[Bug 1117] af-packet driver fix breaks rte_pktmbuf_prepend()

[bugzilla at dpdk.org]

https://bugs.dpdk.org/show_bug.cgi?id=1117

            Bug ID: 1117
           Summary: af-packet driver fix breaks rte_pktmbuf_prepend()
           Product: DPDK
           Version: 21.11
          Hardware: All
                OS: All
            Status: UNCONFIRMED
          Severity: normal
          Priority: Normal
         Component: ethdev
          Assignee: dev at dpdk.org
          Reporter: bly454 at gmail.com
  Target Milestone: ---

The following commit causes rte_pktmbuf_prepend() to fail when prepending the
full headroom reserved when creating the mbuf pool.

https://git.dpdk.org/dpdk-stable/commit/?h=21.11&id=d41d39bcf76900deb6aa000e929fd65734254201

Of issue is if the received frame was originally tagged (and vlan-stripped by
af-socket connection) the above commit does a vlan reinsert action as part of
eth_af_packet_rx(). This moves the SOF forward into the "reserved" space by
four bytes. For example, if the reserved space was 128 bytes and the next stage
of processing is a generic pipeline handling a variety of interface types, the
generic prepend of the reserved 128-byte meta-data space will fail. This means
you cannot use a common pipeline and have to have an exception now for
af-packet rx handling as compared to physical NIC, tap, vhost, etc. Ideally,
the initial SOF offset should be consistent relative to your design's reserved
headroom, regardless of the vlan tagged status of the frame or the interface
type it was received on.

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