netdev - Re: [PATCH net] gso: do GSO for local skb with size bigger than MTU

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Date:	Fri, 05 Dec 2014 08:25:34 +0800
From:	Du Fan <fengyuleidian0615@...il.com>
To:	Jesse Gross <jesse@...ira.com>
CC:	Pravin Shelar <pshelar@...ira.com>, Thomas Graf <tgraf@...g.ch>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	"Du, Fan" <fan.du@...el.com>, Jason Wang <jasowang@...hat.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"fw@...len.de" <fw@...len.de>,
	"dev@...nvswitch.org" <dev@...nvswitch.org>
Subject: Re: [PATCH net] gso: do GSO for local skb with size bigger than MTU


On 2014/12/5 7:23, Jesse Gross wrote:
> On Wed, Dec 3, 2014 at 11:48 PM, Du Fan <fengyuleidian0615@...il.com> wrote:
>> 于 2014年12月04日 06:51, Jesse Gross 写道:
>>> My proposal would be something like this:
>>>    * For L2, reduce the VM MTU to the lowest common denominator on the
>>> segment.
>>>    * For L3, use path MTU discovery or fragment inner packet (i.e.
>>> normal routing behavior).
>>>    * As a last resort (such as if using an old version of virtio in the
>>> guest), fragment the tunnel packet.
>>
>> After some investigation on OpenvSwitch package, it seems before this
>> commit: 06021dcb "datapath: compat: Fix compilation 3.11" OpenvSwitch
>> package is doing GSO on its own.
>>
>> rpl_ip_local_out
>>    -> tnl_skb_gso_segment
>>        ^^^^^^^^^^^^^^^
>>           Perform GSO  in above function
>>      -> ip_local_out
>>            .
>>            .
>>          -> ip_finish_output
>>
>> Which means, when over-MTU-sized skb enter ip_local_out, it's not a gso
>> type skb, so the stack perform ip fragmentation, and send them out.
>> So, over-MTU-sized skb did travel through stack into outside.
>>
>> Why not dropping such OpenvSwitch level GSO operation after 3.10?
> The change in 3.11 was that the tunnel infrastructure used by OVS was
> upstreamed and shared by all implementations. It's not right to
> perform GSO in OVS itself as it prevents the logic from being used by
> other components. Breaking up the packet in OVS also eliminates some
> of the benefits of GSO by shortening the optimized path and prevents
> offloading to hardware.
Thanks for your explanation, I understand its background better now.

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