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

[Guannan Ren]

On 04/23/2013 11:20 PM, 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.
>

    Actually,  virt-manager cached the cpu model and features via 
VIR_DOMAIN_XML_UPDATE_CPU
    after toggling host-model checkbox.
    We can give a hint to user if host cpu changed after 
destroy/start/migration: one is to keep guest
    OS running safely with cached host-model cpu (convert to custom mode 
automatically), the other
    is to use newest host-model cpu.

    "copy XML" is the safest way, but not with best performance.
    Either is okay,  rolling back is last choice.

    Guannan