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

[Russell King - ARM Linux admin <linux@armlinux.org.uk>]

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.

> 
> 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
> 
> 

-- 
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