Index: /trunk/src/VBox/VMM/VMMAll/IEMAll.cpp
===================================================================
--- /trunk/src/VBox/VMM/VMMAll/IEMAll.cpp	(revision 74708)
+++ /trunk/src/VBox/VMM/VMMAll/IEMAll.cpp	(revision 74709)
@@ -434,4 +434,10 @@
     do { return iemVmxVmexitTaskSwitch((a_pVCpu), (a_enmTaskSwitch), (a_SelNewTss)); } while (0)
 
+/**
+ * Invokes the VMX VM-exit handler for MWAIT.
+ */
+# define IEM_VMX_VMEXIT_MWAIT_RET(a_pVCpu, a_fMonitorArmed, a_cbInstr) \
+    do { return iemVmxVmexitInstrMwait((a_pVCpu), (a_fMonitorArmed), (a_cbInstr)); } while (0)
+
 #else
 # define IEM_VMX_IS_ROOT_MODE(a_pVCpu)                                  (false)
@@ -440,7 +446,8 @@
 # define IEM_VMX_IS_PROCCTLS_SET(a_pVCpu, a_cbInstr)                    (false)
 # define IEM_VMX_IS_PROCCTLS2_SET(a_pVCpu, a_cbInstr)                   (false)
-# define IEM_VMX_VMEXIT_TASK_SWITCH_RET(a_pVCpu, a_enmTaskSwitch, a_SelNewTss)  do { return VERR_VMX_IPE_1; } while (0)
 # define IEM_VMX_VMEXIT_INSTR_RET(a_pVCpu, a_uExitReason, a_cbInstr)            do { return VERR_VMX_IPE_1; } while (0)
 # define IEM_VMX_VMEXIT_INSTR_NEEDS_INFO_RET(a_pVCpu, a_uExitReason, a_uInstrId, a_cbInstr)     do { return VERR_VMX_IPE_1; } while (0)
+# define IEM_VMX_VMEXIT_TASK_SWITCH_RET(a_pVCpu, a_enmTaskSwitch, a_SelNewTss)  do { return VERR_VMX_IPE_1; } while (0)
+# define IEM_VMX_VMEXIT_MWAIT_RET(a_pVCpu, a_fMonitorArmed, a_cbInstr)          do { return VERR_VMX_IPE_1; } while (0)
 
 #endif
Index: /trunk/src/VBox/VMM/VMMAll/IEMAllCImpl.cpp.h
===================================================================
--- /trunk/src/VBox/VMM/VMMAll/IEMAllCImpl.cpp.h	(revision 74708)
+++ /trunk/src/VBox/VMM/VMMAll/IEMAllCImpl.cpp.h	(revision 74709)
@@ -7149,9 +7149,14 @@
     }
 
+    /* Check VMX nested-guest intercept. */
+    if (   IEM_VMX_IS_NON_ROOT_MODE(pVCpu)
+        && IEM_VMX_IS_PROCCTLS_SET(pVCpu, VMX_PROC_CTLS_MWAIT_EXIT))
+        IEM_VMX_VMEXIT_MWAIT_RET(pVCpu, EMMonitorIsArmed(pVCpu), cbInstr);
+
     /*
      * Gather the operands and validate them.
      */
-    uint32_t uEax = pVCpu->cpum.GstCtx.eax;
-    uint32_t uEcx = pVCpu->cpum.GstCtx.ecx;
+    uint32_t const uEax = pVCpu->cpum.GstCtx.eax;
+    uint32_t const uEcx = pVCpu->cpum.GstCtx.ecx;
     if (uEcx != 0)
     {
@@ -7171,4 +7176,24 @@
             return iemRaiseGeneralProtectionFault0(pVCpu);
         }
+
+#ifdef VBOX_WITH_NESTED_HWVIRT_VMX
+        /*
+         * If the interrupt-window exiting control is set or a virtual-interrupt is pending
+         * for delivery; and interrupts are disabled the processor does not enter its
+         * mwait state but rather passes control to the next instruction.
+         *
+         * See Intel spec. 25.3 "Changes to Instruction Behavior In VMX Non-root Operation".
+         */
+        if (    IEM_VMX_IS_NON_ROOT_MODE(pVCpu)
+            && !pVCpu->cpum.GstCtx.eflags.Bits.u1IF)
+        {
+            if (   IEM_VMX_IS_PROCCTLS_SET(pVCpu, VMX_PROC_CTLS_INT_WINDOW_EXIT)
+                || VMCPU_FF_IS_PENDING(pVCpu, VMCPU_FF_INTERRUPT_NESTED_GUEST))
+            {
+                iemRegAddToRipAndClearRF(pVCpu, cbInstr);
+                return VINF_SUCCESS;
+            }
+        }
+#endif
     }
 
Index: /trunk/src/VBox/VMM/VMMAll/IEMAllCImplVmxInstr.cpp.h
===================================================================
--- /trunk/src/VBox/VMM/VMMAll/IEMAllCImplVmxInstr.cpp.h	(revision 74708)
+++ /trunk/src/VBox/VMM/VMMAll/IEMAllCImplVmxInstr.cpp.h	(revision 74709)
@@ -3476,4 +3476,23 @@
 
 /**
+ * VMX VM-exit handler for VM-exits due to MWAIT instruction.
+ *
+ * @returns VBox strict status code.
+ * @param   pVCpu               The cross context virtual CPU structure.
+ * @param   fMonitorHwArmed     Whether the address-range monitor hardware is armed.
+ * @param   cbInstr             The instruction length in bytes.
+ */
+IEM_STATIC VBOXSTRICTRC iemVmxVmexitInstrMwait(PVMCPU pVCpu, bool fMonitorHwArmed, uint8_t cbInstr)
+{
+    VMXVEXITINFO ExitInfo;
+    RT_ZERO(ExitInfo);
+    ExitInfo.uReason = VMX_EXIT_MWAIT;
+    ExitInfo.cbInstr = cbInstr;
+    ExitInfo.u64Qual = fMonitorHwArmed;
+    return iemVmxVmexitInstrWithInfo(pVCpu, &ExitInfo);
+}
+
+
+/**
  * VMX VM-exit handler for TPR virtualization.
  *