Don't alter existing host-passthrough CPU config. (and possibly other config.?)
I'm using Boxes for nested virtualisation (Boxes on the outer bare metal), and so need the CPU to be configured as "host-passthrough".
It's possible to do this by creating a VM in Boxes, then editing the CPU and starting the VM in virt-manager (or via virsh).
Boxes then happily interacts with the running VM.
However, if the VM is not already running, whenever Boxes is started, it reconfigures the CPU back to a "nearest match" known CPU, breaking nested virtualisation.
This means the VM has to be manually (re-)started outside Boxes every time, or fixed repeatedly (even if Boxes was launched to use a different VM).
I haven't looked to see what else if anything Boxes reconfigures on start, but it would be great if Boxes could either leave alone the CPU config. of an existing VM, or "understand" host-passthrough as valid (which presumably it does for CPU models as opposed to passthrough).