netdev - VLAN retagging for packets switched between 2 certain ports

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Message-ID: <CA+h21hr4KsDCzEeLD5CtcdXMtY5pOoHGi7-Oig0-gmRKThG30A@mail.gmail.com>
Date:   Thu, 6 Feb 2020 11:02:57 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     netdev <netdev@...r.kernel.org>
Subject: VLAN retagging for packets switched between 2 certain ports

Hi netdev,

I am interested in modeling the following classifier/action with tc filters:
- Match packets with VID N received on port A and going towards port B
- Replace VID with M

Some hardware (DSA switch) I am working on supports this, so it would
be good if I could model this with tc in a way that can be offloaded.
In man tc-flower I found the following matches:
       indev ifname
              Match on incoming interface name. Obviously this makes
sense only for forwarded flows.  ifname is the name of an interface
which must exist at the time of tc invocation.
       vlan_id VID
              Match on vlan tag id.  VID is an unsigned 12bit value in
decimal format.

And there is a generic "vlan" action (man tc-vlan) that supports the
"modify" command.

Judging from this syntax, I would need to add a tc-flower rule on the
egress qdisc of swpB, with indev swpA and vlan_id N.
But what should I do if I need to do VLAN retagging towards the CPU
(where DSA does not give me a hook for attaching tc filters)?

Thanks,
-Vladimir