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Date: Wed, 24 May 2023 08:22:16 -0700
From: Jakub Kicinski <kuba@kernel.org>
To: Simon Horman <simon.horman@corigine.com>
Cc: Paolo Abeni <pabeni@redhat.com>, Louis Peens <louis.peens@corigine.com>,
 David Miller <davem@davemloft.net>, netdev@vger.kernel.org,
 oss-drivers@corigine.com, Willem de Bruijn <willemb@google.com>
Subject: Re: [PATCH net-next] nfp: add L4 RSS hashing on UDP traffic
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On Wed, 24 May 2023 13:30:06 +0200 Simon Horman wrote:
> On Tue, May 23, 2023 at 02:20:05PM -0700, Jakub Kicinski wrote:
> > Yup, that's the exact reason it was disabled by default, FWIW.
> > 
> > The Microsoft spec is not crystal clear on how to handles this:
> > https://learn.microsoft.com/en-us/windows-hardware/drivers/network/rss-hashing-types#ndis_hash_ipv4
> > There is a note saying:
> > 
> >   If a NIC receives a packet that has both IP and TCP headers,
> >   NDIS_HASH_TCP_IPV4 should not always be used. In the case of a
> >   fragmented IP packet, NDIS_HASH_IPV4 must be used. This includes
> >   the first fragment which contains both IP and TCP headers.
> > 
> > While NDIS_HASH_UDP_IPV4 makes no such distinction and talks only about
> > "presence" of the header.
> > 
> > Maybe we should document that device is expected not to use the UDP
> > header if MF is set?  
> 
> Yes, maybe.
> 
> Could you suggest where such documentation should go?

That's the hardest question, perhaps :)

Documentation/networking/scaling.rst and/or OCP NIC spec:

https://ocp-all.groups.io/g/OCP-Networking/topic/nic_software_core_offloads/98930671?p=,,,20,0,0,0::recentpostdate/sticky,,,20,2,0,98930671,previd%3D1684255676674808204,nextid%3D1676673801962532335&previd=1684255676674808204&nextid=1676673801962532335