Re: [Qemu-devel] [PATCH v2 0/3] fix bug about balloon working incorrectly when hotplug memeory

[zhanghailiang <zhang.zhanghailiang@huawei.com>]

On 2014/11/17 18:53, zhanghailiang wrote:
> On 2014/11/17 18:39, Michael S. Tsirkin wrote:
>> On Mon, Nov 17, 2014 at 01:11:07PM +0800, zhanghailiang wrote:
>>> Hi,
>>>
>>> Patch 1 and 2 mainly fix bug about balloon not working correctly when we do
>>> hotplug memory. It takes 'ram_size' as VM's real RAM size which is wrong
>>> after we hotplug memory.
>>>
>>> This bug exists since we begin to support hotplug memory, and it is better
>>> to fix it.
>>>
>>> Patch 3 add some trace events, it helps debugging balloon. If it is unnecessary,
>>> pls feel free to remove it.
>>>
>>> Thanks,
>>> zhanghailiang
>>
>> What about other users of ram_size?
>> Are they all incorrect?
>>
>
> pc-dimm is only supported in x86 target now, and i am not quite sure if hotplug
> memory will break migration. I'll look into it. Thanks.
>

Hi Michael,

I have made a global search in qemu code, ram_size is used mostly for VM's
startup initialization, I think it's all OK except virtio-balloon and
function vmport_cmd_ram_size (I'm not sure about this place :( ).

But, Unfortunately, hotplug memory action breaks migration. :(
I have made a simple test about this:

Source:
# start VM
# hotplug memory:
object_add memory-backend-ram,id=ram1,size=1024M,host-nodes=0,policy=bind
device_add pc-dimm,id=dimm1,memdev=ram1
# migrate VM to Destination

Destination:
# qemu-system-x86_64: Unknown ramblock "ram1", cannot accept migration
qemu: warning: error while loading state for instance 0x0 of device 'ram'
qemu-system-x86_64: load of migration failed: Invalid argument

*further test*:
hot-add CPU also break migration and reports error in destination:
'Unknown savevm section or instance 'cpu_common' 4
qemu-system-x86_64: load of migration failed: Invalid argument'

I think we should support migration after hotplug memory/CPU action,
what's your opinion? ;)
I will try to fix these two problems.

Thanks,
zhanghailiang
>>> v2:
>>> - fix compiling break for other targets that don't support pc-dimm
>>>
>>> zhanghailiang (3):
>>>    pc-dimm: add a function to calculate VM's current RAM size
>>>    virtio-balloon: Fix balloon not working correctly when hotplug memory
>>>    virtio-balloon: Add some trace events
>>>
>>>   hw/mem/pc-dimm.c                | 26 ++++++++++++++++++++++++++
>>>   hw/virtio/virtio-balloon.c      | 21 +++++++++++++++------
>>>   include/exec/cpu-common.h       |  1 +
>>>   stubs/qmp_pc_dimm_device_list.c |  5 +++++
>>>   trace-events                    |  4 ++++
>>>   5 files changed, 51 insertions(+), 6 deletions(-)
>>>
>>> --
>>> 1.7.12.4
>>>
>>
>> .
>>
>
>
>
>
>