Re: [dpdk-dev] [RFC] mbuf: support dynamic fields and flags

["Wiles, Keith" <keith.wiles@intel.com>]

> On Jul 10, 2019, at 12:49 PM, Stephen Hemminger <stephen@networkplumber.org> wrote:
> 
> On Wed, 10 Jul 2019 11:29:07 +0200
> Olivier Matz <olivier.matz@6wind.com> wrote:
> 
>> /**
>>  * Indicate that the metadata field in the mbuf is in use.
>> @@ -738,6 +741,8 @@ struct rte_mbuf {
>> 	 */
>> 	struct rte_mbuf_ext_shared_info *shinfo;
>> 
>> +	uint64_t dynfield1; /**< Reserved for dynamic fields. */
>> +	uint64_t dynfield2; /**< Reserved for dynamic fields. */
>> } __rte_cache_aligned;
> 
> Growing mbuf is a fundamental ABI break and this needs
> higher level approval.  Why not one pointer?
> 
> It looks like you are creating something like FreeBSD m_tag.
> Why not use that?

Changing the mbuf structure causes a big problem for a number reasons as Stephen states.

If we leave the mbuf stucture alone and add this feature to the headroom space between the mbuf structure and the packet. When setting up the mempool/mbuf pool we define a headroom to hold the extra data when the mbuf pool is created or just use the current headroom space. Using this method we can eliminate the mbuf structure change and add the data to the packet buffer. We can do away with dynfield1 and 2 as we know where headroom space begins and ends. Just a thought.

Regards,
Keith