netdev - Re: [PATCH net] gre: Fix regression in gretap TSO support

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Date:	Thu, 30 Oct 2014 08:32:16 -0700
From:	Alexander Duyck <alexander.h.duyck@...hat.com>
To:	Tom Herbert <therbert@...gle.com>
CC:	Neal Cardwell <ncardwell@...gle.com>,
	Pravin Shelar <pshelar@...ira.com>,
	Alexander Duyck <alexander.duyck@...il.com>,
	netdev <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	"H.K. Jerry Chu" <hkchu@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net] gre: Fix regression in gretap TSO support


On 10/30/2014 08:05 AM, Tom Herbert wrote:
> On Thu, Oct 30, 2014 at 7:30 AM, Alexander Duyck
> <alexander.h.duyck@...hat.com> wrote:
>> On 10/30/2014 06:51 AM, Neal Cardwell wrote:
>>> On Thu, Oct 30, 2014 at 1:14 AM, Pravin Shelar <pshelar@...ira.com> wrote:
>>>> On Wed, Oct 29, 2014 at 8:26 PM,  <alexander.duyck@...il.com> wrote:
>>>>> From: Alexander Duyck <alexander.h.duyck@...hat.com>
>>>>>
>>>>> On recent kernels I found that TSO on gretap interfaces didn't work.
>>>>> After
>>>>> bisecting it I found that commit b884b1a4 had introduced a regression in
>>>>> which the Ethernet header was being included in the GRE header length.
>>>>>
>>>>> This change corrects that by basing the GRE header length on the inner
>>>>> mac
>>>>> header in the case of GRE tunnels using transparent Ethernet bridging,
>>>>> and
>>>>> uses the network header for all other GRE tunnel types.
>>>>>
>>>>> Fixes: b884b1a4 ("gre_offload: simplify GRE header length calculation in
>>>>> gre_gso_segment()")
>>> Hmm. There may be other protocols, either now or in the future, where
>>> we want to be able to have a mac header inside the GRE header, rather
>>> than a network header. AFAICT it would be safer to revert b884b1a4,
>>> and go back to the previous code (from c50cd357), where we parse the
>>> GRE header to figure out its length.
>>>
>>> neal
>>
>> The change is consistent with how we handle this in other spots throughout
>> the kernel.  If nothing else you can just search for ETH_P_TEB and you will
>> find multiple spots in the kernel where IP tunnels differentiate between
>> transparent Ethernet bridging and regular IP in IP tunnels by checking for
>> the protocol ETH_P_TEB.
>>
> I'm not sure I understand this. We always use inner mac header in
> __skb_udp_tunnel_segment for computing tunnel length and don't
> distinguish between Ethernet or IP encapsulation. Presumably, in the
> case of IP encapsulation inner mac header is equal to inner network
> header. Why is this different for GRE?
>
> Thanks,
> Tom

I'll dig into that a bit more and see if I can simplify this.  I just 
wasn't sure if the inner mac header was being initialized or not in the 
case of IP in IP tunnels.

Thanks,

Alex
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