On 01/11/2011 08:56 AM, Avi Kivity wrote:
On 01/11/2011 04:36 PM, Anthony Liguori wrote:
They need to use the same device id then. And if they share code,
that indicates that they need to be the same device even more.
No, it really doesn't :-) Cirrus VGA and std VGA share a lot of
code. But that doesn't mean that we treat them as one device.
Cirrus and VGA really are separate devices. They share code because
on evolved from the other, and is backwards compatible with the
other. i8254 and i8254-kvm did not evolve from each other,
Actually, they did, but that's besides the point.
both are implementations of the i8254 spec, and both are 100%
compatible with each other (modulu bugs).
And BTW, there are guest visible differences between the KVM
IOAPIC/PIC/PIT than the QEMU versions. The only reason PIT live
migration works today is because usually delivers all interrupts
quickly. But it actually does maintain state in the work queue that
isn't saved. If PIT tried to implement gradual catchup, there would
be no way not to expose that state to userspace.
Why not? Whatever state the kernel keeps, we expose to userspace and
allow sending it over the wire.
What exactly is the scenario you're concerned about?
Migration between userspace HPET and in-kernel HPET?
One thing I've been considering is essentially migration filters. It
would be a set of rules that essentially were "hpet-kvm.* = hpet.*"
which would allow migration from hpet to hpet-kvm given a translation of
state. I think this sort of higher level ruleset would make it easier
to support migration between versions of the device model.
Of course, that only gives you a forward path. It doesn't give you a
backwards path.
Regards,
Anthony Liguori
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