Re: [dpdk-dev] ovs-dpdk: placing the metadata

[Olivier MATZ <olivier.matz@6wind.com>]

Hi Zoltan,

On 03/24/2015 06:42 PM, Zoltan Kiss wrote:
> Hi,
>
> I've noticed in lib/netdev-dpdk.c that __rte_pktmbuf_init() stores the
> packet metadata right after "struct rte_mbuf", and before the buffer data:
>
>      /* start of buffer is just after mbuf structure */
>      m->buf_addr = (char *)m + sizeof(struct dp_packet);
>
> (struct dp_packet has the rte_mbuf as first member if DPDK enabled)
>
> However, lib/librte_mbuf/rte_mbuf.h seems to codify that the buffer
> should start right after the rte_mbuf:
>
> /**
>   * Given the buf_addr returns the pointer to corresponding mbuf.
>   */
> #define RTE_MBUF_FROM_BADDR(ba)     (((struct rte_mbuf *)(ba)) - 1)
>
> /**
>   * Given the pointer to mbuf returns an address where it's  buf_addr
>   * should point to.
>   */
> #define RTE_MBUF_TO_BADDR(mb)       (((struct rte_mbuf *)(mb)) + 1)
>
> These macros are used for attaching/detaching mbuf's to each other. This
> is the way the code retrieves the direct buffer from an indirect one,
> and vica versa. I think if we want to keep the metadata feature (which I
> guess is quite important), we need to add a pointer to rte_mbuf, which
> helps the direct and indirect structs to find each other. Something like:
>
>      struct rte_mbuf *attach;    /**< Points to the other buffer if this
> one
>                       is (in)direct. Otherwise NULL.  */
>
> What do you think?

I've just sent a patch that should fix this issue.
http://dpdk.org/ml/archives/dev/2015-March/015722.html

Let me know if you have any comment on it.

Regards,
Olivier