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