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

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Message-ID: <20231005101604.33b382d8@kernel.org>
Date: Thu, 5 Oct 2023 10:16:04 -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: [xdp-hints] Re: [RFC bpf-next v2 09/24] xdp: Add VLAN tag hint

On Thu, 5 Oct 2023 18:58:33 +0200 Alexander Lobakin wrote:
> > No unsharing - you can still strip it in the driver.  
> 
> Nobody manually strips VLAN tags in the drivers. You either have HW
> stripping or pass VLAN-tagged skb to the stack, so that skb_vlan_untag()
> takes care of it.

Isn't it just a case of circular logic tho?
We don't optimize the stack for SW stripping because HW does it.
Then HW does it because SW is not optimized.

> > Do you really think that for XDP kfunc call will be cheaper?  
> 
> Wait, you initially asked:
> 
> * discussion about the validity of VLAN stripping as an offload?
> * Do people actually care about having it enabled?
> 
> I did read this as "do we still need HW VLAN stripping in general?", not
> only for XDP. So I replied for "in general" -- yes.
> Forcefully disabling stripping when XDP is active is obscure IMO, let
> the user decide.

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 :(