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

[Andrew Rybchenko <arybchenko@solarflare.com>]

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.

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);
>>>    }
>>>    /**