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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: Mon, 16 Apr 2018 14:29:14 +1000
User-agent: Mutt/1.9.2 (2017-12-15)

On Thu, Apr 12, 2018 at 10:36:10AM +0200, Cédric Le Goater wrote:
> On 04/12/2018 07:16 AM, David Gibson wrote:
> > 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.
> > 
> Yes ... KVM makes it a little uglier. 
> A KVM_DEVICE_DESTROY device is needed to cleanup the VM and a 
> DISABLE_CAP to disconnect the vpcu from the current KVM XIVE/XICS 
> device. I have used an extra arg on ENABLE_CAP for the moment.    
> At the QEMU level, we need to connect/reconnect at reset time to
> handle possible changes in CAS, and at post_load.


> Destroying the MemoryRegion is a bit problematic, I have not
> found a common layout compatible with both the emulated mode 
> (std IO regions) and the KVM mode (ram device regions)

That sounds awkward, I guess we'll discuss the details of this later.

Btw, a secondary advantage of starting off with XIVE only under a
different machine type is that we can declare that one not to be
migration stable until we're ready.  So we can merge something that's
ok to experiment with, but reserve the right to incompatibly change
the migration format until we're confident we're ready and can merge
it into the "stable" machine type.

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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