The compatibility mode hypercall argument translation in Xen 3.3.x through 4.4.x, when running on a 64-bit hypervisor, allows local 32-bit HVM guests to cause a denial of service (host crash) via vectors involving altering the high halves of registers while in 64-bit mode.
https://security.gentoo.org/glsa/201504-04
http://xenbits.xen.org/xsa/advisory-111.html
http://www.securityfocus.com/bid/71332
http://www.debian.org/security/2015/dsa-3140
http://support.citrix.com/article/CTX201794
http://support.citrix.com/article/CTX200288
http://secunia.com/advisories/62672
http://secunia.com/advisories/59937
http://lists.opensuse.org/opensuse-security-announce/2015-02/msg00010.html
http://lists.opensuse.org/opensuse-security-announce/2015-02/msg00005.html