Re: Is it ok for switch TCAMs to depend on the bridge state?

On Thu, Nov 11, 2021 at 11:52:55AM +0000, Vladimir Oltean wrote:

On Sun, Nov 07, 2021 at 01:50:36PM +0200, Ido Schimmel wrote:

quoted

On Tue, Nov 02, 2021 at 11:03:53AM +0000, Vladimir Oltean wrote:

quoted

I've been reviewing a patch set which offloads to hardware some
tc-flower filters with some TSN-specific actions (ingress policing).
The keys of those offloaded tc-flower filters are not arbitrary, they
are the destination MAC address and VLAN ID of the frames, which is
relevant because these TSN policers are actually coupled with the
bridging service in hardware. So the premise of that patch set was that
the user would first need to add static FDB entries to the bridge with
the same key as the tc-flower key, before the tc-flower filters would be
accepted for offloading.

[...]

quoted

I don't have a clear picture in my mind about what is wrong. An airplane
viewer might argue that the TCAM should be completely separate from the
bridging service, but I'm not completely sure that this can be achieved
in the aforementioned case with VLAN rewriting on ingress and on egress,
it would seem more natural for these features to operate on the
classified VLAN (which again, depends on VLAN awareness being turned on).
Alternatively, one might argue that the deletion of a bridge interface
should be vetoed, and so should the removal of a port from a bridge.
But that is quite complicated, and doesn't answer questions such as
"what should you do when you reboot".
Alternatively, one might say that letting the user remove TCAM
dependencies from the bridging service is fine, but the driver should
have a way to also unoffload the tc-flower keys as long as the
requirements are not satisfied. I think this is also difficult to
implement.

Regarding the question in the subject ("Is it ok for switch TCAMs to
depend on the bridge state?"), I believe the answer is yes because there
is no way to avoid it and effectively it is already happening.

To add to your examples and Jakub's, this is also how "ERSPAN" works in
mlxsw. User space installs some flower filter with a mirror action
towards a gretap netdev, but the HW does not do the forwarding towards
the destination.

I don't understand this part. By "forwarding" you mean "mirroring" here,

Yes

and the "destination" is the gretap interface which is offloaded?

No. See more below

quoted

Instead, it relies on the SW to tell it which headers
(i.e., Eth, IP, GRE) to put on the mirrored packet and tell it from
which port the packet should egress. When we have a bridge in the
forwarding path, it means that the offload state of the filter is
affected by FDB updates.

Here you're saying that the gretap interface whose local IP address is
the IP address of a bridge interface that is offloaded by mlxsw, and the
precise egress port is determined by the bridge's FDB? But since you
don't support bridging with foreign interfaces, why would the mirred
rule ever become unoffloaded?

I'm afraid that I don't understand this case very well.

In software, when you mirror to a gretap via act_mirred, the packet is
cloned and transmitted through the gretap netdev. This netdev will then
put a GRE header on the packet, specifying that the next protocol is
Ethernet. It will then put an IP header on the packet with the
configured source and destination IPs and route the packet towards its
destination.

It is possible that routing will determine that the encapsulated packet
should be transmitted via a bridge. In which case, the packet will also
do an FDB lookup in the bridge before determining the egress port.

In hardware, we don't have a representation for the gretap device.
Instead, the hardware is kept very simple and requires the driver to
tell it:

a. Via which port to mirror the packet
b. Which headers to encapsulate the packet with

So the "offload-ability" of the filter is conditioned on software being
able to determine the correct path, which can change with time following
FDB/routes/etc updates.