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

From: David Gibson
Subject: Re: [Qemu-ppc] [PATCH v2 02/19] spapr: introduce a skeleton for the XIVE interrupt controller
Date: Thu, 12 Apr 2018 15:16:53 +1000
User-agent: Mutt/1.9.2 (2017-12-15)

On Mon, Feb 12, 2018 at 09:55:17AM +1100, Benjamin Herrenschmidt wrote:
> 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...

Hrm, the fit is very clunky certainly, but i think we can make it work.

> 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 ?

Sorta.. maybe.. but it would probably get really ugly if we don't
preserve the usual way object lifetimes work.

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

I really think this is the way to go, warts and all.

David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!

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