Index: /trunk/src/VBox/Runtime/r0drv/linux/the-linux-kernel.h
===================================================================
--- /trunk/src/VBox/Runtime/r0drv/linux/the-linux-kernel.h	(revision 55858)
+++ /trunk/src/VBox/Runtime/r0drv/linux/the-linux-kernel.h	(revision 55859)
@@ -379,4 +379,20 @@
 #endif
 
+/** @name Macros for preserving EFLAGS.AC on 3.19+/amd64  paranoid.
+ * The AMD 64 switch_to in macro in arch/x86/include/asm/switch_to.h stopped
+ * restoring flags.
+ * @{ */
+#ifdef CONFIG_X86_SMAP
+# define IPRT_X86_EFL_AC                    RT_BIT(18)
+# define IPRT_LINUX_SAVE_EFL_AC()           RTCCUINTREG fSavedEfl = ASMGetFlags();
+# define IPRT_LINUX_RESTORE_EFL_AC()        ASMSetFlags(fSavedEfl)
+# define IPRT_LINUX_RESTORE_EFL_ONLY_AC()   ASMSetFlags((ASMGetFlags() & ~IPRT_X86_EFL_AC) | (fSavedEfl & IPRT_X86_EFL_AC))
+#else
+# define IPRT_LINUX_SAVE_EFL_AC()           do { } while (0)
+# define IPRT_LINUX_RESTORE_EFL_AC()        do { } while (0)
+# define IPRT_LINUX_RESTORE_EFL_ONLY_AC()   do { } while (0)
+#endif
+/** @} */
+
 /*
  * There are some conflicting defines in iprt/param.h, sort them out here.
Index: /trunk/src/VBox/Runtime/r0drv/linux/thread-r0drv-linux.c
===================================================================
--- /trunk/src/VBox/Runtime/r0drv/linux/thread-r0drv-linux.c	(revision 55858)
+++ /trunk/src/VBox/Runtime/r0drv/linux/thread-r0drv-linux.c	(revision 55859)
@@ -200,8 +200,10 @@
 {
 #ifdef CONFIG_PREEMPT
+    IPRT_LINUX_SAVE_EFL_AC(); /* paranoia */
     AssertPtr(pState);
     Assert(pState->u32Reserved == 42);
     RT_ASSERT_PREEMPT_CPUID_RESTORE(pState);
     preempt_enable();
+    IPRT_LINUX_RESTORE_EFL_ONLY_AC();  /* paranoia */
 
 #else