[Bug 230773] [bhyve] GDT limit needs reset on VMX exit

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

--- Comment #3 from commit-hook at freebsd.org ---
A commit references this bug:

Author: jhb
Date: Thu Oct 11 18:27:20 UTC 2018
New revision: 339312
URL: https://svnweb.freebsd.org/changeset/base/339312

Log:
  Fully restore the GDTR, IDTR, and LDTR after VT-x VM exits.

  The VT-x VMCS only stores the base address of the GDTR and IDTR.  As a
  result, VM exits use a fixed limit of 0xffff for the host GDTR and
  IDTR losing the smaller limits set in when the initial GDT is loaded
  on each CPU during boot.  Explicitly save and restore the full GDTR
  and IDTR contents around VM entries and exits to restore the correct
  limit.

  Similarly, explicitly save and restore the LDT selector.  VM exits
  always clear the host LDTR as if the LDT was loaded with a NULL
  selector and a userspace hypervisor is probably using a NULL selector
  anyway, but save and restore the LDT explicitly just to be safe.

  PR:           230773
  Reported by:  John Levon <levon at movementarian.org>
  Reviewed by:  kib
  Tested by:    araujo
  Approved by:  re (rgrimes)
  MFC after:    1 week

Changes:
  head/sys/amd64/include/cpufunc.h
  head/sys/amd64/vmm/intel/vmx.c

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