Re: [dpdk-dev] [PATCH 2/2] doc: announce new mbuf field for LRO

[Matan Azrad <matan@mellanox.com>]

Hi Olivier

From: Olivier Matz:
> Hi,
> 
> On Tue, Jun 02, 2020 at 06:49:01AM +0000, Matan Azrad wrote:
> > Hi
> >
> > From: Thomas Monjalon
> > > 10/08/2019 23:31, Thomas Monjalon:
> > > > 06/08/2019 20:17, Andrew Rybchenko:
> > > > > On 8/6/19 5:56 PM, Matan Azrad wrote:
> > > > > > The API breakage is because the ``tso_segsz`` field was
> > > > > > documented for LRO.
> > > > > >
> > > > > > The ``tso_segsz`` field in mbuf indicates the size of each
> > > > > > segment in the LRO packet in Rx path and should be provided by
> > > > > > the LRO packet port.
> > > > > >
> > > > > > While the generic LRO packet may aggregate different segments
> > > > > > sizes in one packet, it is impossible to expose this
> > > > > > information for each segment by one field and it doesn't make
> > > > > > sense to expose all the segments sizes in the mbuf.
> > > > > >
> > > > > > A new field may be added as union with the above field to
> > > > > > expose the number of segments aggregated in the LRO packet.
> > > > > >
> > > > > > Signed-off-by: Matan Azrad <matan@mellanox.com>
> > > > > > ---
> > > > > > --- a/doc/guides/rel_notes/deprecation.rst
> > > > > > +++ b/doc/guides/rel_notes/deprecation.rst
> > > > > > +* mbuf: Remove ``tso_segsz`` mbuf field providing for LRO
> > > > > > +support. Use union
> > > > > > +  block for the field memory to be shared with a new field
> > > > > > +``lro_segs_n``
> > > > > > +  indicates the number of segments aggregated in the LRO packet.
> > > > >
> > > > > I think that the number of segments is more logical in the case of LRO.
> > > > > The question (already asked by Konstantin) is why it is needed
> > > > > at all (statistics?). If so, it still makes sense.
> > > > >
> > > > > Segment size is misleading here, since not all segments could be
> > > > > the same size. So,
> > > > >
> > > > > Acked-by: Andrew Rybchenko <arybchenko@solarflare.com>
> > > > >
> > > > > As far as I can see bnxt and qede do not fill it in.
> > > > > mlx5 and vmxnet3 have the number of segments (vmxnet3 has
> > > > > segment size sometimes and sometimes use a function to guess the
> value).
> > > > > So both will win from the change.
> > > > > It looks like virtio does not have number of segments. CC Maxime
> > > > > to comment.
> > > >
> > > > I support improving the API for LRO.
> > > > Unfortunately, the consensus is not strong enough at the moment.
> > >
> > > We had no progress about LRO field in mbuf.
> > > Is it a change we would like to have in 20.11?
> > >
> > +1 to make the change.
> 
> Thinking about this, having the segment size for LRO is useful: it is expected
> to give the size of the segments, except the last one. The advantage of this is
> to be able to resegment exactly at the same MSS on Tx (so it does not break
> PMTU). This is needed if you want to do a bridge or a router with LRO
> enabled.

Yes, you right it may be useful.

I don't familiar with other vendors, but mlx5 HWs supply the number of segments aggregated by the HW and doesn't assume all the head segments are in the same size.

So, we just put in the current field packet_size/num of segments.

So, maybe we need the 2 options as uinion or as 2 separated fields to be more accurate.

> What is described above is more GRO than LRO. But today, it is possible to do
> this with the virtio driver, so removing the segment size would break this use
> case.
> 
> Regards,
> Olivier