Re: [PATCH net-next] net: dsa: allow port mirroring towards foreign interfaces

On Sat, 6 Feb 2021 01:05:21 +0200 Vladimir Oltean wrote:

On Fri, Feb 05, 2021 at 02:42:55PM -0800, Florian Fainelli wrote:

quoted

How does the mirred action deal with that case? How does it know that
packets delivered to the DSA master should be sent towards a foreign
address, do I need to set-up two mirred rules? One that set-ups the
filter on say sw0p0 to redirect egress to eth0 (DSA master) and another
one to ingress filter on eth0 and egress mirror to eth1 (USB ethernet
dongle)?  

[ I should have posted this as RFC, somebody asked me if it's possible,
  I only tested ingress mirroring, saw something come out, and posted this.
  I didn't even study act_mirred.c to see why I got anything at all ]

Let me mark it as RFC, then :)

For ingress mirroring there should be nothing special about the mirror
packets, it's just more traffic in the ingress data path where the qdisc
hook already exists.

For ingress the only possible corner case seems to be if the filter has
SKIP_SW set, then HW will send to CPU but SW will ignore.

That's assuming the frame still comes on the CPU appropriately tagged.

For egress mirroring I don't think there's really any way for the mirred
action to take over the packets from what is basically the ingress qdisc
and into the egress qdisc of the DSA interface such that they will be
redirected to the selected mirror. I hadn't even thought about egress
mirroring. I suppose with more API, we could have DSA do introspection
into the frame header, see it's an egress-mirrored packet, and inject it
into the egress qdisc of the net device instead of doing netif_rx.

IMHO it's not very pretty but FWIW some "SmartNIC" drivers already do
a similar thing. But to be clear that's just an optimization, right?
The SW should still be able to re-process and come to the same
decisions as the switch, provided SKIP_SW was not set?

The idea with 2 mirrors might work however it's not amazing and I was
thinking that if we bother to do something at all, we could as well try
to think it through and come up with something that's seamless for the
user.