netdev - Re: [next-queue RFC 0/4] ethtool: Add support for frame preemption

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Date:   Sun, 17 May 2020 13:51:19 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     David Miller <davem@...emloft.net>
Cc:     Vinicius Costa Gomes <vinicius.gomes@...el.com>,
        intel-wired-lan@...ts.osuosl.org,
        Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
        netdev <netdev@...r.kernel.org>,
        Vladimir Oltean <vladimir.oltean@....com>,
        Po Liu <po.liu@....com>,
        Murali Karicheri <m-karicheri2@...com>,
        Jose Abreu <Jose.Abreu@...opsys.com>
Subject: Re: [next-queue RFC 0/4] ethtool: Add support for frame preemption

On Sun, 17 May 2020 at 01:19, David Miller <davem@...emloft.net> wrote:
>
> From: Vladimir Oltean <olteanv@...il.com>
> Date: Sun, 17 May 2020 00:03:39 +0300
>
> > As to why this doesn't go to tc but to ethtool: why would it go to tc?
>
> Maybe you can't %100 duplicate the on-the-wire special format and
> whatever, but the queueing behavior ABSOLUTELY you can emulate in
> software.
>
> And then you have the proper hooks added for HW offload which can
> do the on-the-wire stuff.
>
> That's how we do these things, not with bolted on ethtool stuff.

When talking about frame preemption in the way that it is defined in
802.1Qbu and 802.3br, it says or assumes nothing about queuing. It
describes the fact that there are 2 MACs per interface, 1 MAC dealing
with some traffic classes and the other dealing with the rest. Queuing
sits completely above the layer where frame preemption happens:
- The queuing layer does not care if packets go to a traffic class
that is serviced by a preemptible MAC or an express MAC
- The MAC does not care by what means have packets been classified to
one traffic class or another.
I have no idea what emulation of queuing behavior you are talking
about. Frame preemption is a MAC hardware feature. Your NIC supports
it or it doesn't. Which means it can talk to a link partner that
supports frame preemption, or it can't.

Thanks,
-Vladimir