netdev - Re: [PATCH net-next] packet: Account for VLAN_HLEN in csum_start when virtio_net

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Date: Thu, 23 Nov 2023 19:19:52 -0500
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
 Mike Pattrick <mkp@...hat.com>, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: netdev@...r.kernel.org, 
 davem@...emloft.net, 
 edumazet@...gle.com, 
 kuba@...nel.org, 
 pabeni@...hat.com, 
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next] packet: Account for VLAN_HLEN in csum_start when
 virtio_net_hdr is enabled

Willem de Bruijn wrote:
> Mike Pattrick wrote:
> > On Thu, Nov 23, 2023 at 4:25 PM Willem de Bruijn
> > <willemdebruijn.kernel@...il.com> wrote:
> > >
> > > Mike Pattrick wrote:
> > > > Af_packet provides checksum offload offsets to usermode applications
> > > > 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 needs
> > > > 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 sending
> > 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.