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Re: [Qemu-ppc] [PATCH v2 02/19] spapr: introduce a skeleton for the XIVE

From: Benjamin Herrenschmidt
Subject: Re: [Qemu-ppc] [PATCH v2 02/19] spapr: introduce a skeleton for the XIVE interrupt controller
Date: Mon, 12 Feb 2018 09:55:17 +1100

On Sun, 2018-02-11 at 19:08 +1100, David Gibson wrote:
> On Thu, Jan 18, 2018 at 08:27:52AM +1100, Benjamin Herrenschmidt wrote:
> > On Wed, 2018-01-17 at 15:39 +0100, Cédric Le Goater wrote:
> > > Migration is a problem. We will need both backend QEMU objects to be 
> > > available anyhow if we want to migrate. So we are back to the current 
> > > solution creating both QEMU objects but we can try to defer some of the 
> > > KVM inits and create the KVM device on demand at CAS time.
> > 
> > Do we have a way to migrate a piece of info from the machine *first*
> > that indicate what type of XICS/XIVE to instanciate ?
> Nope.  qemu migration doesn't work like that.  Yes, it should, and
> everyone knows it, but changing it is a really long term project.

Well, we have a problem then. It looks like Qemu broken migration is
fundamentally incompatible with PAPR and CAS design...

I know we don't migrate the configuration, that's not exactly what I
had in mind tho... Can we have some piece of *data* from the machine be
migrated first, and use it on the target to reconfigure the interrupt
controller before the stream arrives ?

Otherwise, we have indeed no much choice but the horrible wart of
creating both interrupt controllers with only one "active".

> > 
> > > The next problem is the ICP object that currently needs the KVM device 
> > > fd to connect the vcpus ... So, we will need to change that also. 
> > > That is probably the biggest problem today. We need a way to disconnect 
> > > the vpcu from the KVM device and see how we can defer the connection.
> > > I need to make sure this is possible, I can check that without XIVE
> > 
> > Ben.
> > 

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