https://github.com/VirtualBox/virtualbox/commit/efa378dadd192af8fbce331e9ec66fb36d65ad3d
https://bugs.gentoo.org/967656

From efa378dadd192af8fbce331e9ec66fb36d65ad3d Mon Sep 17 00:00:00 2001
From: Jack Doherty <jack.doherty@oracle.com>
Date: Mon, 10 Nov 2025 21:28:06 +0000
Subject: [PATCH] Network/NAT: Fix high CPU usage when using NAT. github:gh-356

svn:sync-xref-src-repo-rev: r171190
--- a/src/VBox/Devices/Network/DrvNAT.cpp
+++ b/src/VBox/Devices/Network/DrvNAT.cpp
@@ -302,7 +302,6 @@ static DECLCALLBACK(void) drvNATRecvWorker(PDRVNAT pThis, void *pBuf, size_t cb)
     rc = RTCritSectLeave(&pThis->DevAccessLock);
     AssertRC(rc);
     ASMAtomicDecU32(&pThis->cPkts);
-    drvNATNotifyNATThread(pThis, "drvNATRecvWorker");
     STAM_PROFILE_STOP(&pThis->StatNATRecv, a);
 }
 
@@ -562,7 +561,7 @@ static DECLCALLBACK(void) drvNATNetworkUp_EndXmit(PPDMINETWORKUP pInterface)
  */
 static void drvNATNotifyNATThread(PDRVNAT pThis, const char *pszWho)
 {
-    RT_NOREF(pszWho);
+    Log3(("Notifying NAT Thread. Culprit: %s\n", pszWho));
 #ifdef RT_OS_WINDOWS
     int cbWritten = send(pThis->ahWakeupSockPair[0], "", 1, NULL);
     if (RT_LIKELY(cbWritten != SOCKET_ERROR))
@@ -713,6 +712,7 @@ static DECLCALLBACK(int) drvNATAsyncIoThread(PPDMDRVINS pDrvIns, PPDMTHREAD pThr
         uint32_t cMsTimeout = DRVNAT_DEFAULT_TIMEOUT;
         slirp_pollfds_fill_socket(pThis->pSlirp, &cMsTimeout, drvNAT_AddPollCb /* SlirpAddPollCb */, pThis /* opaque */);
         cMsTimeout = drvNATTimersAdjustTimeoutDown(pThis, cMsTimeout);
+        Log4Func(("Timeout adjust to: %d\n", cMsTimeout));
 
 #ifdef RT_OS_WINDOWS
         int cChangedFDs = WSAPoll(pThis->aPolls, pThis->cSockets, cMsTimeout);
@@ -721,6 +721,7 @@ static DECLCALLBACK(int) drvNATAsyncIoThread(PPDMDRVINS pDrvIns, PPDMTHREAD pThr
 #endif
         if (RT_LIKELY(!(cChangedFDs >= 0)))
         {
+            Log4Func(("Poll error\n"));
 #ifdef RT_OS_WINDOWS
             int const iLastErr = WSAGetLastError(); /* (In debug builds LogRel translates to two RTLogLoggerExWeak calls.) */
             LogRel(("NAT: RTWinPoll returned error=%Rrc (cChangedFDs=%d)\n", iLastErr, cChangedFDs));
@@ -737,6 +738,7 @@ static DECLCALLBACK(int) drvNATAsyncIoThread(PPDMDRVINS pDrvIns, PPDMTHREAD pThr
 
         Log4Func(("poll\n"));
         slirp_pollfds_poll(pThis->pSlirp, cChangedFDs < 0, drvNAT_GetREventsCb, pThis /* opaque */);
+        Log4Func(("management pipe revents = %d\n", pThis->aPolls[0].revents));
 
         /*
          * Drain the control pipe if necessary.
@@ -747,8 +749,9 @@ static DECLCALLBACK(int) drvNATAsyncIoThread(PPDMDRVINS pDrvIns, PPDMTHREAD pThr
          *       control pipe of all the notifications. If there is an error
          *       on reading the pipe, we try again next time around.
          */
-        if (pThis->aPolls[0].revents & (POLLRDNORM|POLLPRI|POLLRDBAND))   /* POLLPRI won't be seen with WSAPoll. */
+        if (pThis->aPolls[0].revents & (POLLIN|POLLRDNORM|POLLPRI|POLLRDBAND))   /* POLLPRI won't be seen with WSAPoll. */
         {
+            Log4(("Draining control pipe.\n"));
             char achBuf[1024];
             size_t cbRead = 0;
             uint64_t cbWakeupNotifs = ASMAtomicReadU64(&pThis->cbWakeupNotifs);
@@ -791,6 +794,7 @@ static DECLCALLBACK(int) drvNATAsyncIoThread(PPDMDRVINS pDrvIns, PPDMTHREAD pThr
  */
 static DECLCALLBACK(int) drvNATAsyncIoWakeup(PPDMDRVINS pDrvIns, PPDMTHREAD pThread)
 {
+    LogFlowFuncEnter();
     RT_NOREF(pThread);
     PDRVNAT pThis = PDMINS_2_DATA(pDrvIns, PDRVNAT);
 
@@ -1387,6 +1391,7 @@ static void drvNAT_TimerModCb(void *pvTimer, int64_t msNewDeadlineTs, void *pvUs
  */
 static void drvNAT_NotifyCb(void *opaque)
 {
+    LogFlowFuncEnter();
     PDRVNAT pThis = (PDRVNAT)opaque;
     drvNATNotifyNATThread(pThis, "drvNAT_NotifyCb");
 }