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

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Message-ID: <e4bbe997-326f-b6cf-b6d6-f0a24f5aef39@intel.com>
Date: Thu, 5 Oct 2023 18:58:33 +0200
From: Alexander Lobakin <aleksander.lobakin@...el.com>
To: Jakub Kicinski <kuba@...nel.org>
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

From: Jakub Kicinski <kuba@...nel.org>
Date: Wed, 4 Oct 2023 11:08:50 -0700

> 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.

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.

> 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.

Thanks,
Olek