Re: [Qemu-devel] [patch 2/2] target-i386: block migration and savevm if invariant tsc is exposed

[Eduardo Habkost]

On Tue, Apr 29, 2014 at 08:22:36AM +0200, Paolo Bonzini wrote:
> Il 28/04/2014 21:23, Eduardo Habkost ha scritto:
> >> Makes sense.  Basically "-cpu host,migratable=yes" is close to
> >> libvirt's host-model and Alex Graf's proposed "-cpu best".  Should we
> >> call it "-cpu best" and drop migratability of "-cpu host"?
> >
> >"-cpu best" is different from the modes above. It means "use the best
> >existing CPU model (from the pre-defined table) that can run on this
> >host".
> 
> Yes, it's not exactly the same.  In practice the behavior should be close.
> 
> >And it would have the same ambiguities we found on "-cpu host": if a CPU
> >model in the table have invtsc enabled, should it be considered a
> >candidate for "-cpu best", or not?
> 
> No CPU model in the table should have invtsc enabled. :)

I agree with you, but I wouldn't be so sure this will never happen. We
have had conflicts in the past between having CPU models that are useful
defaults for KVM vs CPU models that match real hardware more closely.

This is already the case with the "monitor" flag (that is not supported
by KVM but is supported by TCG, and is present on many CPU models).

(But this can be solved by adding a kvm_default_disable_features array,
like the kvm_default_features array but for features that should be
_disabled_ by default on KVM. I'm adding that to my TODO-list.)

-- 
Eduardo