netdev - Re: [PATCH RFC net-next 0/3] Multi-CPU DSA support

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Date:   Tue, 13 Apr 2021 16:46:32 +0200
From:   Tobias Waldekranz <tobias@...dekranz.com>
To:     Marek Behun <marek.behun@....cz>
Cc:     Vladimir Oltean <olteanv@...il.com>,
        Ansuel Smith <ansuelsmth@...il.com>, netdev@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andriin@...com>,
        Eric Dumazet <edumazet@...gle.com>,
        Wei Wang <weiwan@...gle.com>,
        Cong Wang <cong.wang@...edance.com>,
        Taehee Yoo <ap420073@...il.com>,
        Björn Töpel <bjorn@...nel.org>,
        zhang kai <zhangkaiheb@....com>,
        Weilong Chen <chenweilong@...wei.com>,
        Roopa Prabhu <roopa@...ulusnetworks.com>,
        Di Zhu <zhudi21@...wei.com>,
        Francis Laniel <laniel_francis@...vacyrequired.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC net-next 0/3] Multi-CPU DSA support

On Tue, Apr 13, 2021 at 02:27, Marek Behun <marek.behun@....cz> wrote:
> On Tue, 13 Apr 2021 01:54:50 +0200
> Marek Behun <marek.behun@....cz> wrote:
>
>> I will look into this, maybe ask some follow-up questions.
>
> Tobias,
>
> it seems that currently the LAGs in mv88e6xxx driver do not use the
> HashTrunk feature (which can be enabled via bit 11 of the
> MV88E6XXX_G2_TRUNK_MAPPING register).

This should be set at the bottom of mv88e6xxx_lag_sync_masks.

> If we used this feature and if we knew what hash function it uses, we
> could write a userspace tool that could recompute new MAC
> addresses for the CPU ports in order to avoid the problem I explained
> previously...
>
> Or the tool can simply inject frames into the switch and try different
> MAC addresses for the CPU ports until desired load-balancing is reached.
>
> What do you think?

As you concluded in your followup, not being able to have a fixed MAC
for the CPU seems weird.

Maybe you could do the inverse? Allow userspace to set the masks for an
individual bond/team port in a hash-based LAG, then you can offload that
to DSA. Here there be dragons though, you need to ensure that there is
no intermediate config in which any buckets are enabled on multiple
ports.