Date:	Tue, 15 Jul 2014 13:22:45 -0700
From:	Jerry Chu <hkchu@...gle.com>
To:	Or Gerlitz <or.gerlitz@...il.com>
Cc:	Tom Herbert <therbert@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Or Gerlitz <ogerlitz@...lanox.com>,
	Wolfgang Walter <linux@...m.de>,
	David Miller <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] net-gre-gro: Fix a bug that breaks the
 forwarding path

On Tue, Jul 15, 2014 at 11:21 AM, Or Gerlitz <or.gerlitz@...il.com> wrote:
> On Tue, Jul 15, 2014 at 8:17 PM, Jerry Chu <hkchu@...gle.com> wrote:
>>
>> On Tue, Jul 15, 2014 at 7:06 AM, Or Gerlitz <or.gerlitz@...il.com> wrote:
>> > On Tue, Jul 15, 2014 at 12:30 AM, Jerry Chu <hkchu@...gle.com> wrote:
>> >>
>> >> On Mon, Jul 14, 2014 at 1:38 PM, Or Gerlitz <or.gerlitz@...il.com>
>> >> wrote:
>> >> > On Mon, Jul 14, 2014 at 10:16 PM, Jerry Chu <hkchu@...gle.com> wrote:
>> >> >> On Mon, Jul 14, 2014 at 12:05 PM, Or Gerlitz <or.gerlitz@...il.com>
>> >> >> wrote:
>> >> >>> On Sun, Jul 13, 2014 at 5:46 PM, H.K. Jerry Chu <hkchu@...gle.com>
>> >> >>> wrote:
>> >> >>>> Fixed a bug that was introduced by my GRE-GRO patch
>> >> >>>> (bf5a755f5e9186406bbf50f4087100af5bd68e40 net-gre-gro: Add GRE
>> >> >>>> support to the GRO stack) that breaks the forwarding path
>> >> >>>> because various GSO related fields were not set. The bug will
>> >> >>>> cause on the egress path either the GSO code to fail, or a
>> >> >>>> GRE-TSO capable (NETIF_F_GSO_GRE) NICs to choke. The following
>> >> >>>> fix has been tested for both cases.
>> >> >
>> >> >>> Anything different in this version vs. the one you posted earlier
>> >> >>> on
>> >> >>> February or this is a plain re-post?
>> >> >
>> >> >> I simply moved the patch against the latest net-next and resolved
>> >> >> some small
>> >> >> conflict so yes it's pretty much the same.
>> >> >
>> >> > OK, got it.
>> >> >
>> >> >> Also I don't see the subsequent
>> >> >> discussion on skb->encapsulation affects the validity of this patch
>> >> >> so
>> >> >> i'm resubmitting it. Also the patch has been confirmed to address
>> >> >> the problem
>> >> >> Wolfgang reported last week. Feel free to test against the
>> >> >> configuration (VXLAN?)
>> >> >> you had some question about earlier.
>> >> >
>> >> > Well, re-reading that thread, we were not very decisive there... my
>> >> > comment of setting the inner network header twice in
>> >> > inet_gro_complete
>> >> > doesn't apply only to vxlan but to other tunneling protocols.
>> >>
>> >> Understood, but my reply about it applying to the inner most hdr in the
>> >> end
>> >> is not limited to vxlan either.
>> >>
>> >> > Also, if
>> >> > we really need (why do we? or explained it on the Feb thread) to set
>> >> > skb->encapsulation for the sake of TX in a protocol (GRE) gro
>> >> > complete, looks a bit fishy to me...
>> >>
>> >> Why does it look fishy? When h/w support sLRO on tunneled pkts
>> >> driver will set that bit. So it seems very reasonable for the GRO stack
>> >> that supports tunneled pkts to set that bit too.
>> >
>> >
>> > what do you mean by "h/w support sLRO on tunneled pkts" here? as far
>> > as I understand, the driver should set the bit when they know it's an
>> > encapsulated
>> > packet for which the HW offloaded checksum verification.
>> >
>> > So my fishiness comment was referring to the fact that we see here
>> > that the stack
>> > is setting this bit too sometimes.
>>
>> There are many other places in the "stack" that will set that bit if you
>> grep it. I think many of them are there to make GSO work on tunneled pkts.
>> The code here is no different.
>
>
> I am not near the code now, but AFAIK, the "stack" sets it in the TX path
> and the driver sets it in the RX path, any deviation you see there for this
> convension except for the change introduced by this patch.

Where does the forwarding path (e.g., the case at hand) belong then?

AFAICT the current dev_hard_start_xmit() code pretty much requires
skb->encapsulation to be set on all tunneled pkts in order for the proper
TX offload to be possible.

>
>
>>
>>
>> >
>> >> > but saying this I think brings us
>> >> > back to that incomplete discussion [1]  sounds as this needs some
>> >> > plumbering... Still, this way or another I understand a regression
>> >> > was
>> >> > introduced here and should be fixed.
>> >
>> >> Yep. I thought we've made reasonable progress on the skb->encapsualtion
>> >> even though we can't close all the questions/issues
>> >
>> > where/how exactly (or more or less) this progress comes into play, is it
>> > by the description exchanged in the emails sent over the Feb/Mar thread
>> > or in Tom's 3.15/16/17 patches?
>>
>> Davem wanted a more precise definition of skb->encapsulation and I thought
>> some worthy discussion has happened as part of the thread of the
>> original patch (even though it left with some more questions but
>> that's often the case with complex kernel code, right?)
>
>
> complex as it may be, if the design is valid (e.g doesn't contradict itself,
> etc...) it should be subject to proper documentation and implementation. So
> in that repspect, I read Dave's comment as saying -- guys, this isn't the
> case with that skb encapsulation bit.
>
> For a nearby example, see the documentation of the semantic of the different
> checksum values (e.g none/unnecessary/complete/partial) that the stack and
> drivers are setting, it wasn't very clear/percise since maybe kernel 2.4 and
> only over one of the last kernel cycles on Dec 2013 this patch made it clear
> "net: skbuff: improve comment on checksumming"
>
> Or.
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