linux-kernel - Re: [PATCH v5] af_packet: Handle outgoing VLAN packets without hardware offloading

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Message-ID: <6df76928-be7f-483e-9685-88ee245ef1bf@orange.com>
Date: Wed, 5 Jun 2024 16:22:43 +0200
From: alexandre.ferrieux@...nge.com
To: Chengen Du <chengen.du@...onical.com>,
 Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
 pabeni@...hat.com, kaber@...sh.net, linux-kernel@...r.kernel.org,
 stable@...r.kernel.org
Subject: Re: [PATCH v5] af_packet: Handle outgoing VLAN packets without
 hardware offloading

On 05/06/2024 08:03, Chengen Du wrote:
> On Wed, Jun 5, 2024 at 6:57 AM Willem de Bruijn
> <willemdebruijn.kernel@...il.com> wrote:
> >
> > This adds some parsing overhead in the datapath. SOCK_RAW does not
> > need it, as it can see the whole VLAN tag. Perhaps limit the new
> > branches to SOCK_DGRAM cases? Then the above can also be simplified.
>
> I considered this approach before, but it would result in different
> metadata for SOCK_DGRAM and SOCK_RAW scenarios. This difference makes
> me hesitate because it might be better to provide consistent metadata
> to describe the same packet, regardless of the receiver's approach.
> These are just my thoughts and I'm open to further discussion.

FWIW, I vote for Willem's approach here: there is no problem with having 
different metadata in SOCK_DGRAM and SOCK_RAW, as the underlying parsing efforts 
are different anyway, along with the start offset for BPF.
(No, I'm not super happy to see BPF code reaching out to offset -4096 or so to 
get VLAN as metadata. That just smells like a horrendous kludge.)
To me, it makes plenty of sense to have:
  - SOCK_DGRAM for compatibility (used by everyone today), doing all historical 
shenanigans with VLANs and metadata
  - SOCK_RAW for a modern, new API, making no assumption on encapsulation, and 
presenting an untouched linear frame
  - yes this means different BPF code for the same filter between the two modes

Again, my .02c

-Alex

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