Xen 4.6.x and earlier allows local guest administrators to cause a denial of service (host reboot) via vectors related to multiple mappings of MMIO pages with different cachability settings.
https://security.gentoo.org/glsa/201604-03
http://xenbits.xen.org/xsa/advisory-154.html
http://www.securitytracker.com/id/1035042
http://www.oracle.com/technetwork/topics/security/ovmbulletinjul2016-3090546.html
http://www.debian.org/security/2016/dsa-3519
http://lists.fedoraproject.org/pipermail/package-announce/2016-March/178518.html
http://lists.fedoraproject.org/pipermail/package-announce/2016-February/177990.html