netdev - Re: [PATCH v2 14/14] net: ethernet: mtk_eth_soc: support creating mac address based offload entries

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Message-ID: <ee1d6c89-95f4-bf28-cf25-36b18ffb342f@nbd.name>
Date:   Tue, 12 Apr 2022 15:49:51 +0200
From:   Felix Fietkau <nbd@....name>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     netdev@...r.kernel.org, John Crispin <john@...ozen.org>,
        Sean Wang <sean.wang@...iatek.com>,
        Mark Lee <Mark-MC.Lee@...iatek.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Jiri Pirko <jiri@...dia.com>, Ido Schimmel <idosch@...dia.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Vladimir Oltean <vladimir.oltean@....com>
Subject: Re: [PATCH v2 14/14] net: ethernet: mtk_eth_soc: support creating mac
 address based offload entries


On 12.04.22 15:07, Andrew Lunn wrote:
>> > > > > I'm trying to understand the architecture here.
>> > > > > We have an Ethernet interface and a Wireless interface. The slow
>> > > path
>> > > > is that frames ingress from one of these interfaces, Linux decides
>> > > > what to do with them, either L2 or L3, and they then egress probably
>> > > > out the other interface.
>> > > > > The hardware will look at the frames and try to spot flows? It
>> > > will
>> > > > then report any it finds. You can then add an offload, telling it for
>> > > > a flow it needs to perform L2 or L3 processing, and egress out a
>> > > > specific port? Linux then no longer sees the frame, the hardware
>> > > > handles it, until the flow times out?
>> > > Yes, the hw handles it until either the flow times out, or the corresponding
>> > > offload entry is removed.
>> > > 
>> > > For OpenWrt I also wrote a daemon that uses tc classifier BPF to accelerate
>> > > the software bridge and create hardware offload entries as well via hardware
>> > > TC flower rules: https://github.com/nbd168/bridger
>> > > It works in combination with these changes.
>> > 
>> > What about the bridge? In Linux, it is the software bridge which
>> > controls all this at L2, and it should be offloading the flows, via
>> > switchdev. The egress port you derive here is from the software bridge
>> > FDB?
> 
>> My code uses netlink to fetch and monitor the bridge configuration,
>> including fdb, port state, vlans, etc. and it uses that for the offload path
>> - no extra configuration needed.
> 
> So this is where we get into architecture issues. Do we really want
> Linux to have two ways for setting up L2 networking? It was decided
> that users should not need to know about how to use an accelerator,
> they should not use additional tools, it should just look like
> linux. The user should just add the WiFi netdev to the bridge and
> switchdev will do the rest to offload L2 switching to the hardware.
> 
> You appear to be saying you need a daemon in userspace. That is not
> how every other accelerate works in Linux networking.
> 
> We the Linux network community need to decided if we want this?
The problem here is that it can't be fully transparent. Enabling 
hardware offload for LAN -> WiFi comes at a cost of bypassing airtime 
fairness and mac80211's bufferbloat mitigation.
Some people want this anyway (often but not always for 
benchmark/marketing purposes), but it's not something that I would want 
to have enabled by default simply by a wifi netdev to a bridge.

Initially, I wanted to put more of the state tracking code in the 
kernel. I made the first implementation of my acceleration code as a 
patch to the network bridge - speeding up bridge unicast forwarding 
significantly for any device regardless of hardware support. I wanted to 
build on that to avoid putting a lot of FDB/VLAN related tracking 
directly into the driver.

That approach was immediately rejected and I was told to use BPF instead.

That said, I really don't think it's a good idea to put all the code for 
tracking the bridge state, and all possible forwarding destinations into 
the driver directly.

I believe the combination of doing the bridge state tracking in user 
space + using the standard TC API for programming offloading rules into 
the hardware is a reasonable compromise.

- Felix