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

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Date:   Fri, 8 Dec 2017 21:11:53 +0100
From:   Andreas Hartmann <andihartmann@...19freenet.de>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Michal Kubecek <mkubecek@...e.cz>
Cc:     Jason Wang <jasowang@...hat.com>,
        David Miller <davem@...emloft.net>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: Linux 4.14 - regression: broken tun/tap / bridge network with
 virtio - bisected

On 12/08/2017 at 05:04 PM Willem de Bruijn wrote:
> On Fri, Dec 8, 2017 at 6:40 AM, Michal Kubecek <mkubecek@...e.cz> wrote:
>> On Fri, Dec 08, 2017 at 11:31:50AM +0100, Andreas Hartmann wrote:
>>> On 12/08/2017 at 09:47 AM Michal Kubecek wrote:
>>>> On Fri, Dec 08, 2017 at 08:21:16AM +0100, Andreas Hartmann wrote:
>>>>>
>>>>> 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?
>>>
>>> No - the VMs are not live migrated. They are always running on the same
>>> host - either with kernel < 4.14 or with kernel 4.14.x.
>>
>> This is disturbing... unless I'm mistaken, it shouldn't be possible to
>> have UFO enabled on a virtio device in a VM booted on a host with 4.14
>> kernel.
> 
> Indeed. When working on that revert patch I verified that UFO in
> the guest virtio_net was off before the revert patch, on after.
> 
> Qemu should check host support with tap_probe_has_ufo
> before advertising support to the guest. Indeed, this is exactly
> what broke live migration in virtio_net_load_device at
> 
>     if (qemu_get_byte(f) && !peer_has_ufo(n)) {
>         error_report("virtio-net: saved image requires TUN_F_UFO support");
>         return -1;
>     }
> 
> Which follows
> 
>    peer_has_ufo
>      qemu_has_ufo
>        tap_has_ufo
>          s->has_ufo
> 
> where s->has_ufo was set by tap_probe_has_ufo in net_tap_fd_init.
> 
> Now, checking my qemu git branch, I ran pretty old 2.7.0-rc3. But this
> codepath does not seem to have changed between then and 2.10.1.
> 
> I cherry-picked the revert onto 4.14.3. It did not apply cleanly, but the
> fix-up wasn't too hard. Compiled and booted, but untested otherwise. At
> 
>   https://github.com/wdebruij/linux/commits/v4.14.3-aargh-ufo

I'm just running it at the moment. I didn't face any network hang until
now - although the critical UDP packages have been gone through.
Therefore: looks nice.


Thanks,
Andreas