First, I think your question should be sent to the user mailing list, not
the dev mailing list.
> I have seen that each packet has a headroom memory space (128 bytes long)
> where RSS hashing and other metadata provided by the NIC is stored.
If I'm not mistaken, the headroom is not where metadata provided by the NIC
are stored. Those metadata are stored in the rte_mbuf struct, which is also
128 bytes long.
The headroom area is located AFTER the end of rte_mbuf (at offset 128). By
default the headroom area is also 128 byte long, so the actual packet data
is stored at offset 256.
You can store whatever you want in this headroom area. However those
information are lost as soon as the packet leaves DPDK (the NIC will start
sending at offset 256).