Xen 4.4.x, when running on ARM systems, does not properly restrict access to hardware features, which allows local guest users to cause a denial of service (host or guest crash) via unspecified vectors, related to (1) cache control, (2) coprocessors, (3) debug registers, and (4) other unspecified registers.
http://xenbits.xen.org/xsa/advisory-93.html
http://www.securitytracker.com/id/1030135