Re: [Qemu-devel] [Migration Bug? ] Occasionally, the content of VM's memory is inconsistent between Source and Destination of migration

[Wen Congyang]

On 03/27/2015 04:56 PM, Stefan Hajnoczi wrote:
> On Thu, Mar 26, 2015 at 11:29:43AM +0100, Juan Quintela wrote:
>> Wen Congyang <address@hidden> wrote:
>>> On 03/25/2015 05:50 PM, Juan Quintela wrote:
>>>> zhanghailiang <address@hidden> wrote:
>>>>> Hi all,
>>>>>
>>>>> We found that, sometimes, the content of VM's memory is
>>>>> inconsistent between Source side and Destination side
>>>>> when we check it just after finishing migration but before VM continue to 
>>>>> Run.
>>>>>
>>>>> We use a patch like bellow to find this issue, you can find it from affix,
>>>>> and Steps to reprduce:
>>>>>
>>>>> (1) Compile QEMU:
>>>>>  ./configure --target-list=x86_64-softmmu  --extra-ldflags="-lssl" && make
>>>>>
>>>>> (2) Command and output:
>>>>> SRC: # x86_64-softmmu/qemu-system-x86_64 -enable-kvm -cpu
>>>>> qemu64,-kvmclock -netdev tap,id=hn0-device
>>>>> virtio-net-pci,id=net-pci0,netdev=hn0 -boot c -drive
>>>>> file=/mnt/sdb/pure_IMG/sles/sles11_sp3.img,if=none,id=drive-virtio-disk0,cache=unsafe
>>>>> -device
>>>>> virtio-blk-pci,bus=pci.0,addr=0x4,drive=drive-virtio-disk0,id=virtio-disk0
>>>>> -vnc :7 -m 2048 -smp 2 -device piix3-usb-uhci -device usb-tablet
>>>>> -monitor stdio
>>>>
>>>> Could you try to reproduce:
>>>> - without vhost
>>>> - without virtio-net
>>>> - cache=unsafe is going to give you trouble, but trouble should only
>>>>   happen after migration of pages have finished.
>>>
>>> If I use ide disk, it doesn't happen.
>>> Even if I use virtio-net with vhost=on, it still doesn't happen. I guess
>>> it is because I migrate the guest when it is booting. The virtio net
>>> device is not used in this case.
>>
>> Kevin, Stefan, Michael, any great idea?
> 
> You must use -drive cache=none if you want to use live migration.  It
> should not directly affect memory during migration though.

Otherwise, what will happen? If the user doesn't use cache=none, and
tries to use live migration, qemu doesn't output any message or trigger
an event to notify the user.

Thanks
Wen Congyang

> 
>>>>> We have done further test and found that some pages has been
>>>>> dirtied but its corresponding migration_bitmap is not set.
>>>>> We can't figure out which modules of QEMU has missed setting bitmap
>>>>> when dirty page of VM,
>>>>> it is very difficult for us to trace all the actions of dirtying VM's 
>>>>> pages.
>>>>
>>>> This seems to point to a bug in one of the devices.
> 
> I think you'll need to track down which pages are different.  If you are
> lucky, their contents will reveal what the page is used for.
> 
> Stefan
>