Re: [dpdk-dev] [PATCH v2] mbuf: support dynamic fields and flags

[Olivier Matz <olivier.matz@6wind.com>]

Hi Shahaf,

On Wed, Oct 23, 2019 at 12:00:30PM +0000, Shahaf Shuler wrote:
> Hi Olivier, 
> 
> Thursday, October 17, 2019 5:42 PM, Olivier Matz:
> > Subject: [dpdk-dev] [PATCH v2] mbuf: support dynamic fields and flags
> > 
> > Many features require to store data inside the mbuf. As the room in mbuf
> > structure is limited, it is not possible to have a field for each feature. Also,
> > changing fields in the mbuf structure can break the API or ABI.
> > 
> > This commit addresses these issues, by enabling the dynamic registration of
> > fields or flags:
> > 
> > - a dynamic field is a named area in the rte_mbuf structure, with a
> >   given size (>= 1 byte) and alignment constraint.
> > - a dynamic flag is a named bit in the rte_mbuf structure.
> > 
> > The typical use case is a PMD that registers space for an offload feature,
> > when the application requests to enable this feature.  As the space in mbuf is
> > limited, the space should only be reserved if it is going to be used (i.e when
> > the application explicitly asks for it).
> 
> According to description, the dynamic field enables custom application and supported PMDs to use the dynamic part of the mbuf for their specific needs. 
> However the mechanism to report and activate the field/flag registration comes from the general OFFLOAD flags. 
> 
> Maybe it will be better to an option to query and select dynamic fields for PMD outside of the standard ethdev offload flags? 

It is not mandatory to use the ethdev layer to register a dynamic field
or flag in the mbuf. It is just the typical use case.

It can also be enabled when using a library that have specific needs,
for instance, you call rte_reorder_init(), and it will register the
sequence number dynamic field.

An application that requires a specific mbuf field can also do the
registration by itself.

In other words, when you initialize a subpart that needs a dynamic field
or flag, you have to do the registration there.



> 
> > 
> > The registration can be done at any moment, but it is not possible to
> > unregister fields or flags for now.
> > 
> > Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
> > Acked-by: Thomas Monjalon <thomas@monjalon.net>