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

[Bruce Richardson <bruce.richardson@intel.com>]

On Fri, Jan 11, 2019 at 02:17:04PM +0300, Andrew Rybchenko wrote:
> Olivier, David,
> 
> could you take a look at naming suggested below and share your thoughts.
> My fear is that rte_mbuf_buf_addr() is too generic and true for direct mbuf
> only. That's why I'd like to highlight it in the function name.
> 

I would tend to agree with that concern.

/Bruce

> Thanks,
> Andrew.
> 
> On 1/11/19 2:03 PM, Yongseok Koh wrote:
> > 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);
> > > >    }
> > > >    /**
>