netdev - Re: [PATCH v3 bpf-next 3/8] veth: Avoid drops by oversized packets when XDP is enabled

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Message-ID: <20180724121015.5d9edbf3@cakuba.netronome.com>
Date:   Tue, 24 Jul 2018 12:10:15 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
Cc:     Toshiaki Makita <toshiaki.makita1@...il.com>,
        netdev@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <brouer@...hat.com>
Subject: Re: [PATCH v3 bpf-next 3/8] veth: Avoid drops by oversized packets
 when XDP is enabled

On Tue, 24 Jul 2018 18:39:09 +0900, Toshiaki Makita wrote:
> On 2018/07/24 10:56, Toshiaki Makita wrote:
> > On 2018/07/24 9:27, Jakub Kicinski wrote:  
> >> On Mon, 23 Jul 2018 00:13:03 +0900, Toshiaki Makita wrote:  
> >>> From: Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
> >>>
> >>> All oversized packets including GSO packets are dropped if XDP is
> >>> enabled on receiver side, so don't send such packets from peer.
> >>>
> >>> Drop TSO and SCTP fragmentation features so that veth devices themselves
> >>> segment packets with XDP enabled. Also cap MTU accordingly.
> >>>
> >>> Signed-off-by: Toshiaki Makita <makita.toshiaki@....ntt.co.jp>  
> >>
> >> Is there any precedence for fixing up features and MTU like this?  Most
> >> drivers just refuse to install the program if settings are incompatible.  
> > 
> > I don't know any precedence. I can refuse the program on installing it
> > when features and MTU are not appropriate. Is it preferred?
> > Note that with current implementation wanted_features are not touched so
> > features will be restored when the XDP program is removed. MTU will not
> > be restored though, as I do not remember the original MTU.  
> 
> I just recalled that virtio_net used to refused XDP when guest offload
> features are incompatible but now it dynamically fixup them on
> installing an XDP program.
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=3f93522ffab2d46a36b57adf324a54e674fc9536

That's slightly different AFAIU, because the virtio features weren't
really controllable at runtime at all.  I'm not dead set on leaving the
features be, but I just want to make sure we think this through
properly before we commit to any magic behaviour for ever...

Taking a quick glance at the MTU side, it seems that today if someone
decides to set MTU on one side of a veth pair the packets will simply
get dropped.  So the MTU coupling for XDP doesn't seem in line with
existing behaviour of veth, not only other XDP drivers.