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



On 09/24/2013 02:49 AM, Hu Tao wrote:
> On Mon, Sep 23, 2013 at 02:30:53PM -0400, Cole Robinson wrote:
>> 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?
> 
> Not tested. But the main concern is that guest should not be affected by
> changes caused by host-model during migration, libvirt update, etc.,
> right? It seems that we have only one option left, use preserved
> configuration in such cases, which effectively makes host-model a
> one-time definition and turns it into custom mode at the first time,
> which is almost like the ``copy host CPU configuration'' button, which
> is already there.
> 

Yeah I think that's what Jiri was proposing.

> I'm wondering the purpose of host-model when it was firstly introduced.
> Handling preserved configuration in libvirt will probably make
> host-model a different thing, thus brings compatibility issues.
> 

I think it was meant to be a libvirt approximation of -cpu host, but
unfortunately the current implementation is less complete than -cpu host but
has some of the same problems. Maybe _all_ of the same problems, but I'd have
to look to see if libvirt does something special WRT host-model and migration.

- Cole


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