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

David Miller
2015-06-15T23:25:53Z

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

> 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.