[xdp-hints] XDP-hints via local BTF info

[Jesper Dangaard Brouer <jbrouer@redhat.com>]

Hi Ederson and others,

I've coded up an XDP + AF_XDP application[1] that creates and shares 
XDP-hints via BTF from XDP-prog to AF_XDP.

As explained in README[2] file, API for decoding the BTF data-structures 
in userspace have been placed into separate files lib_xsk_extend.c and 
lib_xsk_extend.h.
The API is based on what Ederson proposed[3], but I have heavily 
modified the API while using it in the AF_XDP app.  I've kept the 
hashmap optimization for now, but ended-up not using it... so I will 
propose removing it(?).
My hope is we can discuss and mature the API together, before proposing 
the API to be included in either libbpf or libxdp.

Notice no kernel changes required.  Everything works today with what BTF 
info are provided in the BPF ELF-object file.
Later, we want to extend the NIC drivers with the same kind of BTF info 
that describe the metadata area (our XDP-hints).  For now, AF_XDP 
userspace will instead get the BTF-info from decoding the ELF-object 
file.  Thus, for now the XDP BPF-prog will be the creator/provider of 
the XDP-hints layout.

The requirement for being a valid XDP-hints data-struct is that the last 
member in the struct is named "btf_id" and have size 4 bytes (32-bit).
With BTF the struct "string" names and member names becomes important, 
and is something that userspace will be "searching" for, to establish if 
a driver/BPF-prog provide a specific XDP-hints data structure.
There is no restriction on what the XDP-hints struct should be named, 
but we might want some conversions when added this to the kernel drivers.

Open to feedback,
-Jesper


  [1] 
https://github.com/xdp-project/bpf-examples/tree/master/AF_XDP-interaction

  [2] 
https://github.com/xdp-project/bpf-examples/blob/master/AF_XDP-interaction/README.org#xdp-hints-via-local-btf-info

  [3] 
https://lore.kernel.org/bpf/20210803010331.39453-15-ederson.desouza@intel.com/