CMPXCHG8B emulation in Xen 3.3.x through 4.7.x on x86 systems allows local HVM guest OS users to obtain sensitive information from host stack memory via a "supposedly-ignored" operand size prefix.
https://support.citrix.com/article/CTX219378
https://security.gentoo.org/glsa/201612-56
http://xenbits.xen.org/xsa/advisory-200.html
http://www.securitytracker.com/id/1037468