Index: /trunk/src/VBox/Devices/PC/DevACPI.cpp
===================================================================
--- /trunk/src/VBox/Devices/PC/DevACPI.cpp	(revision 68244)
+++ /trunk/src/VBox/Devices/PC/DevACPI.cpp	(revision 68245)
@@ -484,4 +484,10 @@
     /** SMBus Host Block Index */
     uint8_t             u8SMBusBlkIdx;
+
+    /** @todo DEBUGGING */
+    uint32_t            uPmTimerOld;
+    uint32_t            uPmTimerA;
+    uint32_t            uPmTimerB;
+    uint32_t            Alignment5;
 } ACPIState;
 
@@ -1756,4 +1762,17 @@
     Log(("acpi: acpiPMTmrRead -> %#x\n", *pu32));
 
+    /** @todo temporary: sanity check against running backwards */
+    uint32_t uOld = ASMAtomicXchgU32(&pThis->uPmTimerOld, *pu32);
+    if (*pu32 - uOld >= 0x10000000)
+    {
+#if defined(IN_RING0)
+        pThis->uPmTimerA = uOld;
+        pThis->uPmTimerB = *pu32;
+        return VERR_TM_TIMER_BAD_CLOCK;
+#elif defined(IN_RING3)
+        AssertReleaseMsgFailed(("acpiPMTmrRead: old=%08RX32, current=%08RX32\n", uOld, *pu32));
+#endif
+    }
+
     NOREF(pvUser); NOREF(Port);
     return rc;
@@ -1761,4 +1780,12 @@
 
 #ifdef IN_RING3
+
+static DECLCALLBACK(void) acpiR3Info(PPDMDEVINS pDevIns, PCDBGFINFOHLP pHlp, const char *pszArgs)
+{
+    RT_NOREF(pszArgs);
+    ACPIState *pThis = PDMINS_2_DATA(pDevIns, ACPIState *);
+    pHlp->pfnPrintf(pHlp,
+                    "timer: old=%08RX32, current=%08RX32\n", pThis->uPmTimerA, pThis->uPmTimerB);
+}
 
 /**
@@ -4081,4 +4108,6 @@
    else
        return PDMDEV_SET_ERROR(pDevIns, rc, N_("Failed to attach LUN #0"));
+
+    PDMDevHlpDBGFInfoRegister(pDevIns, "acpi", "ACPI info", acpiR3Info);
 
     return rc;