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://security.gentoo.org/glsa/201612-56
https://support.citrix.com/article/CTX219378
http://www.debian.org/security/2017/dsa-3847