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Date:   Mon, 2 Sep 2019 20:45:57 +0200
From:   Jiri Pirko <jiri@resnulli.us>
To:     "Allan W. Nielsen" <allan.nielsen@microchip.com>
Cc:     David Miller <davem@davemloft.net>, idosch@idosch.org,
        andrew@lunn.ch, horatiu.vultur@microchip.com,
        alexandre.belloni@bootlin.com, UNGLinuxDriver@microchip.com,
        ivecera@redhat.com, f.fainelli@gmail.com, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 1/2] net: core: Notify on changes to dev->promiscuity.
Message-ID: <20190902184557.GB2312@nanopsycho>
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Mon, Sep 02, 2019 at 08:05:20PM CEST, allan.nielsen@microchip.com wrote:
>The 09/02/2019 19:51, Jiri Pirko wrote:
>> External E-Mail
>> 
>> 
>> Mon, Sep 02, 2019 at 07:42:31PM CEST, allan.nielsen@microchip.com wrote:
>> >Hi Jiri,
>> >
>> >Sorry for joining the discussion this late, but I have been without mail access
>> >for the last few days.
>> >
>> >
>> >The 08/30/2019 08:36, Jiri Pirko wrote:
>> >> Fri, Aug 30, 2019 at 08:02:33AM CEST, davem@davemloft.net wrote:
>> >> >From: Jiri Pirko <jiri@resnulli.us>
>> >> >Date: Fri, 30 Aug 2019 07:39:40 +0200
>> >> >
>> >> >> Because the "promisc mode" would gain another meaning. Now how the
>> >> >> driver should guess which meaning the user ment when he setted it?
>> >> >> filter or trap?
>> >> >> 
>> >> >> That is very confusing. If the flag is the way to do this, let's
>> >> >> introduce another flag, like IFF_TRAPPING indicating that user wants
>> >> >> exactly this.
>> >> >
>> >> >I don't understand how the meaning of promiscuous mode for a
>> >> >networking device has suddenly become ambiguous, when did this start
>> >> >happening?
>> >> 
>> >> The promiscuity is a way to setup the rx filter. So promics == rx filter
>> >> off. For normal nics, where there is no hw fwd datapath,
>> >> this coincidentally means all received packets go to cpu.
>> >> But if there is hw fwd datapath, rx filter is still off, all rxed packets
>> >> are processed. But that does not mean they should be trapped to cpu.
>> >> 
>> >> Simple example:
>> >> I need to see slowpath packets, for example arps/stp/bgp/... that
>> >> are going to cpu, I do:
>> >> tcpdump -i swp1
>> >
>> >How is this different from "tcpdump -p -i swp1"
>> >
>> >> I don't want to get all the traffic running over hw running this cmd.
>> >> This is a valid usecase.
>> >> 
>> >> To cope with hw fwd datapath devices, I believe that tcpdump has to have
>> >> notion of that. Something like:
>> >> 
>> >> tcpdump -i swp1 --hw-trapping-mode
>> >> 
>> >> The logic can be inverse:
>> >> tcpdump -i swp1
>> >> tcpdump -i swp1 --no-hw-trapping-mode
>> >> 
>> >> However, that would provide inconsistent behaviour between existing and
>> >> patched tcpdump/kernel.
>> >> 
>> >> All I'm trying to say, there are 2 flags
>> >> needed (if we don't use tc trap).
>> >
>> >I have been reading through this thread several times and I still do not get it.
>> >
>> >As far as I understand you are arguing that we need 3 modes:
>> >
>> >- tcpdump -i swp1
>> 
>> Depends on default. Promisc is on.
>> 
>> 
>> >- tcpdump -p -i swp1
>> 
>> All traffic that is trapped to the cpu by default, not promisc means
>> only mac of the interface (if bridge for example haven't set promisc
>> already) and special macs. So host traffic (ip of host), bgp, arp, nsnd,
>> etc.
>
>In the case where the interface is enslaved to a bridge, it is put into promisc
>mode, which means that "tcpdump -i swp1" and "tcpdump -p -i swp1" give the same
>result, right?
>
>Is this desirable?

Yes, that is correct and expected. It it might not be bridged, depends
on a usecase.


>
>> >- tcpdump -i swp1 --hw-trapping-mode
>> 
>> Promisc is on, all traffic received on the port and pushed to cpu. User
>> has to be careful because in case of mlxsw this can lead to couple
>> hundred gigabit traffic going over limited pci bandwidth (gigabits).
>> 
>> 
>> >
>> >Would you mind provide an example of the traffic you want to see in the 3 cases
>> >(or the traffic which you do not want to see).
>> >
>> >/Allan
>> >
>> 
>
>-- 
>/Allan