prepend mbuf to another mbuf

[Lombardo, Ed]

Hi,
I have an issue with retransmitting a received packet with encapsulation headers prepended to original received packet when using E810 NIC for transmit.

I have received a packet and is stored in one or more mbufs.  To do encapsulation I am acquiring a mbuf from free pool.  The new mbuf is where I add the L2 Header, IPv4 header and GRE header.  I update the mbuf with rte_mbuf_refcnt_set(new_mbuf, 1) and rte_mbuf_refcnt_update(mbuf, 1); and then fill in the new mbuf metadata like (nb_segs, pkt_len, port, mbuf->next, etc) from the original mbuf.

When I test this feature in VMWare with VMXNET3 vnic it works perfectly, the packet is transmitted with the encapsulation headers ahead of the original packet seen at the endpoint.

When I test same on Intel E810 only the first mbuf of data is transmitted, the original packet data from remaining mbufs is not transmitted.

I compared the mbufs just prior to transmit, byte by byte, in the VMXNET3 and E810 NIC cases and they are identical, the code path is the same.
I also tried dpdk 17.11 and dpdk 22.11 versions with same results.
Also same test fails with Intel X710 and X540 NICs similar to way E810 fails.

I modified the code to insert the encapsulation headers in the headroom of the original mbuf and it worked perfectly.

What could be the issue with the Intel NICs when transmitting a chain of mbufs, where the first mbuf has only the L2 header, IPv4 header and GRE header and remaining mbuf(s) contain the original packet data?

Thanks,
Ed
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