> support that.

Most OSs (including Linux with connected TCP sockets) use non-zero IP
IDs so requiring this would effectively disable GRO.

I think the practical way to go about this is to introduce a new GSO
type for L4-only offload. There are some existing types that we could
immediately convert and kill off with no impact (such as GRE) and some
new protocols that would come for free (such as MPLS) so it would be a
net win. Once the infrastructure is in, it will be easier to evaluate
what else can be simplified on a case by case basis. (i.e. Even
UDP_TUNNEL will have some potential adverse impact from this compared
to explicit support since we'd need to break off the last segment from
a TSO burst where the size isn't an even multiple of the MSS. I guess
the impact is probably small but it would be good to know.)