Re: [Qemu-devel] Re: [PATCH v2 4/9] provide in-kernel apic

[Glauber Costa]

On Fri, Oct 09, 2009 at 05:48:31PM +0100, Jamie Lokier wrote:
> Glauber Costa wrote:
> > On Fri, Oct 09, 2009 at 11:06:41AM +0100, Jamie Lokier wrote:
> > > Glauber Costa wrote:
> > > > > It ensures the two models are compatible.  Since they're the same 
> > > > > device  
> > > > > from the point of view of the guest, there's no reason for them to 
> > > > > have  
> > > > > different representations or to be incompatible.
> > > > 
> > > > live migration between something that has in-kernel irqchip and
> > > > something that has not is likely to be a completely untested
> > > > thing. And this is the only reason we might think of it as the same
> > > > device. I don't see any value in supporting this combination
> > > 
> > > Not just live migration.  ACPI sleep + savevm + loadvm + ACPI resume,
> > > for example.
> > Yes, but in this case too, I'd expect the irqchipness of qemu not to change.
> 
> If I've just been sent an image produced by someone running KVM, and
> my machine is not KVM-capable, or I cannot upgrade the KVM kernel
> module because it's in use by other VMs (had this problem a few
> times), there's no choice but to change the irqchipness.
As gleb mentioned, requiring such a change to happen offline (across a reboot)
is not that much of a pain.

There are thousands of scenarios in which it will have to happen anyway,
including major bumps in qemu version.