CVE 2015-8555
Xen 4.6.x, 4.5.x, 4.4.x, 4.3.x, and earlier do not initialize x86 FPU stack and XMM registers when XSAVE/XRSTOR are not used to manage guest extended register state, which allows local guest domains to obtain sensitive information from other domains via unspecified vectors.
See the 
CVE page on Mitre.org
for more details.
