Re: [PATCH bpf-next v5 1/3] bpf: Add BPF_FIB_LOOKUP_VLAN flag to bpf_fib_lookup() helper

["Toke Høiland-Jørgensen" <toke@redhat.com>]

David Ahern <dsahern@kernel.org> writes:

> On 6/30/26 4:00 AM, Toke Høiland-Jørgensen wrote:
>>> It does not make sense to require a flag to get lookup output. vlan
>>> proto of 0 is not valid, so it is a clear indication that the vlan
>>> output parameters were not set during the lookup.
>> 
>> Okay, so we could just unconditionally set the VLAN fields, but if we
>> start rewriting the ifindex that would be a change of the existing
>> behaviour that could break existing applications, no?
>
> Consistently dealing with upper devices is one of the reasons I never
> sent patches for vlan support.
>
> xdp support is at the driver layer for real (physical) devices. The fib
> lookup is going to return the vlan device index - a virtual device.
> Support for xdp should not be propagated to virtual devices; it goes
> against the intent of xdp. Any trip down this path will have to decide
> how to handle vlan-in-vlan use cases. Where is the line drawn for fast
> networking?

Right, which is why we need building blocks that makes it possible for
XDP programs to do the right thing in the BPF code :)

A helper that resolves the parent could be used for stacked VLANs as
well (just calling the helper multiple times).

>> Specifically, if an XDP application has a table of the interfaces it
>> forwards between, today they'd get a VLAN interface ifindex, which would
>> not be in that table, and the application would return XDP_PASS. Whereas
>> if we change the ifindex and populate the VLAN tag, suddenly the
>> interface would be in the table, but because the application doesn't
>> read the returned VLAN tag, it will end up sending packets out without
>> tagging them, leading to broken forwarding.
>
> I have not followed developments over the past few years. Does XDP have
> support for vlan acceleration in the Tx path now? You really want that
> to deal with vlans and not replicating s/w processing in ebpf.

It does not, no. There's TX metadata for AF_XDP, but VLAN support is not
in there (see include/uapi/linux/if_xdp.h).

Doesn't mean software VLAN handling can't be useful, though; there are
use cases other than the very high end where XDP can speed things up
even if it has to write a VLAN tag or two...

>> So if we don't want the flag, we'd need some other mechanism to resolve
>> the parent ifindex, AFAICT? Maybe a xdp_get_parent_ifindex() kfunc, say?
>> That could also be made generic for other stacked interface types, I
>> suppose.
>> 
>> WDYT?
>
> dealing with stacked devices is hard :-)
>
> What is the return is a bond device or a vlan on a bond device?

Well, bond devices have XDP support, so you can just redirect to those :)

But yeah, each type of stacked device would need to pass different
information through to the XDP program, and the program would need to
support those. Building a single XDP program that supports all of them
will require quite a bit of code, and would probably not perform super
well. But most deployments have distinct subsets of features they need,
so this does not have to be a blocker, IMO?

-Toke