reverted:
--- include/iprt/cdefs.h	(revision 57336)
+++ include/iprt/cdefs.h	(revision 57335)
@@ -949,16 +949,13 @@
 
 /** @def RTCALL
  * The standard calling convention for the Runtime interfaces.
- *
- * @remarks The regparm(0) in the X86/GNUC variant deals with -mregparm=x use in
- *          the linux kernel and potentially elsewhere (3rd party).
  */
 #ifdef _MSC_VER
+# define RTCALL     __cdecl
-# define RTCALL                 __cdecl
 #elif defined(RT_OS_OS2)
+# define RTCALL     __cdecl
+#elif defined(__GNUC__) && defined(IN_RING0) && defined(RT_ARCH_X86) /** @todo consider dropping IN_RING0 here. */
+# define RTCALL     __attribute__((cdecl,regparm(0))) /* regparm(0) deals with -mregparm=x use in the linux kernel. */
-# define RTCALL                 __cdecl
-#elif defined(__GNUC__) && defined(RT_ARCH_X86)
-# define RTCALL                 __attribute__((cdecl,regparm(0)))
 #else
 # define RTCALL
 #endif
@@ -1026,9 +1023,21 @@
  * @param   type    The return type of the function declaration.
  */
 #ifdef __cplusplus
+# if defined(_MSC_VER) || defined(RT_OS_OS2)
+#  define DECLASM(type)          extern "C" type __cdecl
+# elif defined(__GNUC__) && defined(RT_ARCH_X86)
+#  define DECLASM(type)          extern "C" type __attribute__((cdecl,regparm(0)))
+# else
+#  define DECLASM(type)          extern "C" type
+# endif
-# define DECLASM(type)           extern "C" type RTCALL
 #else
+# if defined(_MSC_VER) || defined(RT_OS_OS2)
+#  define DECLASM(type)          type __cdecl
+# elif defined(__GNUC__) && defined(RT_ARCH_X86)
+#  define DECLASM(type)          type __attribute__((cdecl,regparm(0)))
+# else
+#  define DECLASM(type)          type
+# endif
-# define DECLASM(type)           type RTCALL
 #endif
 
 /** @def DECLASMTYPE
@@ -1035,7 +1044,11 @@
  * How to declare an internal assembly function type.
  * @param   type    The return type of the function.
  */
+# if defined(_MSC_VER) || defined(RT_OS_OS2)
+# define DECLASMTYPE(type)      type __cdecl
+#else
+# define DECLASMTYPE(type)      type
+#endif
-#define DECLASMTYPE(type)       type RTCALL
 
 /** @def DECLNORETURN
  * How to declare a function which does not return.
reverted:
--- src/VBox/VMM/VMMR3/PGMDbg.cpp	(revision 57336)
+++ src/VBox/VMM/VMMR3/PGMDbg.cpp	(revision 57335)
@@ -103,8 +103,15 @@
  * @param   cbNeedle        Size of what we're searching for.
  */
 
+#if defined(_MSC_VER) || defined(RT_OS_OS2)
+# define DECLASMCALLBACK(type)          type __cdecl
+#elif defined(__GNUC__) && defined(RT_ARCH_X86)
+# define DECLASMCALLBACK(type)          type __attribute__((cdecl,regparm(0)))
+#else
+# define DECLASMCALLBACK(type)          type
+#endif
+typedef DECLASMCALLBACK(uint8_t const *) FNPGMR3DBGFIXEDMEMSCAN(void const *pvHaystack, uint32_t cbHaystack,
+                                                                void const *pvNeedle, size_t cbNeedle);
-typedef DECLCALLBACK(uint8_t const *) FNPGMR3DBGFIXEDMEMSCAN(void const *pvHaystack, uint32_t cbHaystack,
-                                                             void const *pvNeedle, size_t cbNeedle);
 /** Pointer to an fixed size and step assembly scanner function. */
 typedef FNPGMR3DBGFIXEDMEMSCAN *PFNPGMR3DBGFIXEDMEMSCAN;