netdev - Re: [PATCH net v3 0/3] net/sched: fix actions reading the network header in case of QinQ packets

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Date:   Tue, 4 Jun 2019 10:55:13 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Eli Britstein <elibr@...lanox.com>
Cc:     Davide Caratti <dcaratti@...hat.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        "David S . Miller" <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Shuang Li <shuali@...hat.com>,
        Stephen Hemminger <stephen@...workplumber.org>
Subject: Re: [PATCH net v3 0/3] net/sched: fix actions reading the network
 header in case of QinQ packets

On Sat, Jun 1, 2019 at 9:22 PM Eli Britstein <elibr@...lanox.com> wrote:
>
> I think that's because QinQ, or VLAN is not an encapsulation. There is
> no outer/inner packets, and if you want to mangle fields in the packet
> you can do it and the result is well-defined.

Sort of, perhaps VLAN tags are too short to be called as an
encapsulation, my point is that it still needs some endpoints to push
or pop the tags, in a similar way we do encap/decap.


>
> BTW, the motivation for my fix was a use case were 2 VGT VMs
> communicating by OVS failed. Since OVS sees the same VLAN tag, it
> doesn't add explicit VLAN pop/push actions (i.e pop, mangle, push). If
> you force explicit pop/mangle/push you will break such applications.

>From what you said, it seems act_csum is in the middle of packet
receive/transmit path. So, which is the one pops the VLAN tags in
this scenario? If the VM's are the endpoints, why not use act_csum
there?

Thanks.