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FYI, there is a fun behaviour captured in the sentence: "If FRED virtualization is enabled, NMI virtualization must be enabled in order to properly handle guest NMIs" i.e. hypervisors need to make sure not run the guest with FRED && !vNMI.  AIUI, there's no ERETUx intercept similar to the IRET intercept with with to emulate NMI window tracking in !vNMI mode. I requested that this become a VMRUN consistency check, but was declined.  I've asked that at least the wording change to "undefined behaviour" so something sane can be done for the nested case where L1 tries to do something daft. There are two other issues which are going to be adjusted.  One is the consistency check concerning SS.DPL==3 && INTR_SHADOW==0 (not a valid restriction in later drafts of the spec), and one is "On an intercepted #DB, EXITINFO2 has DR6 register value."  Both do/will (not sure which) behave like Intel, rather than as currently documented. ~Andrew