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Date:   Thu, 23 Nov 2023 19:19:52 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@gmail.com>
To:     Willem de Bruijn <willemdebruijn.kernel@gmail.com>,
        Mike Pattrick <mkp@redhat.com>,
        Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Cc:     netdev@vger.kernel.org, davem@davemloft.net, edumazet@google.com,
        kuba@kernel.org, pabeni@redhat.com, linux-kernel@vger.kernel.org
Message-ID: <655fec28a1c0f_db39d294dc@willemb.c.googlers.com.notmuch>
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References: <20231123183835.635210-1-mkp@redhat.com>
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Subject: Re: [PATCH net-next] packet: Account for VLAN_HLEN in csum_start when
 virtio_net_hdr is enabled
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Willem de Bruijn wrote:
> Mike Pattrick wrote:
> > On Thu, Nov 23, 2023 at 4:25=E2=80=AFPM Willem de Bruijn
> > <willemdebruijn.kernel@gmail.com> wrote:
> > >
> > > Mike Pattrick wrote:
> > > > Af_packet provides checksum offload offsets to usermode applicati=
ons
> > > > through struct virtio_net_hdr when PACKET_VNET_HDR is enabled on =
the
> > > > socket. For skbuffs with a vlan being sent to a SOCK_RAW socket,
> > > > af_packet will include the link level header and so csum_start ne=
eds
> > > > to be adjusted accordingly.
> > >
> > > Is this patch based on observing an incorrect offset in a workload,=

> > > or on code inspection?
> > =

> > Based on an incorrect offset in a workload. The setup involved sendin=
g
> > vxlan traffic though a veth interface configured with a vlan. The
> > vnet_hdr's csum_start value was off by 4, and this problem went away
> > when the vlan was removed.
> > =

> > I'll take another look at this patch.
> =

> This is a vlan device on top of a veth device? On which device and at
> which point (ingress or egress) are you receiving the packet over the
> packet socket?
> =

> From a quick glance, in all cases that I see the VLAN tag is kept in
> skb->vlan_tci, so is never part of the packet payload.
> =

> But checksum offload with VXLAN can be non-trivial on its own. If
> type & SKB_GSO_UDP_TUNNEL_CSUM | SKB_GSO_TUNNEL_REMCSUM, say. Then
> csum_start will point to the checksum in vxlanhdr.

This last statement is incorrect. But SKB_GSO_UDP_TUNNEL_CSUM might
be relevant to unexpected csum_start.