netdev - Re: [PATCH net-next] net: remove abuse of VLAN DEI/CFI bit

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Date:   Tue, 13 Dec 2016 18:21:16 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Michał Mirosław <mirq-linux@...e.qmqm.pl>
Cc:     Stephen Hemminger <stephen@...workplumber.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "moderated list:ETHERNET BRIDGE" <bridge@...ts.linux-foundation.org>,
        "open list:OPENVSWITCH" <dev@...nvswitch.org>
Subject: Re: [PATCH net-next] net: remove abuse of VLAN DEI/CFI bit

On Tue, Dec 13, 2016 at 6:03 PM, Michał Mirosław
<mirq-linux@...e.qmqm.pl> wrote:
> On Tue, Dec 13, 2016 at 05:21:18PM -0800, Stephen Hemminger wrote:
>> On Sat,  3 Dec 2016 10:22:28 +0100 (CET)
>> Michał Mirosław <mirq-linux@...e.qmqm.pl> wrote:
>> > This All-in-one patch removes abuse of VLAN CFI bit, so it can be passed
>> > intact through linux networking stack.
>> >
>> > Signed-off-by: Michał Mirosław <michal.miroslaw@...ndesoftware.pl>
>> > ---
>> >
>> > Dear NetDevs
>> >
>> > I guess this needs to be split to the prep..convert[]..finish sequence,
>> > but if you like it as is, then it's ready.
>> >
>> > The biggest question is if the modified interface and vlan_present
>> > is the way to go. This can be changed to use vlan_proto != 0 instead
>> > of an extra flag bit.
>> >
>> > As I can't test most of the driver changes, please look at them carefully.
>> > OVS and bridge eyes are especially welcome.
>> >
>> > Best Regards,
>> > Michał Mirosław
>> Is the motivation to support 802.1ad Drop Eligability Indicator (DEI)?
>>
>> If so then you need to be more verbose in the commit log, and lots more
>> work is needed. You need to rename fields and validate every place a
>> driver is using DEI bit to make sure it really does the right thing
>> on that hardware. It is not just a mechanical change.
>
> My main motivation is to be able to see the bit intact in tcpdump and be
> able to pass it untouched through at least a veth pair. It would be great
> if all devices didn't do something stupid with the bit, but it's not
> something I am able to make happen.

imo "be able to pass untouched through veth" is not good enough
justification for such invasive patches.
I'm still not sure that all of these changes don't affect user space.