OSX VM limited to 32 vcpu's ?

Everywhere i read that OSX VM's can handle up to 64 vcpu's and QEMU itself way above that, but whatever i try i can not get anything above 32 cores to work.

If i assign 40 vcores , i have to remove the topology line otherwise it wont boot at all above 32. But then once booted if i start anything serious or even a benchmark tool , it dies immediately with this error : malloc: nano zone abandoned because NCPUS mismatch.

If i drop down to 32 all is fine.

Any ideas ?

I am running of a supermicro board with dual 2697v2 xeons

This is the max that will work. Boot fast and all works as expected on any version of OSX i tried from Sierra up till Mojave:

--- cut ---

<vcpu placement='static'>32</vcpu>
  <cputune>
    <vcpupin vcpu='0' cpuset='4'/>
    <vcpupin vcpu='1' cpuset='28'/>
    <vcpupin vcpu='2' cpuset='5'/>
    <vcpupin vcpu='3' cpuset='29'/>
    <vcpupin vcpu='4' cpuset='6'/>
    <vcpupin vcpu='5' cpuset='30'/>
    <vcpupin vcpu='6' cpuset='7'/>
    <vcpupin vcpu='7' cpuset='31'/>
    <vcpupin vcpu='8' cpuset='8'/>
    <vcpupin vcpu='9' cpuset='32'/>
    <vcpupin vcpu='10' cpuset='9'/>
    <vcpupin vcpu='11' cpuset='33'/>
    <vcpupin vcpu='12' cpuset='10'/>
    <vcpupin vcpu='13' cpuset='34'/>
    <vcpupin vcpu='14' cpuset='11'/>
    <vcpupin vcpu='15' cpuset='35'/>
    <vcpupin vcpu='16' cpuset='16'/>
    <vcpupin vcpu='17' cpuset='40'/>
    <vcpupin vcpu='18' cpuset='17'/>
    <vcpupin vcpu='19' cpuset='41'/>
    <vcpupin vcpu='20' cpuset='18'/>
    <vcpupin vcpu='21' cpuset='42'/>
    <vcpupin vcpu='22' cpuset='19'/>
    <vcpupin vcpu='23' cpuset='43'/>
    <vcpupin vcpu='24' cpuset='20'/>
    <vcpupin vcpu='25' cpuset='44'/>
    <vcpupin vcpu='26' cpuset='21'/>
    <vcpupin vcpu='27' cpuset='45'/>
    <vcpupin vcpu='28' cpuset='22'/>
    <vcpupin vcpu='29' cpuset='46'/>
    <vcpupin vcpu='30' cpuset='23'/>
    <vcpupin vcpu='31' cpuset='47'/>
    <emulatorpin cpuset='0-1,24-25'/>
  </cputune>

 <cpu mode='host-passthrough' check='none'/> 

--- end cut ---