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[Qemu-ppc] CPU hotplug, again

From: David Gibson
Subject: [Qemu-ppc] CPU hotplug, again
Date: Tue, 23 Feb 2016 16:24:31 +1100
User-agent: Mutt/1.5.24 (2015-08-30)

Hi Andreas,

I've now found (with Thomas' help) your RFC series for socket/core
based cpu hotplug on x86
(https://github.com/afaerber/qemu-cpu/compare/qom-cpu-x86).  It seems
sensible enough as far as it goes, but doesn't seem to address a bunch
of the things that I was attempting to do with the cpu-package
proposal - and which we absolutely need for cpu hotplug on Power.

1) What interface do you envisage beyond cpu_add?

The patches I see just construct extra socket and core objects, but
still control hotplug (for x86) through the cpu_add interface.  That
interface is absolutely unusable on Power, since it operates on a
per-thread basis, whereas the PAPR guest<->host interfaces can only
communicate information at a per-core granularity.

2) When hotplugging at core or socket granularity, where would the
   code to construct the individual thread objects sit?

Your series has the construction done in both the machine init path
and the hotplug path.  The latter works because hotplug occurs at
thread granularity.  If we're hotplugging at core or socket
granularity what would do the construct?  The core/socket object
itself (in instance_init?  in realize?); the hotplug handler?
something else?

3) How does the management layer determine what is pluggable?

Both the number of pluggable slots, and what it will need to do to
populate them.

4) How do we enforce that toplogies illegal for the platform can't be

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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