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

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Message-ID: <CA+mtBx_AAexNNktyZDFFZwqzgEh_FJbdekvtYVLn7cc2AGLFqA@mail.gmail.com>
Date:	Thu, 30 Oct 2014 08:05:51 -0700
From:	Tom Herbert <therbert@...gle.com>
To:	Alexander Duyck <alexander.h.duyck@...hat.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 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

> Thanks,
>
> Alex
>
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