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

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Message-ID: <d480caba-16e2-da3e-be33-ff4aeb5c6420@mellanox.com>
Date:   Tue, 4 Jun 2019 18:19:50 +0000
From:   Eli Britstein <elibr@...lanox.com>
To:     Cong Wang <xiyou.wangcong@...il.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 6/4/2019 8:55 PM, Cong Wang wrote:
> 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?

In a switchdev mode, we can passthru the VFs to VMs, and have their 
representors in the host, enabling us to manipulate the HW eswitch 
without knowledge of the VMs.

To simplify it, consider the following setup:

v1a <-> v1b and v2a <-> v2b are veth pairs.

Now, we configure v1a.20 and v2a.20 as VLAN devices over v1a/v2a 
respectively (and put the "a" devs in separate namespaces).

The TC rules are on the "b" devs, for example:

tc filter add dev v1b ... action pedit ... action csum ... action 
redirect dev v2b

Now, ping from v1a.20 to v1b.20. The namespaces transmit/receive tagged 
packets, and are not aware of the packet manipulation (and the required 
act_csum).
> Thanks.