netdev - Re: [PATCH bpf-next v4 12/21] xdp: Add checksum hint

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Date: Mon, 31 Jul 2023 09:43:22 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>, Larysa Zaremba
 <larysa.zaremba@...el.com>, bpf <bpf@...r.kernel.org>, Alexei Starovoitov
 <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko
 <andrii@...nel.org>, Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu
 <song@...nel.org>, Yonghong Song <yhs@...com>, John Fastabend
 <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>, Stanislav
 Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>, Jiri Olsa
 <jolsa@...nel.org>, David Ahern <dsahern@...il.com>, Willem de Bruijn
 <willemb@...gle.com>, Jesper Dangaard Brouer <brouer@...hat.com>, Anatoly
 Burakov <anatoly.burakov@...el.com>, Alexander Lobakin
 <alexandr.lobakin@...el.com>, Magnus Karlsson <magnus.karlsson@...il.com>,
 Maryam Tahhan <mtahhan@...hat.com>, xdp-hints@...-project.net, Network
 Development <netdev@...r.kernel.org>, Simon Horman
 <simon.horman@...igine.com>
Subject: Re: [PATCH bpf-next v4 12/21] xdp: Add checksum hint

On Sun, 30 Jul 2023 09:13:02 -0400 Willem de Bruijn wrote:
> > > This levels business is an unfortunate side effect of
> > > CHECKSUM_UNNECESSARY. For a packet with multiple checksum fields, what
> > > does the boolean actually mean? With these levels, at least that is
> > > well defined: the first N checksum fields.  
> >
> > If I understand this correctly this is intel specific feature that
> > other NICs don't have. skb layer also doesn't have such concept.
> > The driver should say CHECKSUM_UNNECESSARY when it's sure
> > or don't pretend that it checks the checksum and just say NONE.  
> 
> I did not know how much this was used, but quick grep for non constant
> csum_level shows devices from at least six vendors.

I thought it was a legacy thing from early VxLAN days.
We used to leave outer UDP csum as 0 before LCO, and therefore couldn't
convert outer to COMPLETE, so inner could not be offloaded/validated.
Should not be all that relevant today.