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

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Aug 29, 2012 at 09:09:16AM -0500, Anthony Liguori wrote:
> Gleb Natapov <gleb@redhat.com> writes:
> 
> > 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.
> 
> Right, I misunderstood from the commit message.  I see now that the
> problem is that bfee7546df51c08e395dc8a7676a5c7f20186fee unconditionally
> enabled kvm_pv_eoi without taking into account migration support.

All that commit does it let user disable pv eoi.
Don't see how reverting it will help ...

> I think for 1.2 we should simply revert the above commit and then we can
> restore it for 1.3 with proper support for migration.
> 
> Regards,
> 
> Anthony Liguori
> 
> >
> > --
> > 			Gleb.