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Date:   Sat, 9 Apr 2022 20:45:57 +0000
From:   Vladimir Oltean <>
To:     Tobias Waldekranz <>
CC:     "" <>,
        Jakub Kicinski <>,
        "David S. Miller" <>,
        Florian Fainelli <>,
        Andrew Lunn <>,
        Vivien Didelot <>,
        Vladimir Oltean <>,
        "" <>,
        Paolo Abeni <>,
        Roopa Prabhu <>,
        Nikolay Aleksandrov <>,
        Jiri Pirko <>, Ido Schimmel <>,
        Mattias Forsblad <>,
        Joachim Wiberg <>
Subject: Re: [PATCH net-next 0/6] Disable host flooding for DSA ports under a

Hi Tobias,

On Sat, Apr 09, 2022 at 09:46:54PM +0200, Tobias Waldekranz wrote:
> On Fri, Apr 08, 2022 at 23:03, Vladimir Oltean <> wrote:
> > For this patch series to make more sense, it should be reviewed from the
> > last patch to the first. Changes were made in the order that they were
> > just to preserve patch-with-patch functionality.
> >
> > A little while ago, some DSA switch drivers gained support for
> > IFF_UNICAST_FLT, a mechanism through which they are notified of the
> > MAC addresses required for local standalone termination.
> > A bit longer ago, DSA also gained support for offloading BR_FDB_LOCAL
> > bridge FDB entries, which are the MAC addresses required for local
> > termination when under a bridge.
> >
> > So we have come one step closer to removing the CPU from the list of
> > destinations for packets with unknown MAC DA.What remains is to check
> > whether any software L2 forwarding is enabled, and that is accomplished
> > by monitoring the neighbor bridge ports that DSA switches have.
> >
> > With these changes, DSA drivers that fulfill the requirements for
> > dsa_switch_supports_uc_filtering() and dsa_switch_supports_mc_filtering()
> > will keep flooding towards the CPU disabled for as long as no port is
> > promiscuous. The bridge won't attempt to make its ports promiscuous
> > anymore either if said ports are offloaded by switchdev (this series
> > changes that behavior). Instead, DSA will fall back by its own will to
> > promiscuous mode on bridge ports when the bridge itself becomes
> > promiscuous, or a foreign interface is detected under the same bridge.
> Hi Vladimir,
> Great stuff! I've added Joachim to Cc. He has been working on a series
> to add support for configuring the equivalent of BR_FLOOD,
> BR_MCAST_FLOOD, and BR_BCAST_FLOOD on the bridge itself. I.e. allowing
> the user to specify how local_rcv is managed in br_handle_frame_finish.
> For switchdev drivers, being able to query whether a bridge will ingress
> unknown unicast to the host or not seems like the missing piece that
> makes this bullet proof. I.e. if you have...
> - No foreign interfaces
> - No promisc
> _and_
> - No BR_FLOOD on the bridge itself
> ..._then_ you can safely disable unicast flooding towards the CPU
> port. The same would hold for multicast and BR_MCAST_FLOOD of course.
> Not sure how close Joachim is to publishing his work. But I just thought
> you two should know about the other one's work :)

I haven't seen Joachim's work and I sure hope he can clarify. It seems
like there is some overlap that I don't currently know what to make of.
The way I see things, BR_FLOOD and BR_MCAST_FLOOD are egress settings,
so I'm not sure how to interpret them when applied to the bridge device
itself. On the other hand, treating IFF_PROMISC/IFF_ALLMULTI on the
bridge device as the knob that decides whether the software bridge wants
to ingress unknown MAC DA packets seems the more appropriate thing to do.

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