Certain instructions need intercepting and emulating by Xen. In some cases Xen emulates the instruction by replaying it, using an executable stub. Some instructions may raise an exception, which is supposed to be handled gracefully.

Certain replayed instructions have additional logic to set up and recover the changes to the arithmetic flags. For replayed instructions where the flags recovery logic is used, the metadata for exception handling was incorrect, preventing Xen from handling the exception gracefully, treating it as fatal instead.