netdev - Re: [RFC bpf-next 1/2] net: bridge: add unstable br_fdb_find_port_from

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Date:   Wed, 26 Jan 2022 12:09:17 +0100
From:   Lorenzo Bianconi <lorenzo@...nel.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Nikolay Aleksandrov <nikolay@...dia.com>,
        bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Lorenzo Bianconi <lorenzo.bianconi@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Ahern <dsahern@...nel.org>,
        Yoshiki Komachi <komachi.yoshiki@...il.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Roopa Prabhu <roopa@...dia.com>,
        "bridge@...ts.linux-foundation.org" 
        <bridge@...ts.linux-foundation.org>,
        Ido Schimmel <idosch@...sch.org>
Subject: Re: [RFC bpf-next 1/2] net: bridge: add unstable
 br_fdb_find_port_from_ifindex helper

> On Mon, Jan 24, 2022 at 10:32 AM Nikolay Aleksandrov <nikolay@...dia.com> 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

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