Hi Tobias,

On Sun, Jul 04, 2021 at 12:04:26AM +0200, Tobias Waldekranz wrote:

On Sat, Jul 03, 2021 at 14:56, Vladimir Oltean [off-list ref] wrote:

quoted

For this series I have taken Tobias' work from here:
https://patchwork.kernel.org/project/netdevbpf/cover/20210426170411.1789186-1-tobias@waldekranz.com/
and made the following changes:
- I collected and integrated (hopefully all of) Nikolay's, Ido's and my
  feedback on the bridge driver changes. Otherwise, the structure of the
  bridge changes is pretty much the same as Tobias left it.
- I basically rewrote the DSA infrastructure for the data plane
  forwarding offload, based on the commonalities with another switch
  driver for which I implemented this feature (not submitted here)
- I adapted mv88e6xxx to use the new infrastructure, hopefully it still
  works but I didn't test that

Hi Vladimir,

Sorry that I have dropped the ball on this series. I have actually had a
v1 of this queued up for a while. Unfortunately I ran into mv88e6xxx
specific problems. (See below)

quoted

The data plane of the software bridge can be partially offloaded to
switchdev, in the sense that we can trust the accelerator to:
(a) look up its FDB (which is more or less in sync with the software
    bridge FDB) for selecting the destination ports for a packet
(b) replicate the frame in hardware in case it's a multicast/broadcast,
    instead of the software bridge having to clone it and send the
    clones to each net device one at a time. This reduces the bandwidth
    needed between the CPU and the accelerator, as well as the CPU time
    spent.

The data path forwarding offload is managed per "hardware domain" - a
generalization of the "offload_fwd_mark" concept which is being
introduced in this series. Every packet is delivered only once to each
hardware domain.

In addition, Tobias said in the original cover letter:

====================
## Overview

   vlan1   vlan2
       \   /
   .-----------.
   |    br0    |
   '-----------'
   /   /   \   \
swp0 swp1 swp2 eth0
  :   :   :
  (hwdom 1)

Up to this point, switchdevs have been trusted with offloading
forwarding between bridge ports, e.g. forwarding a unicast from swp0
to swp1 or flooding a broadcast from swp2 to swp1 and swp0. This
series extends forward offloading to include some new classes of
traffic:

- Locally originating flows, i.e. packets that ingress on br0 that are
  to be forwarded to one or several of the ports swp{0,1,2}. Notably
  this also includes routed flows, e.g. a packet ingressing swp0 on
  VLAN 1 which is then routed over to VLAN 2 by the CPU and then
  forwarded to swp1 is "locally originating" from br0's point of view.

- Flows originating from "foreign" interfaces, i.e. an interface that
  is not offloaded by a particular switchdev instance. This includes
  ports belonging to other switchdev instances. A typical example
  would be flows from eth0 towards swp{0,1,2}.

The bridge still looks up its FDB/MDB as usual and then notifies the
switchdev driver that a particular skb should be offloaded if it
matches one of the classes above. It does so by using the _accel
version of dev_queue_xmit, supplying its own netdev as the
"subordinate" device. The driver can react to the presence of the
subordinate in its .ndo_select_queue in what ever way it needs to make
sure to forward the skb in much the same way that it would for packets
ingressing on regular ports.

Hardware domains to which a particular skb has been forwarded are
recorded so that duplicates are avoided.

The main performance benefit is thus seen on multicast flows. Imagine
for example that:

- An IP camera is connected to swp0 (VLAN 1)

- The CPU is acting as a multicast router, routing the group from VLAN
  1 to VLAN 2.

- There are subscribers for the group in question behind both swp1 and
  swp2 (VLAN 2).

With this offloading in place, the bridge need only send a single skb
to the driver, which will send it to the hardware marked in such a way
that the switch will perform the multicast replication according to
the MDB configuration. Naturally, the number of saved skb_clones
increase linearly with the number of subscribed ports.

As an extra benefit, on mv88e6xxx, this also allows the switch to
perform source address learning on these flows, which avoids having to
sync dynamic FDB entries over slow configuration interfaces like MDIO
to avoid flows directed towards the CPU being flooded as unknown
unicast by the switch.


## RFC

- In general, what do you think about this idea?

- hwdom. What do you think about this terminology? Personally I feel
  that we had too many things called offload_fwd_mark, and that as the
  use of the bridge internal ID (nbp->offload_fwd_mark) expands, it
  might be useful to have a separate term for it.

- .dfwd_{add,del}_station. Am I stretching this abstraction too far,
  and if so do you have any suggestion/preference on how to signal the
  offloading from the bridge down to the switchdev driver?

- The way that flooding is implemented in br_forward.c (lazily cloning
  skbs) means that you have to mark the forwarding as completed very
  early (right after should_deliver in maybe_deliver) in order to
  avoid duplicates. Is there some way to move this decision point to a
  later stage that I am missing?

- BR_MULTICAST_TO_UNICAST. Right now, I expect that this series is not
  compatible with unicast-to-multicast being used on a port. Then
  again, I think that this would also be broken for regular switchdev
  bridge offloading as this flag is not offloaded to the switchdev
  port, so there is no way for the driver to refuse it. Any ideas on
  how to handle this?


## mv88e6xxx Specifics

Since we are now only receiving a single skb for both unicast and
multicast flows, we can tag the packets with the FORWARD command
instead of FROM_CPU. The swich(es) will then forward the packet in
accordance with its ATU, VTU, STU, and PVT configuration - just like
for packets ingressing on user ports.

Crucially, FROM_CPU is still used for:

- Ports in standalone mode.

- Flows that are trapped to the CPU and software-forwarded by a
  bridge. Note that these flows match neither of the classes discussed
  in the overview.

- Packets that are sent directly to a port netdev without going
  through the bridge, e.g. lldpd sending out PDU via an AF_PACKET
  socket.

We thus have a pretty clean separation where the data plane uses
FORWARDs and the control plane uses TO_/FROM_CPU.

The barrier between different bridges is enforced by port based VLANs
on mv88e6xxx, which in essence is a mapping from a source device/port
pair to an allowed set of egress ports.

Unless I am missing something, it turns out that the PVT is not enough
to support multiple (non-VLAN filtering) bridges in multi-chip
setups. While the isolation barrier works, there is no way of correctly
managing automatic learning.

quoted

In order to have a FORWARD
frame (which carries a _source_ device/port) correctly mapped by the
PVT, we must use a unique pair for each bridge.

Fortunately, there is typically lots of unused address space in most
switch trees. When was the last time you saw an mv88e6xxx product
using more than 4 chips? Even if you found one with 16 (!) devices,
you would still have room to allocate 16*16 virtual ports to software
bridges.

Therefore, the mv88e6xxx driver will allocate a virtual device/port
pair to each bridge that it offloads. All members of the same bridge
are then configured to allow packets from this virtual port in their
PVTs.

So while this solution is cute, it does not work in this example:

 CPU
  | .-----.
.-0-1-. .-0-1-.
| sw0 | | sw1 |
'-2-3-' '-2-3-'

- [sw0p2, sw1p2] are attached to one bridge
- [sw0p3, sw1p3] are attached to another bridge
- Neither bridge uses VLAN filtering

Since no VLAN information available in the frames, the source addresses
of FORWARDs sent over the DSA link (sw0p1, sw1p0) cannot possibly be
separated into different FIDs. They will all be placed in the respective
port's default FID. Thus, the two bridges are not isolated with respect
to their FDBs.

My current plan is therefore to start by reworking how bridges are
isolated on mv88e6xxx. Roughly by allocating a reserved VID/FID pair for
each non-filtering bridge. Two of these can be easily managed since both
VID 0 and 4095 are illegal on the wire but allowed in the VTU - after
that it gets tricky. The best scheme I have come up with is to just grab
an unused VID when adding any subsequent non-filtering bridge; in the
event that that VID is requested by a filtering bridge or a VLAN upper,
you move the non-filtering bridge to another currently unused VID.

Does that sound reasonable?

I don't think this patch series makes the problem you are describing any
worse than it already is in mainline, does it?

I mean even with multiple VLAN-unaware bridges spanning the same single
switch chip today, it is still true that you can not have two stations
with the same MAC address, one in one bridge and another in the other
bridge, right?

Do you have an example when this causes issues that need to be addressed
immediately?

I thought the only case where this is a real problem is when you have
multiple CPU ports or multiple DSA links between 2 switches, because
then, if learning is enabled, that same MAC address will bounce between
the 2 ports. For that case, the consensus was that you just can't enable
address learning on those ports, and you let the software manage the FDB
in a way that is compatible with multiple CPU ports / DSA links (install
the MAC DA as a sort of multicast address and let the port forwarding
matrix choose only one of the 2 destinations based on source port).

Lack of FDB partitioning also used to be a problem when the standalone
ports were left to do address learning, but that changed too.

The hardware I am working with simply does not have any way to solve
this either - the FDB is simply not partitionable without VLAN
filtering (we have simple shared VLAN filtering, where the VID is
ignored and the FDB lookup is performed with VID 0, but not anything
more complex). So the simple solution I've been advising for people who
want their MAC addresses to be isolated is to create a single VLAN-aware
bridge and manage the VLAN broadcast domains themselves - that seems to
work and is simple to understand and flexible (note that I am going to
send a patch at some point to prevent the user from partitioning a
sja1105 switch tree into multiple VLAN-aware bridges).

Basically unless I'm misunderstanding something, I think what you're
proposing makes theoretical sense, but without a use case behind it it
might just be too much work with no real life benefit.