[xdp-hints] XDP Redirect and TX Metadata

[Florian Kauer <florian.kauer@linutronix.de>]

Hi,
I am currently implementing an eBPF for redirecting from one physical interface to another. So basically loading the following at enp8s0:

SEC("prog")
int xdp_redirect(struct xdp_md *ctx) {
	/* ... */
	return bpf_redirect(3 /* enp5s0 */, 0);
}

I made three observations that I would like to discuss with you:

1. The redirection only works when I ALSO load some eBPF at the egress interface (enp5s0). It can be just

SEC("prog")
int xdp_pass(struct xdp_md *ctx) {
	return XDP_PASS;
}

but there has to be at least something. Otherwise, only xdp_redirect is called, but xdp_devmap_xmit is not.
It seems somewhat reasonable that the interface where the traffic is redirected to also needs to have the
XDP functionality initialized somehow, but it was unexpected at first. It tested it with an i225-IT (igc driver)
and a 82576 (igb driver). So, is this a bug or a feature?

2. For the RX side, the metadata is documented as "XDP RX Metadata" (https://docs.kernel.org/networking/xdp-rx-metadata.html),
while for TX it is "AF_XDP TX Metadata" (https://www.kernel.org/doc/html/next/networking/xsk-tx-metadata.html).
That seems to imply that TX metadata only works for AF_XDP, but not for direct redirection. Is there a reason for that?

3. At least for the igc, the egress queue is currently selected by using the smp_processor_id.
(https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/net/ethernet/intel/igc/igc_main.c?h=v6.8-rc4#n2453)
For our application, I would like to define the queue on a per-packet basis via the eBPF.
This would allow to steer the traffic to the correct queue when using TAPRIO full hardware offload.
Do you see any problem with introducing a new metadata field to define the egress queue?

Thanks,
Florian