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Re: [Qemu-devel] [Qemu-ppc] [PULL 11/18] pseries: Split CAS PVR negotiat


From: David Gibson
Subject: Re: [Qemu-devel] [Qemu-ppc] [PULL 11/18] pseries: Split CAS PVR negotiation out into a separate function
Date: Wed, 31 May 2017 16:33:21 +1000
User-agent: Mutt/1.8.0 (2017-02-23)

On Mon, May 29, 2017 at 11:14:08PM +0200, Greg Kurz wrote:
> On Thu, 25 May 2017 13:51:25 +1000
> David Gibson <address@hidden> wrote:
> 
> > Guests of the qemu machine type go through a feature negotiation process
> > known as "client architecture support" (CAS) during early boot.  This does
> > a number of things, one of which is finding a CPU compatibility mode which
> > can be supported by both guest and host.
> > 
> > In fact the CPU negotiation is probably the single most complex part of the
> > CAS process, so this splits it out into a helper function.  We've recently
> > made some mistakes in maintaining backward compatibility for old machine
> > types here.  Splitting this out will also make it easier to fix this.
> > 
> > This also adds a possibly useful error message if the negotiation fails
> > (i.e. if there isn't a CPU mode that's suitable for both guest and host).
> > 
> > Signed-off-by: David Gibson <address@hidden>
> > Reviewed-by: Laurent Vivier <address@hidden>
> > Reviewed-by: Greg Kurz <address@hidden>
> > ---
> 
> Any reason for not seing these patches as well in this pull request ?
> 
> pseries: Restore PVR negotiation logic for  pre-2.9 machine types
> pseries: Improve tracing of CPU  compatibility negotiation

Yes.  After more discussion; and comparison with analogous x86 cases
that came up with Igor's NUMA cleanups, I've decided that the
behaviour here while guest visible comes under the heading of a
firmware behaviour change, which we don't typically arrange 100%
matching behaviour for.  Meanwhile, I also found out more things that
suggest matching old behaviour correctly is going to be even messier
than I though.

So, I've decided that leaving the behaviour change in place is the
better course.  Note that it won't affect migration (at least after
the other compat/migration fixes are merged).

I'll reconsider if we observe a real problem in the wild with it.

-- 
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_!
http://www.ozlabs.org/~dgibson

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