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

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Message-ID: <bcb33ba7-b2a3-1fe7-64b2-1e15203e2cce@nbd.name>
Date:   Sat, 12 Nov 2022 12:13:15 +0100
From:   Felix Fietkau <nbd@....name>
To:     Jakub Kicinski <kuba@...nel.org>,
        Vladimir Oltean <olteanv@...il.com>
Cc:     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 12.11.22 05:40, Jakub Kicinski wrote:
> On Sat, 12 Nov 2022 01:37:14 +0200 Vladimir Oltean wrote:
>> Jakub, what do you think? Refcounting or no refcounting?
> 
> I would not trust my word over Felix's.. but since you asked I'd vote
> for refcounted. There's probably a handful of low level redirects
> (generic XDP, TC, NFT) that can happen and steal the packet, and
> keep it alive for a while. I don't think they will all necessarily
> clear the dst.
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.
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?

- Felix