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

[Alexey Kardashevskiy]

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? What
is uint6_t - uint8_t or uint16_t (neither works)?

May be it is a missing barrier - I rebooted machine several times and now
sometime after even 240 seconds (not 210 as before) it works (but most of
the time still does not)...


>> +        if (r) {
>> +            error_report("Error notifiyng host notifier: %d", -r);
>> +            goto err;
>> +        }
>>      }
>>
>>
>>
>>>
>>>>
>>>>
>>>>>>> If to remove vhost=on, it is all good. If to try Fedora19
>>>>>>> (v3.10-something), it all good again - works before and after reboot.
>>>>>>>
>>>>>>>
>>>>>>> And there 2 questions:
>>>>>>>
>>>>>>> 1. does anybody have any clue what might go wrong after reboot?
>>>>>>>
>>>>>>> 2. Is there any good material to read about what exactly and how vhost
>>>>>>> accelerates?
>>>>>>>
>>>>>>> My understanding is that packets from the guest to the real network are
>>>>>>> going as:
>>>>>>> 1. guest's virtio-pci-net does ioport(VIRTIO_PCI_QUEUE_NOTIFY)
>>>>>>> 2. QEMU's net/virtio-net.c calls qemu_net_queue_deliver()
>>>>>>> 3. QEMU's net/tap.c calls tap_write_packet() and this is how the host 
>>>>>>> knows
>>>>>>> that there is a new packet.
>>>>>
>>>>>
>>>>> What about the documentation? :) or the idea?
>>>>>
>>>>>
>>>>>>>
>>>>>>>
>>>>>>> Thanks!
>>>>>>>
>>>>>>>
>>>>>>> This how I run QEMU:
>>>>>>> ./qemu-system-ppc64 \
>>>>>>> -enable-kvm \
>>>>>>> -m 2048 \
>>>>>>> -machine pseries \
>>>>>>> -initrd 1.cpio \
>>>>>>> -kernel vml312_virtio_net_dbg \
>>>>>>> -nographic \
>>>>>>> -vga none \
>>>>>>> -netdev
>>>>>>> tap,id=id3,ifname=tap-id3,script=ifup.sh,downscript=ifdown.sh,vhost=on \
>>>>>>> -device virtio-net-pci,id=id4,netdev=id3,mac=C0:41:49:4b:00:00
>>>>>>>
>>>>>>>
>>>>>>> That is bridge config:
>>>>>>> address@hidden ~]$ brctl show
>>>>>>> bridge name     bridge id               STP enabled     interfaces
>>>>>>> brtest          8000.00145e992e88       no      pin     eth4
>>>>>>>
>>>>>>>
>>>>>>> The ifup.sh script:
>>>>>>> ifconfig $1 hw ether ee:01:02:03:04:05
>>>>>>> /sbin/ifconfig $1 up
>>>>>>> /usr/sbin/brctl addif brtest $1



-- 
Alexey