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

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Date:   Mon, 2 Sep 2019 19:42:31 +0200
From:   "Allan W. Nielsen" <allan.nielsen@...rochip.com>
To:     Jiri Pirko <jiri@...nulli.us>
CC:     David Miller <davem@...emloft.net>, <idosch@...sch.org>,
        <andrew@...n.ch>, <horatiu.vultur@...rochip.com>,
        <alexandre.belloni@...tlin.com>, <UNGLinuxDriver@...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.

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@...emloft.net wrote:
> >From: Jiri Pirko <jiri@...nulli.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
- tcpdump -p -i swp1
- tcpdump -i swp1 --hw-trapping-mode

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