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

Felix Fietkau
2022-04-12T13:51:37Z

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