What may be needed is making sure X86_FEATURE_SME is not set for PV
guests.
And that may be something that Xen will need to control through either
CPUID or MSR support for the PV guests.
Only on newer versions of Xen. On earlier versions (2-3 years old) leaf
0x80000007 is passed to the guest unchanged. And so is MSR_K8_SYSCFG.
-boris