Upstream commit 7ecec8503af37de6be4f96b53828d640a968705f
("xen/setup: Don't relocate p2m over existing one") should be
considered for stable kernels from 4.3 up.
Machines showing the critical memory configuration seem to be
more common these days making it impossible to boot Xen with
a kernel 4.3-4.9 as dom0 without taking special measures.
Juergen