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

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Date:	Tue, 23 Feb 2016 09:42:00 -0800
From:	Tom Herbert <tom@...bertland.com>
To:	Jesse Gross <jesse@...nel.org>
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 9:31 AM, Jesse Gross <jesse@...nel.org> wrote:
> 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.)

Why not just fix the stack to conform to RFC6864? As Edward pointed
out we lose the actual IP ID's in GRO anyway, so attempts to set them
in GSO may be wildly incorrect from the source point of view-- even in
that case were probably better off changing the IP identifier to zero
(okay since we're already breaking the E2E model anyway :-) ).