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



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.

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



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