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

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Message-ID: <CAA93jw4Z-fz_6gTxrjwsifcMy=oAcF0mPT6s=_aGdDU3UgCgcg@mail.gmail.com>
Date:   Mon, 14 Nov 2022 05:18:01 -0800
From:   Dave Taht <dave.taht@...il.com>
To:     Felix Fietkau <nbd@....name>
Cc:     Vladimir Oltean <olteanv@...il.com>,
        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 4:21 AM Felix Fietkau <nbd@....name> wrote:
>
> On 14.11.22 12:55, Vladimir Oltean wrote:
> > On Sat, Nov 12, 2022 at 12:13:15PM +0100, Felix Fietkau wrote:
> >> On 12.11.22 05:40, Jakub Kicinski wrote:
> >> I don't really see a valid use case in running generic XDP, TC and NFT on a
> >> DSA master dealing with packets before the tag receive function has been
> >> run. And after the tag has been processed, the metadata DST is cleared from
> >> the skb.
> >
> > Oh, there are potentially many use cases, the problem is that maybe
> > there aren't as many actual implementations as ideas? At least XDP is,
> > I think, expected to be able to deal with DSA tags if run on a DSA
> > master (not sure how that applies when RX DSA tag is offloaded, but
> > whatever). Marek Behun had a prototype with Marvell tags, not sure how
> > far that went in the end:
> > https://www.mail-archive.com/netdev@vger.kernel.org/msg381018.html
> > In general, forwarding a packet to another switch port belonging to the
> > same master via XDP_TX should be relatively efficient.
> 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.

XDP continues to evolve rapidly, as do its use cases. ( ex:
ttps://github.com/thebracket/cpumap-pping#readme )

What cases did you run into?

specifically planning to be hacking on the mvpp2 switch driver soon.

>
> >> 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.
>
> - Felix



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