On Mon, Jan 24, 2022 at 10:32 AM Nikolay Aleksandrov [off-list ref] wrote:

quoted

+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'.