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 17:53:22 +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 2022-07-17 17:20, Ido Schimmel wrote:
> On Sun, Jul 17, 2022 at 02:21:47PM +0200, netdev@...io-technology.com 
> wrote:
>> 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.
> 
> Good
> 
>> 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.
> 
> Few questions:
> 
> 1. Is it correct to think of this entry as an entry pointing to a
> special /dev/null port?

I guess you can think of it like that. It's internal to the chipset how 
it does it.

> 
> 2. After installing this entry, you no longer get miss violation
> interrupts because packets with this SA incur a mismatch violation
> (src_port != /dev/null) and therefore discarded in hardware?

Yes, and mismatch violations are suppressed in this implementation when 
locking the port.

> 
> 3. What happens to packets with a DA matching the zero-DPV entry, are
> they also discarded in hardware? If so, here we differ from the bridge
> driver implementation where such packets will be forwarded according to
> the locked entry and egress the locked port

I understand that egress will follow what is setup with regard to UC, MC 
and BC, though I haven't tested that. But no replies will get through of 
course as long as the port hasn't been opened for the iface behind the 
locked port.

> 
> 4. The reason for installing this entry is to suppress further miss
> violation interrupts?

Yes, while still HW dropping all ingress packets with the same (SA-mac, 
vlan) on  the port.

> 
> 5. If not replaced, will this entry always age out after the ageing
> time? Not sure what can refresh it given that traffic does not ingress
> from the /dev/null port.

That is where my implementation keeps the entries in a list and removes 
them after the bridge timeout using a kernel worker and jiffies.
So by default they age out after approx. 5 min.

> 
> Thanks