netdev - Re: [PATCH v2 net-next 06/16] net: dsa: add a generic procedure for the flow dissector

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Date:   Sat, 26 Sep 2020 20:05:45 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Vladimir Oltean <vladimir.oltean@....com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, f.fainelli@...il.com,
        vivien.didelot@...il.com, kuba@...nel.org
Subject: Re: [PATCH v2 net-next 06/16] net: dsa: add a generic procedure for
 the flow dissector

On Sat, Sep 26, 2020 at 08:30:58PM +0300, Vladimir Oltean wrote:
> For all DSA formats that don't use tail tags, it looks like behind the
> obscure number crunching they're all doing the same thing: locating the
> real EtherType behind the DSA tag. Nonetheless, this is not immediately
> obvious, so create a generic helper for those DSA taggers that put the
> header before the EtherType.

This is interesting, and opens up the possibility of an optimisation on
the hot path.

net/core/flow_dissector.c:

bool __skb_flow_dissect(const struct net *net,
                        const struct sk_buff *skb,
                        struct flow_dissector *flow_dissector,
                        void *target_container,
                        void *data, __be16 proto, int nhoff, int hlen,
                        unsigned int flags)
{
...

#if IS_ENABLED(CONFIG_NET_DSA)
                if (unlikely(skb->dev && netdev_uses_dsa(skb->dev) &&
                             proto == htons(ETH_P_XDSA))) {
                        const struct dsa_device_ops *ops;
                        int offset = 0;

                        ops = skb->dev->dsa_ptr->tag_ops;
                        if (ops->flow_dissect &&
                            !ops->flow_dissect(skb, &proto, &offset)) {
                                hlen -= offset;
                                nhoff += offset;
                        }
                }
#endif

Given spectra and meltdown etc, jumping through a pointer is expensive
and we try to avoid it on the hot path. Given most of the taggers are
going to use the generic version, maybe add a test here, is
ops->flow_dissect the generic version, and if so, call it directly,
rather than go through the pointer. Or only set ops->flow_dissect if
the generic version cannot be used.

    Andrew