According to this old thread: Guest systems sees CPU of the Host - #17 by nurmagoz
@nurmagoz results show Xen and VBox accessing the majority of host flags. Test by diffing the Dom0/DomU output for cat /proc/cpuinfo
A reference on the inability of guests to manipulate microcode. Different from the paper I read a long time ago and too lazy to dig up.
b) “Qubes and VirtualBox need CPU passthrough enabled, and microcode packages installed, otherwise they’d be vulnerable.”
Yes and I’d add: microcode on Dom0/host (which is not a virtualized Whonix’s concern). Whonix physical build needs the microcode.
Reference is post above with select quotes:
Host passthrough
This is the recommended CPU to use, provided live migration is not required.
The following are important CPU features that should be used on Intel x86 hosts, when available in the host CPU. … In general all of these features are included if using “Host passthrough” or “Host model”.
On kvm64:
their usage is discouraged, as they expose a very limited featureset, … First as mentioned kvm64 is an ancient CPU model, pretty much awful as qemu64. If they [guest kernels] see kvm64 they’ll think the CPU is so old that no mitigations are needed, even if you did turn on spec-ctrl feature. So the guest OS will likely remain vulnerable.