AMD CPU erratum 793 "Specific Combination of Writes to Write Combined Memory Types and Locked Instructions May Cause Core Hang" describes a situation under which a CPU core may hang.
A malicious guest administrator can mount a denial of service attack affecting the whole system.
The vulnerability is applicable only to family 16h model 00h-0fh AMD CPUs. Such CPUs running Xen versions 3.3 onwards are vulnerable. We have not checked earlier versions of Xen. HVM guests can always exploit the vulnerability if it is present. PV guests can exploit the vulnerability only if they have been granted access to physical device(s). Non-AMD CPUs are not vulnerable. CREDITS This issue's security impact was discovered by Jan Beulich.