Re: [libvirt] [PATCH v2] Get the maxvcpus from both the qemuCaps and /dev/kvm

On Thu, 2016-05-05 at 08:52 +0200, Jiri Denemark wrote: > On Wed, May 04, 2016 at 19:27:41 +0200, Andrea Bolognani wrote: > >  > > How about something like > >  > >   <vcpus max='255'> > >     <kvm> > >       <soft_limit>96</soft_limit> > >       <hard_limit>2048</hard_limit> > >     </kvm> > >     <qemu> > >       <hard_limit>255</hard_limit> > >     </qemu> > >   </vcpus> >  > No, domcapabilities are bound to an arch/binary/type/machine-type > combination so providing both kvm and qemu stuff there is wrong. If it's > a result of asking for kvm type, //vcpus@max should contain the kvm > limit. If the type of the domain is qemu, it should contain qemu limit. > And in both cases, the limit is constrained to just the machine type > reported in the domcapabilities XML. The fact is that the QEMU limit *does* influence the actual limit for KVM guests: see the example above, taken from a POWER host, where KVM would happily create 2048 vCPUs but QEMU is limited to 255. I didn't say it explicitly, but it was implied that QEMU limits would only be present for QEMU and KVM guests, and KVM limits only for KVM guests. I guess we could skip all the details and just give the user   <vcpus max='255' suggested='96'/> so they don't have to implement the logic themselves. But reporting the KVM limits without taking QEMU limits into consideration is not the way to go, I think.