Subject: Re: MACVLANs really best solution? How about a bridge with multiple bridge virtual interfaces?

From: Ben Greear <[email-masked]>
Date: Mon, 09 Mar 2009 11:54:39 -0700

Eric W. Biederman wrote:
> Patrick McHardy <[email-masked]> writes:
> 
>> Eric W. Biederman wrote:
>>> Patrick McHardy <[email-masked]> writes:
>>>
>>>> I agree on most points. There is one fundamental operational difference
>>>> however. With macvlan, all MAC addresses are known are therefore can be
>>>> programmed as secondary unicast addresses, while a bridge always uses
>>>> promiscous mode and for unknown addresses needs to flood forward them.
>>>>
>>>> This could be changed in the bridging code of course for bridges
>>>> consisting purely of local devices. Most of the bridging stuff isn't
>>>> needed for macvlans though, so its probably easier to simply perform
>>>> a lookup for local devices in macvlan on transmit, similar to what
>>>> is done on reception.
>>> What I haven't figured out is how you handle the transmit path for
>>> broadcast and multicast ethernet traffic.  How do you test to see if
>>> you have already preformed local transmission?
>> I'm not sure I understand the problem. Whats wrong with doing
>> the same as on transmit, i.e.:
>>
>> - for multicast/broadcast, deliver everywhere (except self)
>>
>> - for unicast, deliver to matching local macvlan device or
>>   underlying device
>>
>>> +static int macvlan_queue_xmit(struct sk_buff *skb, struct net_device *dev)
>>> +{
>>> +	const struct macvlan_dev *vlan = netdev_priv(dev);
>>> +	const struct macvlan_port *port = vlan->port;
>>> +	const struct macvlan_dev *dest;
>>> +	const struct ethhdr *eth;
>>>  -	skb->dev = dev;
>>> -	skb->pkt_type = PACKET_HOST;
>>> +	skb->protocol = eth_type_trans(skb, dev);
>>> +	eth = eth_hdr(skb);
>>>  -	netif_rx(skb);
>>> -	return NULL;
>>> +	dst_release(skb->dst);
>>> +	skb->dst = NULL;
>>> +	skb->mark = 0;
>>> +	secpath_reset(skb);
>>> +	nf_reset(skb);
>>> +
>>> +	if (is_multicast_ether_addr(eth->h_dest)) {
>>> +		macvlan_broadcast(skb, port, dev);
>>> +		return macvlan_xmit_world(skb, dev);
>>> +	}
>>> +
>>> +	dest = macvlan_hash_lookup(port, eth->h_dest);
>>> +	if (dest)
>>> +		return macvlan_unicast(skb, dest);
>>> +			
>>> +	return macvlan_xmit_world(skb, dev);
>>>  }
>> Pretty much like this :)
> 
> Yes.
> 
> There are two tricky parts.
> 
> One problem is that macvlans and the primary hardware device share the
> same transmit queue.  So when I have a broadcast packet on the primary
> devices queue I don't know if I have already sent it out to the
> macvlan devices or not.

A flag could be added to the skb so that we know it originated from
a mac-vlan.  That shouldn't require any extra hooks but just an extra
check in the mac-vlan rx code to drop any pkt received from the
underlying NIC with this flag set.

For broadcasting (or unicasting) to other mac-vlans or the underlying physical
device, the mac-vlan tx logic could check for local delivery before telling the
lower-level NIC to transmit the pkt.

Since we already have a mac hash, we could probably key off of the dest MAC
fairly easily.  For broadcast, it would be a flood.  We could also add a
flag to mac-vlan tx logic to be clever and only send ARP to the mac-vlans
likely to care.  This might not be a good filter for all possible cases, but
for general cases, and thousands of mac-vlans, it would save a lot of work
cheaply.

Thanks,
Ben

-- 
Ben Greear <[email-masked]>
Candela Technologies Inc  http://www.candelatech.com