ovs-dpdk: placing the metadata

[Zoltan Kiss <zoltan.kiss-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>]

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?

Regards,

Zoltan Kiss
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