Re: [Qemu-devel] [RFC PATCH v1 00/13] CPU and Memory hotplug for PowerPC guests

["Andreas Färber" <afaerber@suse.de>]

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Hi,

Am 08.01.2015 um 07:10 schrieb Bharata B Rao:
> This patchset enables CPU and memory hotplug support for PowerPC guests.
> 
> Changes in this patchset (v1):
> 
> - Based on top of Michael Roth's tree
>   (https://github.com/mdroth/qemu/commits/spapr-hotplug-core) which serves
>   as base for his PCI hotplug patches too.
> - Switched to device_add/del semantics instead of cpu-add.

Please don't forget to CC me on this. As previously discussed with Jason
and Christian for s390x, there's certain topology modeling questions
still unsolved for device-based QOM CPU hot-plug. I have an RFC in the
works (again) that hopefully gets us a step closer.

> - Supporting CPU hot unplug now.
> - Added patches to enable memory hotplug.
> - Added ibm,dynamic-reconfiguration-memory support which is needed for
>   memory hotplug.
> 
> v0 - http://lists.nongnu.org/archive/html/qemu-devel/2014-09/msg00752.html
> 
> CPU hotplug
> -----------
> - Works with BE guest, has issues with LE guest. Has been tested on BE host
>   only.
> - Adding a core (and all its threads) in response to device_add command.
>   Similarly removing a core via device_del will remove all the threads.

Earlier discussions concluded that hot-plug needs to happen on a socket
level, not core. If you're assuming the socket to have one core (as we
were planning for s390x), that doesn't make much of a difference
number-wise, but it does in modeling terms. Think what you can
physically plug onto the mainboard, that's the granularity that
realized=true/false is going to operate on. A virtual socket may well
correspond to a thread on some socket/node of the host, but you cannot
add threads to a core or cores to a chip at runtime.

On x86 this may become a functional limitation of what is possible
through cpu-add, so better avoid that mistake for ppc from the start.

Regards,
Andreas

> - Using Gu Zheng's "reclaim vCPU objects" patch to remove and reuse
>   the vCPU objects after CPUs removal.
>   (Gu Zheng's original patch:
>    http://lists.gnu.org/archive/html/qemu-devel/2014-12/msg01829.html)
[snip]

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