Re: [dpdk-dev] Packet Cloning

[Matt Laswell <laswell@infiniteio.com>]

Since Padam is going to be altering payload, he likely cannot use that API.
The rte_pktmbuf_clone() API doesn't make a copy of the payload.  Instead,
it gives you a second mbuf whose payload pointer points back to the
contents of the first (and also increments the reference counter on the
first so that it isn't actually freed until all clones are accounted for).
This is very fast, which is good.  However, since there's only really one
buffer full of payload, changes in the original also affect the clone and
vice versa.  This can have surprising and unpleasant side effects that may
not show up until you are under load, which is awesome*.

For what it's worth, if you need to be able to modify the copy while
leaving the original alone, I don't believe that there's a good solution
within DPDK.   However, writing your own API to copy rather than clone a
packet mbuf isn't difficult.

-- 
Matt Laswell
infinite io, inc.
laswell@infiniteio.com

* Don't ask me how I know how much awesome fun this can be, though I
suspect you can guess.

On Thu, May 28, 2015 at 9:52 AM, Stephen Hemminger <
stephen@networkplumber.org> wrote:

> On Thu, 28 May 2015 17:15:42 +0530
> Padam Jeet Singh <padam.singh@inventum.net> wrote:
>
> > Hello,
> >
> > Is there a function in DPDK to completely clone a pkt_mbuf including the
> segments?
> >
> > I am trying to build a packet mirroring application which sends packet
> out through two separate interfaces, but the packet payload needs to be
> altered before send.
> >
> > Thanks,
> > Padam
> >
> >
>
> Isn't this what you want?
>
> /**
>  * Creates a "clone" of the given packet mbuf.
>  *
>  * Walks through all segments of the given packet mbuf, and for each of
> them:
>  *  - Creates a new packet mbuf from the given pool.
>  *  - Attaches newly created mbuf to the segment.
>  * Then updates pkt_len and nb_segs of the "clone" packet mbuf to match
> values
>  * from the original packet mbuf.
>  *
>  * @param md
>  *   The packet mbuf to be cloned.
>  * @param mp
>  *   The mempool from which the "clone" mbufs are allocated.
>  * @return
>  *   - The pointer to the new "clone" mbuf on success.
>  *   - NULL if allocation fails.
>  */
> static inline struct rte_mbuf *rte_pktmbuf_clone(struct rte_mbuf *md,
>                 struct rte_mempool *mp)
>