Message-ID: <20150616211147.GB2135@nanopsycho.orion>
Date:	Tue, 16 Jun 2015 23:11:47 +0200
From:	Jiri Pirko <jiri@...nulli.us>
To:	Scott Feldman <sfeldma@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	Netdev <netdev@...r.kernel.org>,
	"simon.horman@...ronome.com" <simon.horman@...ronome.com>,
	Roopa Prabhu <roopa@...ulusnetworks.com>,
	"Arad, Ronen" <ronen.arad@...el.com>,
	"Fastabend, John R" <john.r.fastabend@...el.com>,
	"andrew@...n.ch" <andrew@...n.ch>,
	Florian Fainelli <f.fainelli@...il.com>,
	Guenter Roeck <linux@...ck-us.net>,
	davidch <davidch@...adcom.com>,
	"stephen@...workplumber.org" <stephen@...workplumber.org>
Subject: Re: [RFC PATCH net-next 0/4] switchdev: avoid duplicate packet
 forwarding

Tue, Jun 16, 2015 at 06:47:47PM CEST, sfeldma@...il.com wrote:
>On Mon, Jun 15, 2015 at 11:04 PM, Jiri Pirko <jiri@...nulli.us> wrote:
>> 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.
>
>It would be great if we could use dev->index, but fwd_mark is really
>to mark device ports that belong to a group.  In the case of a bridge,
>the device ports in the bridge should all have the same mark.  And
>another device's ports in the same bridge would have a different mark
>(so we can't use the bridge's dev->ifindex).  On ingress, the skb is
>marked with the ingress port's mark.  If the skb is to be forwarded
>out an egress port, the skb mark is compared with egress port's mark.
>If marks compare, then the device has already forwarded the pkt so the
>kernel can consume_skb to avoid duplicate pkts on the wire.
>
>So what we need is a unique mark for device ports within a fwding
>group, such as a bridge.

Yep, have a group of netdevs, pick one of them and use it's ifindex for
the whole group.


>
>I'm investigating Dave's suggestion to use IDR.  I think this will work...
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