Re: [RFC 0/8] mbuf: structure reorganization

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

Hi everyone,

> >
> > In my opinion, if we have the room in the first cache line, we should
> > put it there. The only argument I see against is "we may find something
> > more important in the future, and we won't have room for it in the
> > first cache line". I don't feel we should penalize today's use cases for
> > hypothetic future use cases.
> >
> >
> >
> >> 2. timestamp normalization point
> >>      inside PMD RX vs somewhere later as user needs it (extra
> >> function in dev_ops?).
> >
> > This point could be changed. My initial proposition tries to provide a
> > generic API for timestamp. Let me remind it here:
> >
> > a- the timestamp is in nanosecond
> > b- the reference is always the same for a given path: if the timestamp
> >    is set in a PMD, all the packets for this PMD will have the same
> >    reference, but for 2 different PMDs (or a sw lib), the reference
> >    would not be the same.
> >
> > We may remove a-, and just have:
> >  - the reference and the unit are always the same for a given path: if
> >    the timestamp is set in a PMD, all the packets for this PMD will have
> >    the same reference and unit, but for 2 different PMDs (or a sw lib),
> >    they would not be the same.
> >
> > In both cases, we would need a conversion code (maybe in a library) if
> > the application wants to work with timestamps from several sources. The
> > second solution removes the normalization code in the PMD when not
> > needed, it is probably better.
> 
> I agree.

One question - does that mean that application would need to
keep a track from what PMD each particular packet came to do the normalization?
Konstantin

> 
> >
> > About having the timestamp in the packet data, I don't think it is
> > a good solution for a generic API in DPDK. The timestamp is a metadata,
> > it has to go in the mbuf metadata. The packet data should not be
> > modified when the timestamp is enabled.
> 
> Good NICs already do that based on the packet type (e.g. NTP/PTP packets).
> 
> >
> > But this would not prevent to have driver-specific features to do that.
> > In that case, the application will be aware that it is using this
> > specific driver and that it will receive a timestamp in the packet data.
> >
> > To summarize, the generic API could be:
> > - an ethdev API to enable the timestamp in a PMD for received packets
> > - a mbuf flag "timestamp present"
> > - a mbuf 64b field to store the timestamp value
> >
> > Additionally, a driver-specific API can be added for a given PMD.
> > Example:
> > - the generic timestamp ethdev is disabled (or not supported)
> > - a driver-specific feature "put timestamp in packet" is enabled
> > It would have no additional cost compared to what we have today, since
> > the timestamp in mbuf is not read/written.
> >
> 
> Thanks for the writeup. This sounds like a reasonable approach to me.
> 
> Do you still want to call the 64bit field "timestamp" or rename it to
> something neutral and document that it is used together with the mbuf
> flags?