Re: [libvirt] [PATCH v2 3/6] Add support for cpu mode attribute

On 01/12/2012 04:02 AM, Jiri Denemark wrote: > The mode can be either of "custom" (default), "host-model", > "host-passthrough". The semantics of each mode is described in the > following examples: > > --- > Notes: > Version 2: > - added documentation > - fixed XML examples and schema > Thanks, that's what I was waiting for in v1. > +++ b/docs/formatdomain.html.in > + <dt><code>host-model</code></dt> > + <dd>The <code>host-model</code> mode is essentially a shortcut to > + copying host CPU definition from capabilities XML into domain XML. > + Since the CPU definition is copied just before starting a domain, > + exactly the same XML can be used on different hosts while still > + providing the best guest CPU each host supports. Neither > + <code>match</code> attribute nor any <code>feature</code> elements > + can be used in this mode. Specifying CPU model is not supported > + either, but <code>model</code>'s <code>fallback</code> attribute may > + still be used. Libvirt does not model every aspect of each CPU so > + the guest CPU will not match the host CPU exactly. On the other > + hand, the ABI provided to the guest is reproducible. During > + migration, complete CPU model definition is transferred to the > + destination host so the migrated guest will see exactly the same CPU > + model even if the destination host contains more capable CPUs.</dd> Question - if I start on a less-powerful CPU, then live migrate to a more powerful CPU, then restart the guest, did the migration lock me in to the less-powerful capabilities, or does restarting the guest then pick up on the more-powerful capabilities? I'm guessing the former (once you migrate, you want to be locked in), because otherwise, the act of migration followed by guest reboot would make Windows guests susceptible to reactivation. In which case, I think rewording the last sentence might help: