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

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Message-ID: <0be2e89e-8a08-e52c-fecd-3064262c2ecb@gmail.com>
Date: Thu, 5 Oct 2023 11:20:49 -0600
From: David Ahern <dsahern@...il.com>
To: Jakub Kicinski <kuba@...nel.org>,
 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,
 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 10/5/23 11:16 AM, Jakub Kicinski wrote:
> 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 :(

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.