Xen PV Guest Non-SELFSNOOP CPU Memory Corruption

Xen: PV guest on non-SELFSNOOP CPUs can validate non-coherent L2 pagetable I'm not sure whether there are any major users of unshimmed Xen PV left, but https://xenbits.xen.org/docs/unstable/support-matrix.html says it's still a security-supported usecase for 64-bit guests. Tested on Debian's Xen...