On 4 Jun 2019, at 9:43, Jesper Dangaard Brouer wrote:

On Mon, 3 Jun 2019 09:38:51 -0700
Jonathan Lemon [off-list ref] wrote:

quoted

Currently, the AF_XDP code uses a separate map in order to
determine if an xsk is bound to a queue.  Instead of doing this,
have bpf_map_lookup_elem() return the queue_id, as a way of
indicating that there is a valid entry at the map index.

Just a reminder, that once we choose a return value, there the
queue_id, then it basically becomes UAPI, and we cannot change it.

Yes - Alexei initially wanted to return the sk_cookie instead, but
that's 64 bits and opens up a whole other can of worms.

Can we somehow use BTF to allow us to extend this later?

I was also going to point out that, you cannot return a direct pointer
to queue_id, as BPF-prog side can modify this... but Daniel already
pointed this out.

So, I see three solutions here (for this and Toke's patchset also,
which is encountering the same problem).

1) add a scratch register (Toke's approach)
2) add a PTR_TO_<type>, which has the access checked.  This is the most
   flexible approach, but does seem a bit overkill at the moment.
3) add another helper function, say, bpf_map_elem_present() which just
   returns a boolean value indicating whether there is a valid map entry
   or not.

I was starting to do 2), but wanted to get some more feedback first.
-- 
Jonathan