Re: [PATCH net-next 3/3] net: dsa: mv88e6xxx: defautl to multicast and unicast flooding

[Florian Fainelli <f.fainelli@gmail.com>]

On 2/17/2019 8:34 AM, Russell King - ARM Linux admin wrote:
> On Sun, Feb 17, 2019 at 02:27:16PM +0000, Russell King - ARM Linux admin wrote:
>> On Sun, Feb 17, 2019 at 02:25:17PM +0000, Russell King wrote:
>>> Switches work by learning the MAC address for each attached station by
>>> monitoring traffic from each station.  When a station sends a packet,
>>> the switch records which port the MAC address is connected to.
>>>
>>> With IPv4 networking, before communication commences with a neighbour,
>>> an ARP packet is broadcasted to all stations asking for the MAC address
>>> corresponding with the IPv4.  The desired station responds with an ARP
>>> reply, and the ARP reply causes the switch to learn which port the
>>> station is connected to.
>>>
>>> With IPv6 networking, the situation is rather different.  Rather than
>>> broadcasting ARP packets, a "neighbour solicitation" is multicasted
>>> rather than broadcasted.  This multicast needs to reach the intended
>>> station in order for the neighbour to be discovered.
>>>
>>> Once a neighbour has been discovered, and entered into the sending
>>> stations neighbour cache, communication can restart at a point later
>>> without sending a new neighbour solicitation, even if the entry in
>>> the neighbour cache is marked as stale.  This can be after the MAC
>>> address has expired from the forwarding cache of the DSA switch -
>>> when that occurs, there is a long pause in communication.
>>>
>>> Our DSA implementation for mv88e6xxx switches has defaulted to having
>>> multicast and unicast flooding disabled.  As per the above description,
>>> this is fine for IPv4 networking, since the broadcasted ARP queries
>>> will be sent to and received by all stations on the same network.
>>> However, this breaks IPv6 very badly - blocking neighbour solicitations
>>> and later causing connections to stall.
>>>
>>> The defaults that the Linux bridge code expect from bridges are that
>>> unknown unicast frames and unknown multicast frames are flooded to
>>> all stations, which is at odds to the defaults adopted by our DSA
>>> implementation for mv88e6xxx switches.
>>>
>>> This commit enables by default flooding of both unknown unicast and
>>> unknown multicast frames.  This means that mv88e6xxx DSA switches now
>>> behave as per the bridge(8) man page, and IPv6 works flawlessly through
>>> such a switch.
>>
>> Note that there is the open question whether this affects the case where
>> each port is used as a separate network interface: that case has not yet
>> been tested.
> 
> I've checked with a mv88e6131 on the clearfog gt8k board.  lan1
> connected to my lan with plenty of traffic on, and configured as
> part of a bridge.  lan2 connected to the zii board, but not part
> of the bridge.  Monitoring lan2 from the zii board shows no traffic
> that was received from lan1.
> 
> So it looks fine.

With the current state whereby we do not have the necessary hooks to
perform filtering on non-bridged/standalone ports, this is entirely fine
indeed.

In the future this is part of something I want to address because it is
IMHO highly undesirable to have non-bridged ports be flooded with
unknown multicast or unknown unicast for that matter because that makes
them deviate from a standard NIC interface. Unknown unicast is not
necessarily a low hanging fruit, but still, if we have switches capable
of filtering, we might as well make use of that. Of course, one
difficulty is that we must not break running tcpdump on those DSA slave
network interfaces.

> 
>>
>>>
>>> Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
>>> ---
>>>  drivers/net/dsa/mv88e6xxx/chip.c | 9 +++++----
>>>  1 file changed, 5 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/drivers/net/dsa/mv88e6xxx/chip.c b/drivers/net/dsa/mv88e6xxx/chip.c
>>> index b75a865a293d..eb5e3d88374f 100644
>>> --- a/drivers/net/dsa/mv88e6xxx/chip.c
>>> +++ b/drivers/net/dsa/mv88e6xxx/chip.c
>>> @@ -2144,13 +2144,14 @@ static int mv88e6xxx_setup_message_port(struct mv88e6xxx_chip *chip, int port)
>>>  static int mv88e6xxx_setup_egress_floods(struct mv88e6xxx_chip *chip, int port)
>>>  {
>>>  	struct dsa_switch *ds = chip->ds;
>>> -	bool flood;
>>>  
>>> -	/* Upstream ports flood frames with unknown unicast or multicast DA */
>>> -	flood = dsa_is_cpu_port(ds, port) || dsa_is_dsa_port(ds, port);
>>> +	/* Linux bridges are expected to flood unknown multicast and
>>> +	 * unicast frames to all ports - as per the defaults specified
>>> +	 * in the iproute2 bridge(8) man page. Not doing this causes
>>> +	 * stalls and failures with IPv6 over Marvell bridges. */
>>>  	if (chip->info->ops->port_set_egress_floods)
>>>  		return chip->info->ops->port_set_egress_floods(chip, port,
>>> -							       flood, flood);
>>> +							       true, true);
>>>  
>>>  	return 0;
>>>  }
>>> -- 
>>> 2.7.4
>>>
>>>
>>
>> -- 
>> RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
>> FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up
>> According to speedtest.net: 11.9Mbps down 500kbps up
> 

-- 
Florian