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



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.

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.


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