[dpdk-dev] rte_mbuf: documentation, meta-data, and inconsistencies

daniel chapiesky dchapiesky2 at gmail.com
Fri Aug 29 02:00:59 CEST 2014

rte_muf: Inconsistency in the programmer's guide or the code.....?

>From the DPDK 1.7.0 programmer's guide we read:

    "For a newly allocated mbuf, the area at which the data begins in the
message buffer
    is RTE_PKTMBUF_HEADROOM bytes after the beginning of the buffer, which
is cache

In the file ./lib/librte_mbuf/rte_mbuf.c and function rte_pktmbuf_init() we

    m->pkt.data = (char*) m->buf_addr + RTE_MIN(RTE_PKTMBUF_HEADROOM,

Where RTE_PKTMBUF_HEADROOM is configured to be 128 bytes from the file


Question 1:

    Does the above invocation of RTE_MIN() cause the programmer's guide
    to be inaccurate?

    (Saying "in practice it does not matter..." is not an answer)

Question 2:

    Why is "packet metadata" implicitly sharing the same bytes of the mbuf
headroom area,
    instead of being explicitly kept from being overwritten by a call to

        rte_pktmbuf_prepend(pkt, 100);

    presuming my metadata is at least 32 bytes long????

    The above command would place the packet data area starting at the last
4 bytes of my metadata....

    (Saying "you can change CONFIG_RTE_PKTMBUF_HEADROOM at compile time" is
not an answer because it
     effects all mbufs not just the one I with my meta-data...)

Question 3:

    Why do we write:

        #define MBUF_SIZE (2048 + sizeof(struct rte_mbuf) +

        rte_mempool_create(s, 1024, MBUF_SIZE, 32,0,
                        rte_pktmbuf_pool_init, NULL,
                        rte_pktmbuf_init, NULL,
                        socketid, flags);

    instead of:

        #define MBUF_SIZE (2048 + sizeof(struct rte_mbuf))

        rte_mempool_create(s, 1024, MBUF_SIZE, 32,0,
                        rte_pktmbuf_pool_init, NULL,
                        rte_pktmbuf_init, NULL,
                        socketid, flags);

    and have rte_pktmbuf_init() enforce ( + RTE_PKTMBUF_HEADROOM )


My reason for asking the above questions stems from my work on a Packet

I came to understand that there really isn't an *explicit* declaration
of "allocate this much meta-data space for each packet".

The programmer's guide and mbuf.h describe of headroom and tailroom as a
place to edit
the packet data. An example would be to wrap the packet for tunnelling by
and appending the data area to be large enought to contain a new header and

In fact, while the programmer's guide mentions packet meta-data a few
times, there is no
section which actually describes ****how to make and access**** your very
own packet meta-data.
This addition would be very nice.

The tables in the Packet Framework Pipeline examples all use keys injected
into the meta-data
of the mbuf at RX time to compare a rule against that key (explicitly
stating the "offset
into packet meta-data") and not allowing "offset into packet data".

I actually like this setup because it allows the meta-data key to be
securely analyzed
and copied from the packet data - keeping malformed packets out of the
decision making
process of the tables.

But, in the end, sharing the meta-data area with the packet headroom seems
to be a very
bad idea.


Daniel Chapiesky

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