Add commentary about the new "asmlinkage_protect()" macro

[~andy/linux] / include / asm-x86 / linkage.h

diff --git a/include/asm-x86/linkage.h b/include/asm-x86/linkage.h
index 31739c7d66a94930868ed1037bed18df47d3c1e1..c048353f4b8558f73b8939980c8de947a5fd0513 100644 (file)
--- a/include/asm-x86/linkage.h
+++ b/include/asm-x86/linkage.h
@@ -8,12 +8,45 @@
 
 #ifdef CONFIG_X86_32
 #define asmlinkage CPP_ASMLINKAGE __attribute__((regparm(0)))
-#define prevent_tail_call(ret) __asm__ ("" : "=r" (ret) : "0" (ret))
 /*
  * For 32-bit UML - mark functions implemented in assembly that use
  * regparm input parameters:
  */
 #define asmregparm __attribute__((regparm(3)))
+
+/*
+ * Make sure the compiler doesn't do anything stupid with the
+ * arguments on the stack - they are owned by the *caller*, not
+ * the callee. This just fools gcc into not spilling into them,
+ * and keeps it from doing tailcall recursion and/or using the
+ * stack slots for temporaries, since they are live and "used"
+ * all the way to the end of the function.
+ *
+ * NOTE! On x86-64, all the arguments are in registers, so this
+ * only matters on a 32-bit kernel.
+ */
+#define asmlinkage_protect(n, ret, args...) \
+       __asmlinkage_protect##n(ret, ##args)
+#define __asmlinkage_protect_n(ret, args...) \
+       __asm__ __volatile__ ("" : "=r" (ret) : "0" (ret), ##args)
+#define __asmlinkage_protect0(ret) \
+       __asmlinkage_protect_n(ret)
+#define __asmlinkage_protect1(ret, arg1) \
+       __asmlinkage_protect_n(ret, "g" (arg1))
+#define __asmlinkage_protect2(ret, arg1, arg2) \
+       __asmlinkage_protect_n(ret, "g" (arg1), "g" (arg2))
+#define __asmlinkage_protect3(ret, arg1, arg2, arg3) \
+       __asmlinkage_protect_n(ret, "g" (arg1), "g" (arg2), "g" (arg3))
+#define __asmlinkage_protect4(ret, arg1, arg2, arg3, arg4) \
+       __asmlinkage_protect_n(ret, "g" (arg1), "g" (arg2), "g" (arg3), \
+                             "g" (arg4))
+#define __asmlinkage_protect5(ret, arg1, arg2, arg3, arg4, arg5) \
+       __asmlinkage_protect_n(ret, "g" (arg1), "g" (arg2), "g" (arg3), \
+                             "g" (arg4), "g" (arg5))
+#define __asmlinkage_protect6(ret, arg1, arg2, arg3, arg4, arg5, arg6) \
+       __asmlinkage_protect_n(ret, "g" (arg1), "g" (arg2), "g" (arg3), \
+                             "g" (arg4), "g" (arg5), "g" (arg6))
+
 #endif
 
 #ifdef CONFIG_X86_ALIGNMENT_16