linux-kernel - Re: [PATCH net-next v3 1/4] net: dsa: add support for DSA rx offloading via metadata dst

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Date:   Mon, 14 Nov 2022 14:17:32 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Felix Fietkau <nbd@....name>
Cc:     Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>, Andrew Lunn <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v3 1/4] net: dsa: add support for DSA rx
 offloading via metadata dst

On Mon, Nov 14, 2022 at 01:06:42PM +0100, Felix Fietkau wrote:
> In that case it likely makes sense to disable DSA tag offloading whenever
> driver XDP is being used.
> Generic XDP probably doesn't matter much. Last time I tried to use it and
> ran into performance issues, I was told that it's only usable for testing
> anyway and there was no interest in fixing the cases that I ran into.

Don't know about generic XDP performance, sorry. But I think it's
reasonable that XDP programs written for a DSA switch should also work
if the DSA master has no driver support for XDP. At least until it gains
driver level support.

> > > How about this: I send a v4 which uses skb_dst_drop instead of skb_dst_set,
> > > so that other drivers can use refcounting if it makes sense for them. For
> > > mtk_eth_soc, I prefer to leave out refcounting for performance reasons.
> > > Is that acceptable to you guys?
> > 
> > I don't think you can mix refcounting at consumer side with no-refcounting
> > at producer side, no?
> skb_dst_drop checks if refcounting was used for the skb dst pointer.
> 
> > I suppose that we could leave refcounting out for now, and bug you if
> > someone comes with a real need later and complains. Right now it's a bit
> > hard for me to imagine all the possibilities. How does that sound?
> Sounds good. I think I'll send v4 anyway to deal with the XDP case and to
> switch to skb_dst_drop.

Sounds good.