netdev - Re: [xdp-hints] Re: [RFC bpf-next v2 09/24] xdp: Add VLAN tag hint

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Message-ID: <20231005110635.7020d23b@kernel.org>
Date: Thu, 5 Oct 2023 11:06:35 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: David Ahern <dsahern@...il.com>
Cc: Alexander Lobakin <aleksander.lobakin@...el.com>, Larysa Zaremba
 <larysa.zaremba@...el.com>, bpf@...r.kernel.org, ast@...nel.org,
 daniel@...earbox.net, andrii@...nel.org, martin.lau@...ux.dev,
 song@...nel.org, yhs@...com, john.fastabend@...il.com, kpsingh@...nel.org,
 sdf@...gle.com, haoluo@...gle.com, jolsa@...nel.org, 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,
 netdev@...r.kernel.org, Willem de Bruijn <willemdebruijn.kernel@...il.com>,
 Alexei Starovoitov <alexei.starovoitov@...il.com>, Simon Horman
 <simon.horman@...igine.com>, Tariq Toukan <tariqt@...lanox.com>, Saeed
 Mahameed <saeedm@...lanox.com>, Maciej Fijalkowski
 <maciej.fijalkowski@...el.com>
Subject: Re: [xdp-hints] Re: [RFC bpf-next v2 09/24] xdp: Add VLAN tag hint

On Thu, 5 Oct 2023 11:20:49 -0600 David Ahern wrote:
> > Every time I'm involved in conversations about NIC datapath host
> > interfaces I cringe at this stupid VLAN offload. Maybe I'm too
> > daft to understand it's amazing value but we just shift 2B from
> > the packet to the descriptor and then we have to worry about all
> > the corner cases that come from vlan stacking :(  
> 
> 4B (vlan tci + protocol).
> 
> VLAN stripping in S/W and pushing the header on Tx is measurable and
> does have a noticeable performance impact.
> 
> XDP programs need to co-exist with enabled offloads. If the tag is not
> stripped, XDP program needs to handle it. If the tag is stripped, the
> XDP program needs to access to the value.

Well, I thought I'd ask :) I'm not opposed.

But if either of you have the data on how much slower well-implemented
Rx stripping in the driver is than putting the info in the descriptor,
I'd be very interested.

Tx is a different situation.