CPU Mode and Nested VMs

Hi guys, thank you in advance for your thoughts!

I have a Ubuntu KVM/VM client running on unraid, it in turn is a VirtualBox host which then has several VMs running on that. This all works fine for the most part, however I do have a question regarding virtualization of the CPU.

BTW, I tried getting VirtualBox running natively on Unraid with little success, I think I read all the post for this here, plus posts from other sources and still never got it to work. So, this is why I have this setup in the first place. If anyone has VirtualBox running on unraid, please let me know.

I first started out with "Host Passthrough" which (i would think) exposed the CPU to the Ubuntu Client, which in turn exposed that to the VirtualBox clients. For this test, I had 8 of my 8 cores allocated to the Ubuntu VM and have allocated a single core to the VirtualBox Client.

I noticed that when a started a process within a VirtualBox windows Client, it pinned one of the processors on the unraid box. To me, this behavior makes sense because it is just using the "passed-through" cores as if it were naively owned by the VM client and it is simply using only one core because I only allocated one core to the VirtualBox client.

What would be nice is if I allocation 1 core to the VirtualBox client that it would use 1/8th the processing power spread across the 8 host cores I have allocated. I thought that, in order for this to work that way, I would have to not pass through the CPU and virtualize the CPU layer. I changed my Ubuntu VM settings to do this, I allocated 8 cores to it but it is not set to "qemu64"

  <cpu mode='custom' match='exact' check='none'>
    <model fallback='forbid'>qemu64</model>
  </cpu>
 

for fun I also added <vcpu placement='static'>32</vcpu> to give me 32 virtual cores

so, my thinking was that if this is truly virtual that if I allocate just 4 of the 32 cores (1/8th again) to a VirtualBox client, that when is runs something, it would use 4/32 (or 1/8) of the 8 "real" cores and the workload would be spread across the 8 real cores.

This does not seem to be what is happening, when I run a process on the VirtualBox client and watch the usage on the unraid server, it is still pinning 1 core and all the others seem to be pretty much idle.

This could just be how qemu64 CPU virtualization layer is coded or maybe I am misunderstanding what CPU virtualization is completely.

Any input would be helpful.