9723 provide support for VMM's GDT handling
Reviewed by: Robert Mustacchi <rm@joyent.com>
Reviewed by: Patrick Mooney <patrick.mooney@joyent.com>

@@ -1288,11 +1288,33 @@
         kpti_safe_cr3 = (uint64_t)getcr3();
 }
 
 #endif  /* __xpv */
 
+#ifndef __xpv
 /*
+ * As per Intel Vol 3 27.5.2, the GDTR limit is reset to 64Kb on a VM exit, so
+ * we have to manually fix it up ourselves.
+ *
+ * The caller may still need to make sure that it can't go off-CPU with the
+ * incorrect limit, before calling this (such as disabling pre-emption).
+ */
+void
+reset_gdtr_limit(void)
+{
+        ulong_t flags = intr_clear();
+        desctbr_t gdtr;
+
+        rd_gdtr(&gdtr);
+        gdtr.dtr_limit = (sizeof (user_desc_t) * NGDT) - 1;
+        wr_gdtr(&gdtr);
+
+        intr_restore(flags);
+}
+#endif /* __xpv */
+
+/*
  * In the early kernel, we need to set up a simple GDT to run on.
  *
  * XXPV Can dboot use this too?  See dboot_gdt.s
  */
 void