Date:	Wed, 22 Jun 2016 18:22:50 +0000
From:	Yuval Mintz <Yuval.Mintz@...gic.com>
To:	Alexander Duyck <alexander.duyck@...il.com>
CC:	Manish Chopra <manish.chopra@...gic.com>,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Ariel Elior <Ariel.Elior@...gic.com>,
	Tom Herbert <tom@...bertland.com>,
	"Hannes Frederic Sowa" <hannes@...hat.com>
Subject: Re: [PATCH net-next 0/5] qed/qede: Tunnel hardware GRO support

>>>> This series adds driver support for the processing of tunnel
>>>> [specifically vxlan/geneve/gre tunnels] packets which are
>>>> aggregated [GROed] by the hardware before driver passes
>>>> such packets upto the stack.
>>> First off I am pretty sure this isn't GRO.  This is LRO.
>> Nopes. This is GRO - each MSS-sized frame will arrive on its
>> own frag, whereas the headers [both  internal & external]
>> would arrive on the linear part of the SKB.

> No it is LRO, it just very closely mimics GRO.  If it is in hardware
> it is LRO.  GRO is extensible in software and bugs can be fixed in
> kernel, LRO is not extensible and any bugs in it are found in hardware
> or the driver and would have to be fixed there.  It all comes down to
> bug isolation.  If we find that disabling LRO makes the bug go away we
> know the hardware aggregation has an issue.  Both features should not
> operate on the same bit.  Otherwise when some bug is found in your
> implementation it will be blamed on GRO when the bug is actually in
> LRO.
I'm not aware of a definition stating GRO *has* to be extensible in SW;
AFAIK the LRO/GRO distinction revolves around multiple areas [criteria
for aggregation, method of aggregation, etc.].

> I realize this causes some pain when routing or bridging as LRO is
> disabled but that is kind of the point.  We don't want the hardware to
> be mangling frames when we are attempting to route packets between any
> two given devices.
Actually, while I might disagree on whether this is LRO/GRO, I don't think
there's any problem for us to base this on the LRO feature - I.e., when we
started working on qede we didn't bother implementing LRO as we understood
it was deprecated, but this encapsulated aggregation is configurable; If it
makes life easier for everyone if we make the configuration based on the LRO
configuration, so that when LRO is disabled we won't have this turned on
it can be easily done.

>>> Also I don't know if you have been paying attention to recent
>>> discussions on the mailing list but the fact is GRO over UDP tunnels
>>> is still a subject for debate.  This patch takes things in the
>>> opposite direction of where we are currently talking about going with
>>> GRO.  I've added Hannes and Tom to this discussion just to make sure I
>>> have the proper understanding of all this as my protocol knowledge is
>>> somewhat lacking.
>> I guess we're on the exact opposite side of the discussion - I.e., we're
>> the vendor that tries pushing offload capabilities to the device.
>> Do notice however that we're not planning on pushing anything new
>> feature-wise, just like we haven't done anything for regular TCP GRO -
>> All we do is allow our HW/FW to aggregate packets instead of stack.

> If you have been following the list then arguably you didn't fully
> understand what has been going on.  I just went through and cleaned up
> all the VXLAN, GENEVE, and VXLAN-GPE mess that we have been creating
> and tried to get this consolidated so that we could start to have
> conversations about this without things being outright rejected.  I
> feel like you guys have just prooven the argument for the other side
> that said as soon as we start support any of it, the vendors were
> going to go nuts and try to stuff everything and the kitchen sink into
> the NICs.  The fact is I am starting to think they were right.
You hurt my feeling; I started going nuts ages ago ;-)
But seriously, this isn't really anything new but rather a step forward in
the direction we've already taken - bnx2x/qede are already performing
the same for non-encapsulated TCP.
And while I understand why you're being suspicious of such an addition,
I don't entirely see how it affects any of that discussion - I already yielded
that we can make this configurable, so if any routing decision would be
added that result in packets NOT supposed to be aggregated, the feature
can be turned off [at worst, at the expanse of it not having its benefit
on other connections].

>>> Ideally we need to be able to identify that a given packet terminates
>>> on a local socket in our namespace before we could begin to perform
>>> any sort of mangling on the local packets.  It is always possible that
>>> we could be looking at a frame that uses the same UDP port but is not
>>> the tunnel protocol if we are performing bridging or routing.  The
>>> current GRO implementation fails in that regard and there are
>>> discussions between several of us on how to deal with that.  It is
>>> likely that we would be forcing GRO for tunnels to move a bit further
>>>> up the stack if bridging or routing so that we could verify that the
>>> frame is not being routed back out before we could aggregate it.

>> I'm aware of the on-going discussion, but I'm not sure this should
>> bother us greatly - the aggregation is done based on the
>> inner TCP connection; I.e., it's guaranteed all the frames belong to
>> the same TCP connection. While HW requires the UDP port for the
>> initial classification effort, it doesn't take decision solely on that.

> I realize that UDP port is only one piece of this.  Exactly what
> fields are being taken into account.  That information would be useful
> as we could then go though and determine what the probability is of us
> having a false match between any two packets in a given UDP flow.
While we can surely do that, I think that's really beside the point;
I.e., I don't expect any of you to debug issues arising from a bad HW/FW
implementation. But if you REALLY want, I can go and ask for exact
details.

> Also I would still argue this is LRO.  If we are doing routing it
> should be disabled for us to later re-enable if we feel it is safe.
> Having a feature like this enabled by default with GRO is a really bad
> idea.
Which is exactly what happens in qede/bnx2x for regular TCP.
[I understand this area is a much hotter potato at the moment then
simple GRO when we've submitted that content for both drivers;
But I won't mention it again - the sins of the fathers are not excuses
for any wrong-doings today].

>>> Also I would be interested in knowing how your hardware handles
>>> tunnels with outer checksums.  Is it just ignoring the frames in such
>>> a case, ignoring the checksum, or is it actually validating the frames
>>> and then merging the resulting checksum?

>> HW is validating both inner and outer csum; if it wouldn't be able to
>> due to some limitation, it would not try to pass the packet as a GRO
>> aggregation but rather as regular seperated packets.
>> But I don't believe it merges the checksum, only validate each
>> independently [CSUM_UNNECESSARY].

>So it is mangling the packets then. 
Obviously - you can't aggregate things without touching them.

> If this were flagged as LRO at
> least if bridging or routing is enabled you won't be mangling the
> frames without the user having to specifically go back through and
> re-enable LRO.

- Alex
    

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