From mboxrd@z Thu Jan  1 00:00:00 1970
From: Du Fan <fengyuleidian0615@gmail.com>
Subject: Re: [PATCH net] gso: do GSO for local skb with size bigger than MTU
Date: Fri, 05 Dec 2014 08:25:34 +0800
Message-ID: <5480FB7E.6020905@gmail.com>
References: <20141201135225.GA16814@casper.infradead.org> <20141202154839.GB5344@t520.home> <20141202170927.GA9457@casper.infradead.org> <20141202173401.GB4126@redhat.com> <20141202174158.GB9457@casper.infradead.org> <CAEP_g=-86Z6pxNow-wjnbx_v9er_TSn6x5waigqVqYHa7tEQJw@mail.gmail.com> <20141203090339.GA9299@redhat.com> <CAEP_g=9C+D3gbjJ4n1t6xuyjqEAMYi4ZfqPoe92UAoQJH-UsKg@mail.gmail.com> <20141203183859.GB16447@redhat.com> <CAEP_g=_Y2YQg0wDRJLXsNH6p3fOc5G0KSJb12x_b4OE238ophg@mail.gmail.com> <20141203220244.GA8822@casper.infradead.org> <CAEP_g=90nC2HhmBNKh-hKJ5MJ85Z-_ER14roDxMsZAKog+dFhw@mail.gmail.com> <548011DE.4000208@gmail.com> <CAEP_g=9AH_RShKujVeUmty6BKPJZt=u_-h8NnUL1kX+tSYdBxQ@mail.gmail.com>
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Cc: Pravin Shelar <pshelar@nicira.com>, Thomas Graf <tgraf@suug.ch>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	"Du, Fan" <fan.du@intel.com>, Jason Wang <jasowang@redhat.com>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	"davem@davemloft.net" <davem@davemloft.net>,
	"fw@strlen.de" <fw@strlen.de>,
	"dev@openvswitch.org" <dev@openvswitch.org>
To: Jesse Gross <jesse@nicira.com>
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On 2014/12/5 7:23, Jesse Gross wrote:
> On Wed, Dec 3, 2014 at 11:48 PM, Du Fan <fengyuleidian0615@gmail.com>=
 wrote:
>> =E4=BA=8E 2014=E5=B9=B412=E6=9C=8804=E6=97=A5 06:51, Jesse Gross =E5=
=86=99=E9=81=93:
>>> 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 thi=
s
>> commit: 06021dcb "datapath: compat: Fix compilation 3.11" OpenvSwitc=
h
>> 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.