netdev - Re: switchdev offload & ecmp

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Message-ID: <eb9e0c4a-5862-d109-3272-38430b69f707@6wind.com>
Date:   Tue, 16 May 2017 22:22:02 +0200
From:   Nicolas Dichtel <nicolas.dichtel@...nd.com>
To:     Ido Schimmel <idosch@...lanox.com>
Cc:     Jiri Pirko <jiri@...nulli.us>,
        Nikolay Aleksandrov <nikolay@...ulusnetworks.com>,
        Roopa Prabhu <roopa@...ulusnetworks.com>,
        netdev <netdev@...r.kernel.org>
Subject: Re: switchdev offload & ecmp

Le 16/05/2017 à 16:11, Ido Schimmel a écrit :
> On Tue, May 16, 2017 at 02:57:47PM +0200, Nicolas Dichtel wrote:
>>>> I suspect that there can be scenarii where some packets of a flow are forwarded
>>>> by the driver and some other are forwarded by the kernel.
>>>
>>> Can you elaborate? The kernel only sees specific packets, which were
>>> trapped to the CPU. See:
>>> https://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next.git/tree/drivers/net/ethernet/mellanox/mlxsw/spectrum.c#n2996
>> Ok, this part was not clear for me, thank you for the pointer.
>>
>> So, when an arp resolution is needed, the packets are not trapped to the CPU,
>> the device manages the queue itself?
> 
> There are two cases here. If you need an ARP resolution following a hit
> of a directly connected route and this neighbour isn't in the device's
> table, then packet is trapped (HOST_MISS_IPV4 in above list) to the CPU
> and triggers ARP resolution in the kernel. Eventually a NETEVENT will be
> sent and the neighbour will be programmed to the device.
> 
> If you need an ARP resolution of a nexthop, then this is a bit
> different. If you have an ECMP group with several nexthops, then once
> one of them is resolved, packets will be forwarded using it. To make
> sure other nexthops will also be resolved we try to periodically refresh
> them. Otherwise packets will always be forwarded using a single nexthop,
> as the kernel won't have motivation to resolve the others.
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/net/ethernet/mellanox/mlxsw/spectrum_router.c#n987
> 
> In case no nexthops can be resolved, then packets will be trapped to the
> CPU (RTR_INGRESS0 in above list) and forwarded by the kernel.
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/net/ethernet/mellanox/mlxsw/spectrum_router.c#n1896
> 
Ok, thank you for the details.

Regards,
Nicolas