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

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Date: Tue, 3 Oct 2023 15:09:39 +0200
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: Jakub Kicinski <kuba@...nel.org>, Larysa Zaremba
	<larysa.zaremba@...el.com>
CC: <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

From: Jakub Kicinski <kuba@...nel.org>
Date: Tue, 3 Oct 2023 05:35:19 -0700

> On Wed, 27 Sep 2023 09:51:09 +0200 Larysa Zaremba wrote:
>> Implement functionality that enables drivers to expose VLAN tag
>> to XDP code.
>>
>> VLAN tag is represented by 2 variables:
>> - protocol ID, which is passed to bpf code in BE
>> - VLAN TCI, in host byte order
> 
> 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.

Thanks,
Olek