The adjustments made for XSA-379 as well as those subsequently becoming
XSA-387 still left a race window, when a HVM or PVH guest does a grant
table version change from v2 to v1 in parallel with mapping the status
page(s) via XENMEM_add_to_physmap. Some of the status pages may then be
freed while mappings of them would still be inserted into the guest's
secondary (P2M) page tables.