The Xen Project reports:
VMX refuses attempts to enter a guest with an instruction pointer which doesn't satisfy certain requirements. In particular, the instruction pointer needs to be canonical when entering a guest currently in 64-bit mode. This is the case even if the VM entry information specifies an exception to be injected immediately (in which case the bad instruction pointer would possibly never get used for other than pushing onto the exception handler's stack). Provided the guest OS allows user mode to map the virtual memory space immediately below the canonical/non-canonical address boundary, a non-canonical instruction pointer can result even from normal user mode execution. VM entry failure, however, is fatal to the guest. Malicious HVM guest user mode code may be able to crash the guest.