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

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

Hi,

On Wed, Jul 10, 2019 at 05:14:33PM +0000, Wang, Haiyue wrote:
> Hi,
> 
> Sounds cool, just have some questions inline.
> 
> > -----Original Message-----
> > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Olivier Matz
> > Sent: Wednesday, July 10, 2019 17:29
> > To: dev@dpdk.org
> > Subject: [dpdk-dev] [RFC] 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).
> > 
> > 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>

(...)

> > +/**
> > + * @file
> > + * RTE Mbuf 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 module addresses this issue, 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).
> > + *
> > + * The registration can be done at any moment, but it is not possible
> > + * to unregister fields or flags for now.
> > + *
> > + * Example of use:
> > + *
> > + * - RTE_MBUF_DYN_<feature>_(ID|SIZE|ALIGN) are defined in this file
> 
> Does it means that all PMDs define their own 'RTE_MBUF_DYN_<feature>_(ID|SIZE|ALIGN)'
> here ? In other words, each PMD can expose its private DYN_<feature> here for public
> using ?

For generic fields, I think they should be declared in this file. For
instance, if we decide to replace the current m->timestamp field by a
dynamic field, we should add like this:

#define RTE_MBUF_DYN_TIMESTAMP_ID "rte_timestamp"
#define RTE_MBUF_DYN_TIMESTAMP_SIZE sizeof(uint64_t)
#define RTE_MBUF_DYN_TIMESTAMP_ALIGN __alignof__(uint64_t)

If the feature is PMD-specific, the defines could be exposed in a
PMD header.

> How about adding another eth_dev_ops API definitions to show the PMD's supporting feature
> names, sizes, align in run time for testpmd ? And also another eth_dev_ops API for showing
> the data saved in rte_mbuf by 'dump_pkt_burst' ? Adding a new command for testpmd to set
> the dynamic feature may be good for PMD test.
> 
> > + * - If the application asks for the feature, the PMD use
> 
> How does the application ask for the feature ? By ' rte_mbuf_dynfield_register()' ?

No change in this area. If we take again the timestamp example, the
feature is asked by the application through the ethdev layer by passing
DEV_RX_OFFLOAD_TIMESTAMP to port or queue configuration.

> 
> > + *   rte_mbuf_dynfield_register() to get the dynamic offset and stores
> > + *   in a global variable.
> 
> In case, the PMD calls 'rte_mbuf_dynfield_register()' for 'dyn_feature' firstly, this
> means that PMD requests the dynamic feature itself if I understand correctly. Should
> PMD calls 'rte_mbuf_dynfield_lookup' for 'dyn_feature' to query the name exists, the
> size and align are right as expected ? If exists, but size and align are not right, may
> be for PMD change its definition, then PMD can give a warning or error message. If name
> exists, both size and align are expected, then PMD think that the application request
> the right dynamic features.

The PMD should only call rte_mbuf_dynfield_register() if the application
requests the feature (through ethdev, or through another mean if it's a
PMD-specific feature). The goal is to only reserve the area in the mbuf
for features that are actually needed.

Hope this is clearer now. I think I need to enhance the documentation in
next version ;)

Thanks for the feedback.