netdev - Re: [PATCH v2 net] net: bridge: Do not offload IGMP/MLD messages

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Message-ID: <67753866-5237-4758-9bf3-d6a8611ac179@gmail.com>
Date: Wed, 16 Jul 2025 10:29:55 -0400
From: Joseph Huang <joseph.huang.2024@...il.com>
To: Ido Schimmel <idosch@...dia.com>, Joseph Huang <Joseph.Huang@...min.com>
Cc: netdev@...r.kernel.org, Nikolay Aleksandrov <razor@...ckwall.org>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Simon Horman <horms@...nel.org>, Vladimir Oltean <vladimir.oltean@....com>,
 Tobias Waldekranz <tobias@...dekranz.com>,
 Florian Fainelli <f.fainelli@...il.com>, bridge@...ts.linux.dev,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 net] net: bridge: Do not offload IGMP/MLD messages

On 7/16/2025 2:26 AM, Ido Schimmel wrote:
> On Mon, Jul 14, 2025 at 11:01:00AM -0400, Joseph Huang wrote:
>> Do not offload IGMP/MLD messages as it could lead to IGMP/MLD Reports
>> being unintentionally flooded to Hosts. Instead, let the bridge decide
>> where to send these IGMP/MLD messages.
>>
>> Consider the case where the local host is sending out reports in response
>> to a remote querier like the following:
>>
>>         mcast-listener-process (IP_ADD_MEMBERSHIP)
>>            \
>>            br0
>>           /   \
>>        swp1   swp2
>>          |     |
>>    QUERIER     SOME-OTHER-HOST
>>
>> In the above setup, br0 will want to br_forward() reports for
>> mcast-listener-process's group(s) via swp1 to QUERIER; but since the
>> source hwdom is 0, the report is eligible for tx offloading, and is
>> flooded by hardware to both swp1 and swp2, reaching SOME-OTHER-HOST as
>> well. (Example and illustration provided by Tobias.)
>>
>> Fixes: 472111920f1c ("net: bridge: switchdev: allow the TX data plane forwarding to be offloaded")
>> Signed-off-by: Joseph Huang <Joseph.Huang@...min.com>
> 
> I don't have personal experience with this offload, but it makes sense
> to not offload the replication of control packets to the underlying
> device and instead let the CPU handle it. These shouldn't be sent at an
> high rate anyway.
> 
> 
> I think you can just early return if the packet is IGMP/MLD. Something
> like:
> 
> diff --git a/net/bridge/br_switchdev.c b/net/bridge/br_switchdev.c
> index 95d7355a0407..9a910cf0256e 100644
> --- a/net/bridge/br_switchdev.c
> +++ b/net/bridge/br_switchdev.c
> @@ -17,6 +17,9 @@ static bool nbp_switchdev_can_offload_tx_fwd(const struct net_bridge_port *p,
>   	if (!static_branch_unlikely(&br_switchdev_tx_fwd_offload))
>   		return false;
>   
> +	if (br_multicast_igmp_type(skb))
> +		return false;
> +
>   	return (p->flags & BR_TX_FWD_OFFLOAD) &&
>   	       (p->hwdom != BR_INPUT_SKB_CB(skb)->src_hwdom);
>   }

Talking about these packets being low rate, should I add unlikely() like so:

diff --git a/net/bridge/br_switchdev.c b/net/bridge/br_switchdev.c
index 95d7355a0407..9a910cf0256e 100644
--- a/net/bridge/br_switchdev.c
+++ b/net/bridge/br_switchdev.c
@@ -17,6 +17,9 @@ static bool nbp_switchdev_can_offload_tx_fwd(const 
struct net_bridge_port *p,
   	if (!static_branch_unlikely(&br_switchdev_tx_fwd_offload))
   		return false;

+	if (unlikely(br_multicast_igmp_type(skb)))
+		return false;
+
   	return (p->flags & BR_TX_FWD_OFFLOAD) &&
   	       (p->hwdom != BR_INPUT_SKB_CB(skb)->src_hwdom);
   }

Thanks,
Joseph