[libvirt-users] setvcpus libvirt 1.2.9

Eric Blake eblake at redhat.com
Wed Nov 5 14:05:00 UTC 2014

On 11/05/2014 02:56 PM, Thomas Stein wrote:
> Am 05.11.14 14:48, schrieb Eric Blake:
>> On 11/05/2014 01:20 PM, Thomas Stein wrote:
>>> Am 05.11.14 11:48, schrieb Martin Kletzander:
>>>> On Wed, Nov 05, 2014 at 11:35:37AM +0100, Thomas Stein wrote:
>>>>> Hello.
>>>>> I'm struggeling a little bit with the setvcpus command. I'm trying now
>>>>> to get this running for a linux VM. Here is what i have:
>>>>> <vcpu placement='auto' current='2'>4</vcpu>
>> This is one case where xen may be a bit more powerful than current qemu
>> support.  Xen supports booting a guest with a promise of a maximum cpu
>> larger than what it boots with; but I don't know that qemu does that
>> well yet.  For now, with qemu, you pretty much have to boot with all
>> cpus enabled, then after the guest is up and running, disable the cpus
>> you don't plan to use right away.  CPU hotplug/hot-unplug is still a
>> work in progress in qemu (for example, some versions of qemu support
>> hotplug but not hot-unplug).  I haven't played much with it myself to
>> know what minimum version of libvirt and qemu will do what you want, so
>> you may have to do some experimentation.
> Thanks. I let you know if my experiments lead to some discoveries.

In fact, one of the reasons that 'setvcpus --guest' was added in the
guest agent was precisely because direct qemu support for cpu hot-unplug
was not ready yet, and faking things by requiring the guest to do the
work instead of qemu was an attempt to paper around the lack of qemu
support.  However, guest unplug is different than qemu unplug on several
fronts; for instance, rebooting the guest will forget what settings the
previous run had been using (that is, libvirt will NOT update XML to
reflect guest settings, because guest interaction is assumed to be
untrusted; while if qemu hot-unplug works, libvirt would update the XML
to match).

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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