netdev - Re: [PATCH net 1/2] net/af_packet: adjust network header position for VLAN tagged packets

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Date:   Wed, 20 Apr 2022 08:59:58 +0800
From:   Hangbin Liu <liuhangbin@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Maxim Mikityanskiy <maximmi@...lanox.com>,
        Mike Pattrick <mpattric@...hat.com>, netdev@...r.kernel.org,
        Eric Dumazet <edumazet@...gle.com>,
        virtualization@...ts.linux-foundation.org,
        Balazs Nemeth <bnemeth@...hat.com>,
        Flavio Leitner <fbl@...hat.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jason Wang <jasowang@...hat.com>
Subject: Re: [PATCH net 1/2] net/af_packet: adjust network header position
 for VLAN tagged packets

On Tue, Apr 19, 2022 at 10:26:09AM -0400, Michael S. Tsirkin wrote:
> > > There are also some duplicated codes in these *_snd functions.
> > > I think we can move them out to one single function.
> > 
> > Please don't refactor this code. It will complicate future backports
> > of stable fixes.
> 
> Hmm I don't know offhand which duplication this refers to specifically
> so maybe it's not worth addressing specifically but generally not
> cleaning up code just because of backports seems wrong ...

Yes, packet_snd() and tpacket_snd() share same addr/msg checking logic that
I think we can clean up.

> > > > stretching the definition of the flags to include VLAN is acceptable
> > > > (unlike outright tunnels), but even then I would suggest for net-next.
> > >
> > > As I asked, I'm not familiar with virtio code. Do you think if I should
> > > add a new VIRTIO_NET_HDR_GSO_VLAN flag? It's only a L2 flag without any L3
> > > info. If I add something like VIRTIO_NET_HDR_GSO_VLAN_TCPV4/TCPV6/UDP. That
> > > would add more combinations. Which doesn't like a good idea.
> > 
> > I would prefer a new flag to denote this type, so that we can be
> > strict and only change the datapath for packets that have this flag
> > set (and thus express the intent).
> > 
> > But the VIRTIO_NET_HDR types are defined in the virtio spec. The
> > maintainers should probably chime in.
> 
> Yes, it's a UAPI extension, not to be done lightly. In this case IIUC
> gso_type in the header is only u8 - 8 bits and 5 of these are already
> used.  So I don't think the virtio TC will be all that happy to burn up
> a bit unless a clear benefit can be demonstrated. 
> 
> I agree with the net-next proposal, I think it's more a feature than a
> bugfix. In particular I think a Fixes tag can also be dropped in that
> IIUC GSO for vlan packets didn't work even before that commit - right?

Right. virtio_net_hdr GSO with vlan doesn't work before.
I will post this to net-next.

Thanks
Hangbin