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

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Date: Wed, 4 Oct 2023 11:08:50 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Alexander Lobakin <aleksander.lobakin@...el.com>
Cc: 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>, 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>, <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: [RFC bpf-next v2 09/24] xdp: Add VLAN tag hint

On Tue, 3 Oct 2023 15:09:39 +0200 Alexander Lobakin wrote:
> > Sorry for a random chime-in but was there any discussion about 
> > the validity of VLAN stripping as an offload?
> > 
> > I always thought this is a legacy "Windows" thing which allowed
> > Windows drivers to operate on VLAN-tagged networks even before
> > the OS itself understood VLANs...  Do people actually care about
> > having it enabled?  
> 
> On MIPS routers, I actually have some perf gains from having it enabled.
> So they do, I'd say. Mediatek even has DSA tag stripping. Both save you
> some skb->data push-pulls, csum corrections when CHECKSUM_COMPLETE, skb
> unsharing in some cases, reduce L3/L4 headers cacheline spanning etc.

No unsharing - you can still strip it in the driver.
Do you really think that for XDP kfunc call will be cheaper?