netdev - Re: [PATCH net-next RFC v2] switchdev: bridge: drop hardware forwarded packets

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Date:	Fri, 20 Mar 2015 11:13:31 -0700
From:	Scott Feldman <sfeldma@...il.com>
To:	John Fastabend <john.r.fastabend@...el.com>
Cc:	Roopa Prabhu <roopa@...ulusnetworks.com>,
	"David S. Miller" <davem@...emloft.net>,
	Jiří Pírko <jiri@...nulli.us>,
	"Arad, Ronen" <ronen.arad@...el.com>,
	Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next RFC v2] switchdev: bridge: drop hardware
 forwarded packets

On Fri, Mar 20, 2015 at 10:11 AM, John Fastabend
<john.r.fastabend@...el.com> wrote:
> On 03/20/2015 09:58 AM, roopa@...ulusnetworks.com wrote:
>> From: Roopa Prabhu <roopa@...ulusnetworks.com>
>>
>> On a Linux bridge with bridge forwarding offloaded to switch ASIC,
>> there is a need to not re-forward frames that have already been
>> forwarded in hardware.
>>
>> Typically these are broadcast or multicast frames forwarded by the
>> hardware to multiple destination ports including sending a copy of
>> the packet to the cpu (kernel e.g. an arp broadcast).
>> The bridge driver will try to forward the packet again, resulting in
>> two copies of the same packet.
>>
>> These packets can also come up to the kernel for logging when they hit
>> a LOG acl rule in hardware. In such cases, you do want the packet
>> to go through the bridge netfilter hooks. Hence, this patch adds the
>> required checks just before the packet is being xmited.
>>
>> v2:
>>       - Add a new hw_fwded flag in skbuff to indicate that the packet
>>       is already hardware forwarded. Switch driver will set this flag.
>>       I have been trying to avoid having this flag in the skb
>>       and thats why this patch has been in my tree for long. Cant think
>>       of other better alternatives. Suggestions are welcome. I have put
>>       this under CONFIG_NET_SWITCHDEV to minimize the impact.
>>
>> Signed-off-by: Roopa Prabhu <roopa@...ulusnetworks.com>
>> Signed-off-by: Wilson Kok <wkok@...ulusnetworks.com>
>> ---
>
> Interesting. I completely avoid this problem by not instantiating a
> software bridge ;) When these pkts come up the stack I either use a
> raw socket to capture them, put a 'tc' ingress rule to do something,
> or have OVS handle them in some special way. It seems to me that this
> is where the sw/hw model starts to break when you have these magic
> bits to handle the packets differently.
>
> How do you know to set the skb bit? Do you have some indicator in the
> descriptor? I don't have any good way to learn this on my hardware. But
> I can assume if it reached the CPU it was because of some explicit rule.

I was wondering that also, since there was no example.

This features seems like it belongs in the bridge.  We already have
BR_FLOOD to indicate whether unknown unicast traffic is flooded to a
bridge port.  Can we add another BR_FLOOD_BCAST (or some name) for
this new feature?  You would set/clear this flag on the bridge
(master) port.  The default is set.  And now:

- #define BR_AUTO_MASK          (BR_FLOOD | BR_LEARNING)
+ #define BR_AUTO_MASK          (BR_FLOOD | BR_FLOOD_BCAST | BR_LEARNING)

Does this work for your use-case, Roopa?

-scott
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