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

[Alexey Kardashevskiy]

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);
+        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


-- 
Alexey