Re: [dpdk-dev] [PATCH v3 1/2] mbuf: add function returning default buffer address

[Andrew Rybchenko <arybchenko@solarflare.com>]

On 1/10/19 9:35 PM, Yongseok Koh wrote:
> This patch introduces two new functions - rte_mbuf_buf_addr() and
> rte_mbuf_data_addr_default().
>
> rte_mbuf_buf_addr() reutrns the default buffer address of given mbuf which
> comes after mbuf structure and private data.
>
> rte_mbuf_data_addr_default() returns the default address of mbuf data
> taking the headroom into account.
>
> Signed-off-by: Yongseok Koh <yskoh@mellanox.com>
> ---
>
> v3:
> * rename functions
>
> v2:
> * initial implementation
>
>   lib/librte_mbuf/rte_mbuf.h | 43 ++++++++++++++++++++++++++++++++++++++++---
>   1 file changed, 40 insertions(+), 3 deletions(-)
>
> diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
> index bc562dc8a9..486566fc28 100644
> --- a/lib/librte_mbuf/rte_mbuf.h
> +++ b/lib/librte_mbuf/rte_mbuf.h
> @@ -788,8 +788,47 @@ rte_mbuf_from_indirect(struct rte_mbuf *mi)
>   }
>   
>   /**
> + * Return the default buffer address of the mbuf.
> + *
> + * @param mb
> + *   The pointer to the mbuf.
> + * @param mp
> + *   The pointer to the mempool of the mbuf.
> + * @return
> + *   The pointer of the mbuf buffer.
> + */
> +static inline char * __rte_experimental
> +rte_mbuf_buf_addr(struct rte_mbuf *mb, struct rte_mempool *mp)

struct rte_mbuf has pool member. So, I don't understand why mp
argument is required. I guess there is a reason, but it should be
explained in comments. I see motivation in rte_mbuf_to_baddr()
description, but rte_mbuf_buf_add() does not explain it.

Also right now the function name looks like simple get accessor for
buf_addr and I'd expect to seem one line implementation may be
with extra debug checks: return mb->buf_addr.

May be rte_mbuf_direct_buf_addr() ?
If so, similar below rte_mbuf_direct_data_addr_default().

> +{
> +	char *buffer_addr;
> +
> +	buffer_addr = (char *)mb + sizeof(*mb) + rte_pktmbuf_priv_size(mp);
> +	return buffer_addr;
> +}
> +
> +
> +/**
> + * Return the default address of the beginning of the mbuf data.
> + *
> + * @param mb
> + *   The pointer to the mbuf.
> + * @return
> + *   The pointer of the beginning of the mbuf data.
> + */
> +static inline char * __rte_experimental
> +rte_mbuf_data_addr_default(struct rte_mbuf *mb)
> +{
> +	return rte_mbuf_buf_addr(mb, mb->pool) + RTE_PKTMBUF_HEADROOM;
> +}
> +
> +/**
>    * Return the buffer address embedded in the given mbuf.
>    *
> + * Note that accessing mempool pointer of a mbuf is expensive because the
> + * pointer is stored in the 2nd cache line of mbuf. If mempool is known, it
> + * is better not to reference the mempool pointer in mbuf but calling
> + * rte_mbuf_buf_addr() would be more efficient.
> + *
>    * @param md
>    *   The pointer to the mbuf.
>    * @return
> @@ -798,9 +837,7 @@ rte_mbuf_from_indirect(struct rte_mbuf *mi)
>   static inline char *
>   rte_mbuf_to_baddr(struct rte_mbuf *md)
>   {
> -	char *buffer_addr;
> -	buffer_addr = (char *)md + sizeof(*md) + rte_pktmbuf_priv_size(md->pool);
> -	return buffer_addr;
> +	return rte_mbuf_buf_addr(md, md->pool);
>   }
>   
>   /**