Various Intel CPU models have an erratum listed under the title "Processor May Hang When Executing Code In an HLE Transaction". It describes a potential hang when using instructions with the XACQUIRE prefix on the host physical memory range covering the first 4 MiB starting at the 1GiB boundary. A malicious or buggy guest may cause a CPU to hang, resulting in a DoS (Denial of Service) affecting the entire host. References: https://xenbits.xen.org/xsa/advisory-282.html