netdev - Re: [PATCH v4 net-next 3/6] drivers: net: dsa: add locked fdb entry flag to drivers

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Date:   Sun, 17 Jul 2022 14:21:47 +0200
From:   netdev@...io-technology.com
To:     Ido Schimmel <idosch@...dia.com>
Cc:     Vladimir Oltean <olteanv@...il.com>, davem@...emloft.net,
        kuba@...nel.org, netdev@...r.kernel.org,
        Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>, Jiri Pirko <jiri@...nulli.us>,
        Ivan Vecera <ivecera@...hat.com>,
        Roopa Prabhu <roopa@...dia.com>,
        Nikolay Aleksandrov <razor@...ckwall.org>,
        Shuah Khan <shuah@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        linux-kernel@...r.kernel.org, bridge@...ts.linux-foundation.org,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v4 net-next 3/6] drivers: net: dsa: add locked fdb entry
 flag to drivers

On 2022-07-13 14:39, Ido Schimmel wrote:
> On Wed, Jul 13, 2022 at 09:09:58AM +0200, netdev@...io-technology.com 
> wrote:

> 
> What are "Storm Prevention" and "zero-DPV" FDB entries?

They are both FDB entries that at the HW level drops all packets having 
a specific SA, thus using minimum resources.
(thus the name "Storm Prevention" aka, protection against DOS attacks. 
We must remember that we operate with CPU based learning.)

> 
> There is no decision that I'm aware of. I'm simply trying to understand
> how FDB entries that have 'BR_FDB_ENTRY_LOCKED' set are handled in
> mv88e6xxx and other devices in this class. We have at least three
> different implementations to consolidate:
> 
> 1. The bridge driver, pure software forwarding. The locked entry is
> dynamically created by the bridge. Packets received via the locked port
> with a SA corresponding to the locked entry will be dropped, but will
> refresh the entry. On the other hand, packets with a DA corresponding 
> to
> the locked entry will be forwarded as known unicast through the locked
> port.
> 
> 2. Hardware implementations like Spectrum that can be programmed to 
> trap
> packets that incurred an FDB miss. Like in the first case, the locked
> entry is dynamically created by the bridge driver and also aged by it.
> Unlike in the first case, since this entry is not present in hardware,
> packets with a DA corresponding to the locked entry will be flooded as
> unknown unicast.
> 
> 3. Hardware implementations like mv88e6xxx that fire an interrupt upon
> FDB miss. Need your help to understand how the above works there and
> why. Specifically, how locked entries are represented in hardware (if 
> at
> all) and what is the significance of not installing corresponding
> entries in hardware.
> 

With the mv88e6xxx, a miss violation with the SA occurs when there is no 
entry. If you then add a normal entry with the SA, the port is open for 
that SA of course. The zero-DPV entry is an entry that ensures that 
there is no more miss violation interrupts from that SA, while dropping 
all entries with the SA.