[libvirt] cpu affinity, isolcpus and cgroups

Hi, i am currently looking into realtime VMs using libvirt. My first starting point was reserving a couple of cores using isolcpus and later tuning the affinity to place my vcpus on the reserved pcpus. My first observation was that libvirt ignores isolcpus. Affinity masks of new qemus will default to all cpus and will not be inherited from libvirtd. A comment in the code suggests that this is done on purpose. After that i changed the code to use only the available cpus by default. But taskset was still showing all 'f's on my qemus. Then i traced my change down to sched_setaffinity assuming that some other mechanism might have reverted my hack, but it is still in place. Libvirt is setting up cgroups and now my suspicion is that cgroups and taskset might not work well together. Looks like the last 16 cores are not used. But if i use taskset to ask for the affinity mask i get all 32 cpus. I know that is not strictly libvirt but also a kernel question, still you guys are probably able to point me to what i am missing here. regards, Henning