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. The particular case of context switch to a vCPU of a
PCID-enabled guest left open a time window between the full TLB flush,
and the actual address space switch, during which additional TLB
entries (from the address space about to be switched away from) can be
accumulated, which will not subsequently be purged.