Re: [Qemu-devel] Re: [PATCH v2 3/9] provide in-kernel ioapic

[Jamie Lokier]

Gleb Natapov wrote:
> On Thu, Oct 08, 2009 at 06:42:07PM +0200, Avi Kivity wrote:
> > On 10/08/2009 06:34 PM, Gleb Natapov wrote:
> > >So suppose I have simple watchdog device that required to be poked every
> > >second, otherwise it resets a computer. On migration we have to migrate
> > >time elapsed since last poke, but if device doesn't expose it to
> > >software in any way you are saying we can recreate is some other way?
> > 
> > The time is exposed (you can measure it by poking the device and
> The time yes, not its internal representation. What if one implementation 
> stores how much time passed and another how much time left.
> One counts in wall clack another only when guest runs. etc... and
> this is a device with only one write only register.

In that case you can decide between calling it two different devices
(which have the same guest-visible behaviour but are not
interchangable), or calling them different implementations of one
device - by adding a little more code to save state in a common format.

(Although they may have to be different devices for qemu
configuration, it's ok for them to have the same PCI IDs and for the
guest not to know the difference)

For your watchdog example, it's not hard to pick a saved state which
works for both.

ioapic will be harder to find a useful common saved state, and there
might need to be an *optional hint* section (e.g. for selecting the
next CPU to get an interrupt), but I think it would be worth it in
this case.  Being able to load a KVM image into TCG and vice versa is
quite useful sometimes.  E.g. I've had to do some OS installs using
TCG at first, then switch to KVM later for performance.

-- Jamie