Re: [RFC PATCH 00/12] qemu: add support to hot-plug/unplug cpu device

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On Wed, 2015-01-21 at 09:49 +0100, Peter Krempa wrote:
> On Wed, Jan 21, 2015 at 16:00:52 +0800, Zhu Guihua wrote:
> > If you apply the folowing patchset
> > [PATCH v3 0/7] cpu: add device_add foo-x86_64-cpu support
> > https://lists.nongnu.org/archive/html/qemu-devel/2015-01/msg01552.html,
> > and [PATCH v2 00/11] cpu: add i386 cpu hot remove support
> > https://lists.nongnu.org/archive/html/qemu-devel/2015-01/msg01557.html,
> > qemu can support hotplug and hot-unplug cpu device.
> > 
> > So this patch series will make libvirt support hotplug and hot-unplug cpu
> > device for qemu driver, and now only supports one cpu driver which is
> > 'qemu64-x86_64-cpu'.
> > 
> > The cpu device's xml like this:
> > <cpu driver='qemu64-x86_64-cpu' apic_id='3'>
> >
> 
> Libvirt already implements vCPU hotplug and unplug via
> virDomainSetVcpusFlags() API and implements this API also for the qemu
> driver. Granted that the existing code uses older comands but that can
> be tweaked to support the new stuff too.
> 

Now Libvirt only supported vCPU hotplug by qemu command 'cpu-add', but
vCPU hot-unplug has not been implemented for qemu driver.

> Additionally the new <cpu> subelement of <devices> doesn't seem
> necessary:
> 
> - There's only one model that can be plugged

there are more model will be plugged in next version.

> - Configuring the APIC id doesn't make much sense for users

I agree. I will not consider the APIC id in future.

> - nothing else can be configured
> 

Yes, only cpu model shoud be configured.

> I have following questions though:
> 
> Is there a difference in specifying the cpu via the -device option and
> via the legacy -cpu argument?
> 

Of course, -device has more functions than -cpu.

> Is there a plan to have more than one cpu "model" that can be plugged?
> 

Yes, all x86 cpu model will be supported. This is an RFC patchset, so
the feature is realized in a simple way.

> Can more than one cpu model be plugged to the same VM at the same time?
> 

Yes, it can.

> In general I don't think it's necessary to add the new device type and
> we should rather improve the existing API to support the new device
> type.
> 

I think it's necessary to support more cpu model. When user hotplug a
cpu, it is essential to point the cpu model.

And I think improve the existing API would change too much. 
The feature is implemented by command 'device_add', so I think it should
also keep consistent with the other devices' hotplug.

Regards,
Zhu

> More comments will be inline in the patches.
> 
> Peter


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