I gave linux59 5.9.0-1 from unstable a try. The virtualbox modules (dkms) do not compile for it. That is not realy a surprise.
But gnome-boxes, which uses libvirt, is not working either. It does not even show me the boxes I already have setup. When I boot back into kernel 5.8 the boxes are still there, but with kernel 5.9 I do not see them and I can not create new boxes.
the module crashes with kernel 5.9. Without that option, gnome-boxes is running fine.
This is an interesting difference to kernel 5.8. With kernel 5.8 this option is mandatory for Ryzen CPUs otherwise it is not starting the vms with this error:
AMD CPU doesn't support hyperthreading. Please configure -smp options properly.