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Re: [virt-tools-list] [PATCH v2 0/3] add cpu mode 'host-model' support



On 09/23/2013 07:17 AM, Hu Tao wrote:
> On Tue, Apr 23, 2013 at 05:20:12PM +0200, Jiri Denemark wrote:
>> On Tue, Apr 23, 2013 at 10:47:58 -0400, Cole Robinson wrote:
>>> On 04/23/2013 08:06 AM, Martin Kletzander wrote:
>>>> On 04/23/2013 01:56 PM, Guannan Ren wrote:
>>>>> On 04/23/2013 07:37 PM, Martin Kletzander wrote:
>>>>>> On 04/20/2013 10:09 PM, Cole Robinson wrote:
>>>>>>> On 04/18/2013 03:47 AM, Guannan Ren wrote:
>>>>>>>> v1 to v2:
>>>>>>>>    removed UPDATE_CPU flag checking
>>>>>>>>    renamed helper function name from reset() to clear_attrs()
>>>>>>>>    change the check box to be labeled 'Use host CPU model'
>>>>>>>>    remove the lightbulb icon, use tooltip instead
>>>>>>>>    reword the tooltip from Cole's
>>>>>>>>    remove the WARN image icon from UI
>>>>>>>>
>>>>>>>> Add a checkbox for 'host-model' mode and removed 'Copy host CPU
>>>>>>>> configuration'
>>>>>>>> button.
>>>>>>>>
>>>>>> Sorry for not catching this thread earlier, but IIUC, the 'host-model'
>>>>>> doesn't make up for the button.  XML is saved with 'host-model' then,
>>>>>> right?
>>>>>>
>>>>>> Unfortunately, I can't see that easily right now as git virt-manager
>>>>>> consistently crashes for me on all VMs and bare metal as well and I made
>>>>>> that one of my priorities in order to speed up the bug hunt on it.
>>>>>>
>>>>>
>>>>>     Martin, I am using virt-manager git head now, it seems fine for me.
>>>>>     Is there anything wrong about 'host-model', I can't quite follow you
>>>>> here.
>>>>>
>>>>>     Guannan
>>>>>
>>>>
>>>> I was just wondering if dropping the button isn't a bad idea, some guest
>>>> OS might have problems when it is ran on different CPU, which is what
>>>> might happen with host-model after destroy/start, but would be avoided
>>>> with 'Copy host configuration'.  I'm not saying 'host-model' is wrong,
>>>> we definitely want the support for that.
>>>>
>>>
>>> Hmm, how would host-model change CPU between destroy/start... like a libvirt
>>> update supporting more flags? I didn't think about that, and it is
>>> problematic. Libvirt goes to great lengths to try and preserve hardware config
>>> for a VM across libvirt updates, host-model potentially throws that out the
>>> window...
>>>
>>> Unless there's some clever way of getting around that it makes me think
>>> host-model just doesn't fit in the UI. Trying to explain all the nuances of
>>> this stuff in the current UI is impossible, so until we come up with something
>>> different we should go with the safest bet, which is only providing the old
>>> button press behavior.
>>
>> I agree that currently copying host CPU XML into guest CPU is safest
>> than using host-model (which is just a shortcut for it but the config is
>> not preserved after domain shutdown). However, host-model will be
>> improved (hopefully soon) to provide more. And I think we (libvirt)
>> should come up with something that would preserve the configuration,
>> too.
> 
> If we preserve cpu configurations when host-model is specified, what to
> do with situations where the preserved configurations are different with
> what host-model gets?
> 
> - VM is copied to another host with a different cpu. The new cpu may
>   have all features in preserved configurations, or may not. Using
>   preserved configurations may fail to start VM.
> 
> - VM is migrated to another host with a different cpu. Same as above.
> 

Yes, without host-model handling those bits for us, virt-manager would need to
explicitly handle it. Thankfully libvirt already has APIs that could help us
here. But really I'm less concerned with cross host, non-uniform hardware
migration compatibility than I am with a libvirt upgrade implicitly changing
guest hardware.

> - libvirt is updated to support more flags. It's better to update the
>   preserved configuration.
> 

If libvirt supports more flags, we update libvirt, restart VM, guest sees the
CPU is different... wouldn't this cause Windows reactivation?

- Cole


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