Re: [Qemu-devel] status of cpu hotplug work for x86_64?

[Igor Mammedov]

On Fri, 2 May 2014 14:10:35 +0200
Vasilis Liaskovitis <address@hidden> wrote:

> Hi,
> 
> On Mon, Apr 28, 2014 at 11:58:38AM -0600, Chris Friesen wrote:
> > Hi,
> > 
> > I'm trying to figure out what the current status is for cpu hotplug
> > and hot-remove on x86_64.
> > 
> > As far as I can tell, it seems like currently there is a QMP
> > "cpu-add" command but no matching remove...is that correct?
> 
> correct. "cpu-add" is the way to hot-add CPUs.
> There is no support for cpu hot-remove at this point. 
> 
> The latest patchset for cpu hot-remove that I know of is:
> http://lists.gnu.org/archive/html/qemu-devel/2013-12/msg04266.html
> 
> If I understand correctly the biggest hurdle is supporting vcpu destruction
> during the VM lifetime on the kvm host side. The corresponding kvm patches 
> have
> not been accepted:
> http://comments.gmane.org/gmane.comp.emulators.kvm.devel/114347
> 
> So I have the same question as you: Is there any plan to support cpu 
> hot-remove?
Besides above mentioned series and KVM side of the problem there are
several not yet addressed QEMU issues:
  - remove dependency on cpu_index for X86 cpu (arbitrary CPU add/rm)
  - need an interface to specify apic id when adding CPU
    -  clean way would be -device x86-cpu-foo,{apic-id=xxx || 
node=foo,socket=x,core=y,thread=z}
    -  x86 cpu subclasses by Eduardo were merged in the last release so
       on the road to "-device x86-cpu-foo" only conversion to of CPU features 
to properties
       is left (I need to rebase and respin it)
  - fix vmstate register/unregister (migration and arbitrary CPU add/rm)

As for plans QEMU side of work could be done without KVM support first and then
once KVM would be able to unplug VCPUs it could be added to QEMU without much 
issues.


> 
> thanks,
> 
> - Vasilis