On Mon, Jul 18, 2016 at 04:01:18PM +0200, Peter Krempa wrote: > On Mon, Jul 18, 2016 at 19:19:20 +1000, David Gibson wrote: > > We've recently added a new device_add based cpu hotplug > > implementation, with the spapr machine type being the first user. In > > order to overcome the limitations of the old cpu_add interface, it > > works very differently. That's going to require a new interface in > > libvirt to properly use the new interface in qemu, which will in turn > > require work further up the stack to make use of it. > > > > While that's certainly necessary in the long run, it would be nice if > > we could have at least something working with the old cpu_add > > interface. This patch is an attempt to do so. > > > > This patch implements cpu-add on machines which don't support it > > directly but which do support the query-hotpluggable-cpus interface. > > Essentially, > > cpu-add <n> > > will be treated as a device_add of the n-th entry in the list provided > > by query-hotpluggable-cpus. > > Does this have the same implications when libvirt will need to re-create > a matching qemu process for migration target? (Will we need to specify > -device cpu,... for every CPU 'entity' added this way on the command > line?) > > When using cpu_add libvirt recreates the process by just increasing the > number of active cpus specified via '-smp'.
As long as the cpus are added "in order" (which is necessary to avoid
other problems anyway), then increasing the value to -smp is
sufficient.
...but working out how much to increase -smp may be an issue. With
Power -smp will need to be increased by #threads for each cpu-add
instead of just 1 as it is now on x86.
> In case where libvirt would need to add the -device entries this would
> create just problems with compatibility. Libvirt needs to add XML
> specification of the cpus in order to allow tracking which cpus need to
> be added on the commandline.
>
> > This means that on spapr cpu-add will add a whole core, not a single
> > vcpu as on x86. That's arguably cheating on the cpu-add semantics, but
> > AFAICT those were never terribly well defined anyway.
>
> Peter
>
--
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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