Why invtsc (CPUID_APM_INVTSC) is unmigratable?

[Xiaoyao Li]

I'm wondering why CPUID_APM_INVTSC is set as unmigratable_flags. Could 
anyone explain it?

When the host supports invtsc, it can be exposed to guest.
When the src VM has invtsc exposed, what will forbid it to be migrated 
to a dest that also supports VMs with invtsc exposed?