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

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Date: Mon, 7 Aug 2023 17:08:36 +0200
From: Larysa Zaremba <larysa.zaremba@...el.com>
To: Jakub Kicinski <kuba@...nel.org>
CC: Willem de Bruijn <willemdebruijn.kernel@...il.com>, Alexei Starovoitov
	<alexei.starovoitov@...il.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 Mon, Jul 31, 2023 at 09:43:22AM -0700, Jakub Kicinski wrote:
> 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.

Sorry for the delayed response.
Thanks a lot for this feedback, it became a gateway to deepen my understanding 
of checksumming in kernel pretty significantly.