Stephen Hemminger [off-list ref] writes:

On Sat, 27 Jun 2020 01:45:24 +0300
Petr Machata [off-list ref] wrote:

quoted

The Spectrum hardware allows execution of one of several actions as a
result of queue management decisions: tail-dropping, early-dropping,
marking a packet, or passing a configured latency threshold or buffer
size. Such packets can be mirrored, trapped, or sampled.

Modeling the action to be taken as simply a TC action is very attractive,
but it is not obvious where to put these actions. At least with ECN marking
one could imagine a tree of qdiscs and classifiers that effectively
accomplishes this task, albeit in an impractically complex manner. But
there is just no way to match on dropped-ness of a packet, let alone
dropped-ness due to a particular reason.

Would a BPF based hook be more flexible and reuse more existing
infrastructure?

This does reuse the existing infrastructure though: filters, actions,
shared blocks, qdiscs invoking blocks, none of that is new.

And BPF can still be invoked though classifier and / or action bpf. It
looks like you get the best of both worlds here: something symbolic for
those of us that use the filter infrastructure, and a well-defined hook
for those of us who like the BPF approach.