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

[Yongseok Koh <yskoh@mellanox.com>]

On Fri, Jan 11, 2019 at 11:14:22AM +0300, Andrew Rybchenko wrote:
> 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.

Well, I don't like to put same comment here and there but I'll add small comment
here.

> 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.

This func is suggested by David and Olivier because same code is being repeated
in multiple locations. This can be used to initialize a mbuf when mb->buf_addr is
null. And second use-case (this is my use-case) is to get the buf_addr without
accessing the mbuf struct when mempool of mbuf is known, e.g. Rx buffer
replenishment. It is definitely beneficial for performance, especially RISC
cores.

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

Regarding naming, people have different tastes. As it is acked by Olivier and
David, I'll keep the names.

Thanks,
Yongseok

> > +{
> > +	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);
> >   }
> >   /**
>