Subject: Re: [RFC bpf-next 1/2] net: bridge: add unstable br_fdb_find_port_from_ifindex helper

From: Lorenzo Bianconi <[email-masked]>
Date: Wed, 26 Jan 2022 12:09:17 +0100

> On Mon, Jan 24, 2022 at 10:32 AM Nikolay Aleksandrov <[email-masked]> wrote:
> > >
> > > +int br_fdb_find_port_from_ifindex(struct xdp_md *xdp_ctx,
> > > +                               struct bpf_fdb_lookup *opt,
> > > +                               u32 opt__sz)
> > > +{
> > > +     struct xdp_buff *ctx = (struct xdp_buff *)xdp_ctx;
> > > +     struct net_bridge_port *port;
> > > +     struct net_device *dev;
> > > +     int ret = -ENODEV;
> > > +
> > > +     BUILD_BUG_ON(sizeof(struct bpf_fdb_lookup) != NF_BPF_FDB_OPTS_SZ);
> > > +     if (!opt || opt__sz != sizeof(struct bpf_fdb_lookup))
> > > +             return -ENODEV;
> > > +
> > > +     rcu_read_lock();
> > > +
> > > +     dev = dev_get_by_index_rcu(dev_net(ctx->rxq->dev), opt->ifindex);
> > > +     if (!dev)
> > > +             goto out;
> 
> imo that is way too much wrapping for an unstable helper.
> The dev lookup is not cheap.
> 
> With all the extra checks the XDP acceleration gets reduced.
> I think it would be better to use kprobe/fentry on bridge
> functions that operate on fdb and replicate necessary
> data into bpf map.
> Then xdp prog would do a single cheap lookup from that map
> to figure out 'port'.

ack, right. This is a very interesting approach. I will investigate it. Thanks.

Regards,
Lorenzo