Re: [Qemu-devel] [PATCHv4 3/4] cpuid: disable pv eoi for 1.1 and older compat types

[Gleb Natapov <gleb@redhat.com>]

On Wed, Aug 29, 2012 at 08:36:30AM -0500, Anthony Liguori wrote:
> "Michael S. Tsirkin" <mst@redhat.com> writes:
> 
> > In preparation for adding PV EOI support, disable PV EOI by default for
> > 1.1 and older machine types, to avoid CPUID changing during migration.
> >
> > PV EOI can still be enabled/disabled by specifying it explicitly.
> >     Enable for 1.1
> >     -M pc-1.1 -cpu kvm64,+kvm_pv_eoi
> >     Disable for 1.2
> >     -M pc-1.2 -cpu kvm64,-kvm_pv_eoi
> >
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> 
> As best I can tell, we're papering over an ABI breakage in KVM.
> 
> If an old QEMU attempts to do a live migration on a new kernel,
> migrating to an QEMU on a different box with an older kernel, it will
> fail because there is state that isn't being migrated.
> 
> This ought to be fixed in the kernel by making these features
> whitelisted by userspace.
> 
What do you mean. Userspace and only userspace decides what cpuid bits
will be seen by a guest. Currently userspace enables all PV cpuid bits
it finds.

--
			Gleb.