Forgot to answer this part...

quoted

It would probably require a bit of refactoring in the kernel data
structures so they can be used without being tied to an skb. David Ahern
did something similar for the fib. For the routing table case, that
resulted in a significant speedup: About 2.5x-3x the performance when
using it via XDP (depending on the number of routes in the table).

I'm curious about how much the helper function can improve the
performance compared to XDP programs which emulates kernel feature
without using such helpers. 2.5x-3x sounds a bit slow as XDP to me,
but it can be routing specific problem.

That's specific to routing; the numbers we got were roughly consistent
with the routing table lookup performance reported here:
https://vincent.bernat.ch/en/blog/2017-ipv4-route-lookup-linux

I.e., a fib lookup takes something on the order of 30-50 ns, which
eats up quite a bit of the time budget for forwarding...

-Toke