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

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Date:	Tue, 16 Jun 2015 08:04:27 +0200
From:	Jiri Pirko <jiri@...nulli.us>
To:	David Miller <davem@...emloft.net>
Cc:	sfeldma@...il.com, netdev@...r.kernel.org,
	simon.horman@...ronome.com, roopa@...ulusnetworks.com,
	ronen.arad@...el.com, john.r.fastabend@...el.com, andrew@...n.ch,
	f.fainelli@...il.com, linux@...ck-us.net, davidch@...adcom.com,
	stephen@...workplumber.org
Subject: Re: [RFC PATCH net-next 0/4] switchdev: avoid duplicate packet
 forwarding

Tue, Jun 16, 2015 at 01:25:51AM CEST, davem@...emloft.net wrote:
>From: sfeldma@...il.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.

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


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