Re: [RFC PATCH net-next 0/4] switchdev: avoid duplicate packet forwarding

Tue, Jun 16, 2015 at 01:25:51AM CEST, davem@davemloft.net wrote:

From: sfeldma@gmail.com
Date: Sat, 13 Jun 2015 11:04:26 -0700

quoted

The switchdev port driver must do two things:

1) Generate a fwd_mark for each switch port, using some unique key of the
   switch device (and optionally port).  This is a one-time operation done
   when port's netdev is setup.

2) On packet ingress from port, mark the skb with the ingress port's
   fwd_mark.  If the device supports it, it's useful to only mark skbs
   which were already forwarded by the device.  If the device does not
   support such indication, all skbs can be marked, even if they're
   local dst.

Two new 32-bit fields are added to struct sk_buff and struct netdevice to
hold the fwd_mark.  I've wrapped these with CONFIG_NET_SWITCHDEV for now. I
tried using skb->mark for this purpose, but ebtables can overwrite the
skb->mark before the bridge gets it, so that will not work.

In general, this fwd_mark can be used for any case where a packet is
forwarded by the device and a copy is sent to the CPU, to avoid the kernel
re-forwarding the packet.  sFlow is another use-case that comes to mind,
but I haven't explored the details.

Generally I'm against adding new fields fo sk_buff but I'm trying to be
open minded. :-)

About the per-device fwd_mark, if the key attribute is uniqueness,
let's just do it right and use something like lib/idr.c to generate
truly unique indices at probe time for all devices using this
facility.  I like that better than having them be unique by a happy
accident.

We already have per-device uniqueue key. dev->ifindex.
That should be good for fwd_mark purposes I believe.