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

[Andrew Rybchenko <arybchenko@solarflare.com>]

On 10.07.2019 12:29, Olivier Matz wrote:
> 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>

I like the idea.

I think it would be very useful to measure performance impact. Since it is
core structure which is heavily used on datapath, performance impact is
required to make decision to go or not to go. If acceptable, more fields
can be converted to dynamic: timestamp, user data, sequence number,
timesync data etc. Rules on which fields should be static and which
dynamic are required. Timestamp, for example, is located in the first
cache line. Do we need a way prioritize some dynamic fields to be located
(if possible) in the first cache line? Or is it better simply move some 
static
to the first cache line instead?

I think rules should be better defined and imposed, if possible, when
dynamic fields may be registered. Which entities are allowed to register
dynamic fields? Do we need to keep track which entity has registered
which dynamic fields? What to expect if a dynamic field is registered
after port start (the field is registered, but most likely not filled in)?
What to expect on port restart?