On Tue, Jun 23, 2015 at 12:08:28PM -0300, Eduardo Habkost wrote: > On Tue, Jun 23, 2015 at 02:32:00PM +0200, Andreas Färber wrote: > > Am 08.06.2015 um 22:18 schrieb Jiri Denemark: > > >> To help libvirt in the transition, a x86-cpu-model-dump script is > > >> provided, > > >> that will generate a config file that can be loaded using -readconfig, > > >> based on > > >> the -cpu and -machine options provided in the command-line. > > > > > > Thanks Eduardo, I never was a big fan of moving (or copying) all the CPU > > > configuration data to libvirt, but now I think it actually makes sense. > > > We already have a partial copy of CPU model definitions in libvirt > > > anyway, but as QEMU changes some CPU models in some machine types (and > > > libvirt does not do that) we have no real control over the guest CPU > > > configuration. While what we really want is full control to enforce > > > stable guest ABI. > > > > That sounds like FUD to me. Any concrete data points where QEMU does not > > have a stable ABI for x86 CPUs? That's what we have the pc*-x.y machines > > for. > > What Jiri is saying that the CPUs change depending on -mmachine, not > that the ABI is broken by a given machine. > > The problem here is that libvirt needs to provide CPU models whose > runnability does not depend on the machine-type. If users have a VM that > is running in a host and the VM machine-type changes,
How does it change, and why? > the VM should be > still runnable in that host. QEMU doesn't provide that, our CPU models > may change when we introduce new machine-types, so we are giving them a > mechanism that allows libvirt to implement the policy they need. I don't mind wrt CPU specifically, but we absolutely do change guest ABI in many ways when we change machine types. > -- > Eduardo
