Use of Process Context Identifiers (PCID) was introduced into Xen in
order to improve performance after XSA-254 (and in particular its
Meltdown sub-issue).  This enablement implied changes to the TLB
flushing logic.  One aspect which was overlooked is the safety of
switching between shadow pagetables, which previously relied on the
unconditional flushing of a write to CR3.

With PCID enabled, a switch of shadow pagetable for a 64bit PV guest
fails to invalidate the linear mappings of the previous shadow
pagetable.  As a result, subsequent accesses to the shadow pagetables
may be deemed to be safe by the shadow logic (based on the old shadow
pagetable) but fault when made in practice.