vm vcpu uses isolated cores

Hello, I am trying to understand why qemu vm CPU threads uses isolated cpus. I have a host which isolates some cpus using isolcpu like isolcpus=1,2,3,4,5,7,8,9,10,11. unfortunately, vcpupin does not mask out these cpus (vcpupin is still something like ffffffff). When I log in to the system, seems qemu cpu thread only runs on these isolcpus. I do not quite understand this behavior, because I think by using isolcpu, kernel schedule will exclude these cpus and thus vcpu thread shouldn't use these cores unless taskset explicitly got called.. So my question is how does cpu thread got scheduled on isolated cpus? Thank you. -- Best Regards, Jiatong Shen