[Bug 230773] GDT limit needs reset on VMX exit

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=230773

            Bug ID: 230773
           Summary: GDT limit needs reset on VMX exit
           Product: Base System
           Version: CURRENT
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: levon at movementarian.org

Intel vol 3 27.5.2:

"The base addresses for GDTR and IDTR are loaded from the GDTR base-address
field and the IDTR base-address field, respectively. If the processor supports
the Intel 64 architecture and the processor supports N < 64 linear-address
bits, each of bits 63:N of each base address is set to the value of bit N–1 of
that base address. The GDTR and IDTR limits are each set to FFFFH."

As far as I could tell, bhyve doesn't do this in current FreeBSD, leaving the
GDT limit significantly larger than the previous NGDT-based one.

We found and fixed this in our port as seen here:

https://github.com/joyent/illumos-joyent/commit/1d0338f3f33eec2ed45ec5a6cae30c399a3ef769#diff-5b270bdc09d0e88fd39cfed0ccd0c44b

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