Re: [Qemu-devel] vhost-net issue: does not survive reboot on ppc64

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Dec 26, 2013 at 09:13:31PM +1100, Alexey Kardashevskiy wrote:
> On 12/25/2013 08:52 PM, Michael S. Tsirkin wrote:
> > On Wed, Dec 25, 2013 at 12:36:12PM +1100, Alexey Kardashevskiy wrote:
> >> On 12/25/2013 02:43 AM, Michael S. Tsirkin wrote:
> >>> On Wed, Dec 25, 2013 at 01:15:29AM +1100, Alexey Kardashevskiy wrote:
> >>>> On 12/24/2013 08:40 PM, Michael S. Tsirkin wrote:
> >>>>> On Tue, Dec 24, 2013 at 02:09:07PM +1100, Alexey Kardashevskiy wrote:
> >>>>>> On 12/24/2013 03:24 AM, Michael S. Tsirkin wrote:
> >>>>>>> On Mon, Dec 23, 2013 at 02:01:13AM +1100, Alexey Kardashevskiy wrote:
> >>>>>>>> On 12/23/2013 01:46 AM, Alexey Kardashevskiy wrote:
> >>>>>>>>> On 12/22/2013 09:56 PM, Michael S. Tsirkin wrote:
> >>>>>>>>>> On Sun, Dec 22, 2013 at 02:01:23AM +1100, Alexey Kardashevskiy wrote:
> >>>>>>>>>>> Hi!
> >>>>>>>>>>>
> >>>>>>>>>>> I am having a problem with virtio-net + vhost on POWER7 machine - it does
> >>>>>>>>>>> not survive reboot of the guest.
> >>>>>>>>>>>
> >>>>>>>>>>> Steps to reproduce:
> >>>>>>>>>>> 1. boot the guest
> >>>>>>>>>>> 2. configure eth0 and do ping - everything works
> >>>>>>>>>>> 3. reboot the guest (i.e. type "reboot")
> >>>>>>>>>>> 4. when it is booted, eth0 can be configured but will not work at all.
> >>>>>>>>>>>
> >>>>>>>>>>> The test is:
> >>>>>>>>>>> ifconfig eth0 172.20.1.2 up
> >>>>>>>>>>> ping 172.20.1.23
> >>>>>>>>>>>
> >>>>>>>>>>> If to run tcpdump on the host's "tap-id3" interface, it shows no trafic
> >>>>>>>>>>> coming from the guest. If to compare how it works before and after reboot,
> >>>>>>>>>>> I can see the guest doing an ARP request for 172.20.1.23 and receives the
> >>>>>>>>>>> response and it does the same after reboot but the answer does not come.
> >>>>>>>>>>
> >>>>>>>>>> So you see the arp packet in guest but not in host?
> >>>>>>>>>
> >>>>>>>>> Yes.
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>>> One thing to try is to boot debug kernel - where pr_debug is
> >>>>>>>>>> enabled - then you might see some errors in the kernel log.
> >>>>>>>>>
> >>>>>>>>> Tried and added lot more debug printk myself, not clear at all what is
> >>>>>>>>> happening there.
> >>>>>>>>>
> >>>>>>>>> One more hint - if I boot the guest and the guest does not bring eth0 up
> >>>>>>>>> AND wait more than 200 seconds (and less than 210 seconds), then eth0 will
> >>>>>>>>> not work at all. I.e. this script produces not-working-eth0:
> >>>>>>>>>
> >>>>>>>>>
> >>>>>>>>> ifconfig eth0 172.20.1.2 down
> >>>>>>>>> sleep 210
> >>>>>>>>> ifconfig eth0 172.20.1.2 up
> >>>>>>>>> ping 172.20.1.23
> >>>>>>>>>
> >>>>>>>>> s/210/200/ - and it starts working. No reboot is required to reproduce.
> >>>>>>>>>
> >>>>>>>>> No "vhost" == always works. The only difference I can see here is vhost's
> >>>>>>>>> thread which may get suspended if not used for a while after the start and
> >>>>>>>>> does not wake up but this is almost a blind guess.
> >>>>>>>>
> >>>>>>>>
> >>>>>>>> Yet another clue - this host kernel patch seems to help with the guest
> >>>>>>>> reboot but does not help with the initial 210 seconds delay:
> >>>>>>>>
> >>>>>>>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> >>>>>>>> index 69068e0..5e67650 100644
> >>>>>>>> --- a/drivers/vhost/vhost.c
> >>>>>>>> +++ b/drivers/vhost/vhost.c
> >>>>>>>> @@ -162,10 +162,10 @@ void vhost_work_queue(struct vhost_dev *dev, struct
> >>>>>>>> vhost_work *work)
> >>>>>>>>                 list_add_tail(&work->node, &dev->work_list);
> >>>>>>>>                 work->queue_seq++;
> >>>>>>>>                 spin_unlock_irqrestore(&dev->work_lock, flags);
> >>>>>>>> -               wake_up_process(dev->worker);
> >>>>>>>>         } else {
> >>>>>>>>                 spin_unlock_irqrestore(&dev->work_lock, flags);
> >>>>>>>>         }
> >>>>>>>> +       wake_up_process(dev->worker);
> >>>>>>>>  }
> >>>>>>>>  EXPORT_SYMBOL_GPL(vhost_work_queue);
> >>>>>>>>
> >>>>>>>>
> >>>>>>>
> >>>>>>> Interesting. Some kind of race? A missing memory barrier somewhere?
> >>>>>>
> >>>>>> I do not see how. I boot the guest and just wait 210 seconds, nothing
> >>>>>> happens to cause races.
> >>>>>>
> >>>>>>
> >>>>>>> Since it's all around startup,
> >>>>>>> you can try kicking the host eventfd in
> >>>>>>> vhost_net_start.
> >>>>>>
> >>>>>>
> >>>>>> How exactly? This did not help. Thanks.
> >>>>>>
> >>>>>> diff --git a/hw/net/vhost_net.c b/hw/net/vhost_net.c
> >>>>>> index 006576d..407ecf2 100644
> >>>>>> --- a/hw/net/vhost_net.c
> >>>>>> +++ b/hw/net/vhost_net.c
> >>>>>> @@ -229,6 +229,17 @@ int vhost_net_start(VirtIODevice *dev, NetClientState
> >>>>>> *ncs,
> >>>>>>          if (r < 0) {
> >>>>>>              goto err;
> >>>>>>          }
> >>>>>> +
> >>>>>> +        VHostNetState *vn = tap_get_vhost_net(ncs[i].peer);
> >>>>>> +        struct vhost_vring_file file = {
> >>>>>> +            .index = i
> >>>>>> +        };
> >>>>>> +        file.fd =
> >>>>>> event_notifier_get_fd(virtio_queue_get_host_notifier(dev->vq));
> >>>>>> +        r = ioctl(vn->dev.control, VHOST_SET_VRING_KICK, &file);
> >>>>>
> >>>>> No, this sets the notifier, it does not kick.
> >>>>> To kick you write 1 there:
> >>>>> 	uint6_t  v = 1;
> >>>>> 	write(fd, &v, sizeof v);
> >>>>
> >>>>
> >>>> Please, be precise. How/where do I get that @fd? Is what I do correct?
> >>>
> >>> Yes.
> >>>
> >>>> What
> >>>> is uint6_t - uint8_t or uint16_t (neither works)?
> >>>
> >>> Sorry, should have been uint64_t.
> >>
> >>
> >> Oh, that I missed :-) Anyway, this does not make any difference. Is there
> >> any cheap&dirty way to make vhost-net kernel thread always awake? Sending
> >> it signals from the user space does not work...
> > 
> > You can run a timer in qemu and signal the eventfd from there
> > periodically.
> > 
> > Just to restate, tcpdump in guest shows that guest sends arp packet,
> > but tcpdump in host on tun device does not show any packets?
> 
> 
> Ok. Figured it out about disabling interfaces in Fedora19. I was wrong,
> something is happening on the host's TAP - the guest sends ARP request, the
> response is visible on the TAP interface but not in the guest.

Okay. So problem is on host to guest path then.
Things to try:

1. trace handle_rx [vhost_net]
2. trace tun_put_user [tun]
3. I suspect some host bug in one of the features.
Let's try to disable some flags with device property:
you can get the list by doing:
./x86_64-softmmu/qemu-system-x86_64 -device virtio-net-pci,?|grep on/off
Things I would try turning off is guest offloads (ones that start with guest_)
event_idx,any_layout,mq.
Turn them all off, if it helps try to find the one that helped.

-- 
MST