netdev - Re: [PATCH v3 1/2] net: core: Notify on changes to dev->promiscuity.

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Date:   Fri, 30 Aug 2019 08:13:27 +0200
From:   Jiri Pirko <jiri@...nulli.us>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Horatiu Vultur <horatiu.vultur@...rochip.com>,
        alexandre.belloni@...tlin.com, UNGLinuxDriver@...rochip.com,
        davem@...emloft.net, allan.nielsen@...rochip.com,
        ivecera@...hat.com, f.fainelli@...il.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 1/2] net: core: Notify on changes to dev->promiscuity.

Thu, Aug 29, 2019 at 04:37:32PM CEST, andrew@...n.ch wrote:
>> Wait, I believe there has been some misundestanding. Promisc mode is NOT
>> about getting packets to the cpu. It's about setting hw filters in a way
>> that no rx packet is dropped.
>> 
>> If you want to get packets from the hw forwarding dataplane to cpu, you
>> should not use promisc mode for that. That would be incorrect.
>
>Hi Jiri
>
>I'm not sure a wireshark/tcpdump/pcap user would agree with you. They
>want to see packets on an interface, so they use these tools. The fact
>that the interface is a switch interface should not matter. The
>switchdev model is that we try to hide away the interface happens to
>be on a switch, you can just use it as normal. So why should promisc
>mode not work as normal?

It does, disables the rx filter. Why do you think it means the same
thing as "trap all to cpu"? Hw datapath was never considered by
wireshark.

In fact, I have usecase where I need to see only slow-path traffic by
wireshark, not all packets going through hw. So apparently, there is a
need of another wireshark option and perhaps another flag
IFF_HW_TRAPPING?.

tcpdump -i eth0
tcpdump -i eth0 --no-promiscuous-mode
tcpdump -i eth0 --hw-trapping-mode


> 
>> If you want to get packets from the hw forwarding dataplane to cpu, you
>> should use tc trap action. It is there exactly for this purpose.
>
>Do you really think a wireshark/tcpdump/pcap user should need to use
>tc trap for the special case the interface is a switch port? Doesn't that
>break the switchdev model?
>
>tc trap is more about fine grained selection of packets. Also, it
>seems like trapped packets are not forwarded, which is not what you
>would expect from wireshark/tcpdump/pcap.
>
>      Andrew