netdev - Re: Linux 4.14 - regression: broken tun/tap / bridge network with virtio

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Date:   Fri, 8 Dec 2017 09:47:51 +0100
From:   Michal Kubecek <mkubecek@...e.cz>
To:     Andreas Hartmann <andihartmann@...19freenet.de>
Cc:     Jason Wang <jasowang@...hat.com>,
        David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: Linux 4.14 - regression: broken tun/tap / bridge network with
 virtio - bisected

On Fri, Dec 08, 2017 at 08:21:16AM +0100, Andreas Hartmann wrote:
> 
> Thanks for this hint - I'm not using xdp. Therefore I rechecked my
> bisect and detected a mistake. The rebisect now leads to
> 
> 
> 
> [v2,RFC,11/13] net: Remove all references to SKB_GSO_UDP. [1]
> 
> 
> 
> For the repeated bisect, I switched back to the original qemu 2.6.2
> (instead of 2.10.1), because problems can be seen reliably with 2.6.2.
> 
> All my VMs are using virtio_net. BTW: I couldn't see the problems
> (sometimes, the VM couldn't be stopped at all) if all my VMs are using
> e1000 as interface instead.
> 
> This finding now matches pretty much the responsible UDP-package which
> caused the stall. I already mentioned it here [2].
> 
> To prove it, I reverted from the patch series "[PATCH v2 RFC 0/13]
> Remove UDP Fragmentation Offload support" [3]
> 
> 11/13 [v2,RFC,11/13] net: Remove all references to SKB_GSO_UDP. [4]
> 12/13 [v2,RFC,12/13] inet: Remove software UFO fragmenting code. [5]
> 13/13 [v2,RFC,13/13] net: Kill NETIF_F_UFO and SKB_GSO_UDP. [6]
> 
> and applied it to Linux 4.14.4. It compiled fine and is running fine.
> The vnet doesn't die anymore. Yet, I can't say if the qemu stop hangs
> are gone, too.
> 
> Obviously, there is something broken with the new UDP handling. Could
> you please analyze this problem? I could test some more patches ... .

Any chance your VMs were live migrated from pre-4.14 host kernel? If
this is the case, you should try commit 0c19f846d582 ("net: accept UFO
datagrams from tuntap and packet"). Or disabling UFO in the guest should
work around the issue.

                                                          Michal Kubecek