x86/nmi: Fix section mismatch warnings on 32-bit

It was reported that compiling for 32-bit caused a bunch of
section mismatch warnings:

VDSOSYM arch/x86/vdso/vdso32-syms.lds
LD arch/x86/vdso/built-in.o
LD arch/x86/built-in.o

WARNING: arch/x86/built-in.o(.data+0x5af0): Section mismatch in
reference from the variable test_nmi_ipi_callback_na.10451 to
the function .init.text:test_nmi_ipi_callback() [...]

WARNING: arch/x86/built-in.o(.data+0x5b04): Section mismatch in
reference from the variable nmi_unk_cb_na.10399 to the function
.init.text:nmi_unk_cb() The variable nmi_unk_cb_na.10399
references the function __init nmi_unk_cb() [...]

Both of these are attributed to the internal representation of
the nmiaction struct created during register_nmi_handler. The
reason for this is that those structs are not defined in the
init section whereas the rest of the code in nmi_selftest.c is.

To resolve this, I created a new #define,
register_nmi_handler_initonly, that tags the struct as
__initdata to resolve the mismatch. This #define should only be
used in rare situations where the register/unregister is called
during init of the kernel.

Big thanks to Jan Beulich for decoding this for me as I didn't
have a clue what was going on.

Reported-by: Witold Baryluk <[e-mail]>
Tested-by: Witold Baryluk <[e-mail]>
Cc: Jan Beulich <[e-mail]>
Signed-off-by: Don Zickus <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/include/asm/nmi.h

···
 	__register_nmi_handler((t), &fn##_na);	\
 })
 
+/*
+ * For special handlers that register/unregister in the
+ * init section only.  This should be considered rare.
+ */
+#define register_nmi_handler_initonly(t, fn, fg, n)		\
+({							\
+	static struct nmiaction fn##_na __initdata = {		\
+		.handler = (fn),			\
+		.name = (n),				\
+		.flags = (fg),				\
+	};						\
+	__register_nmi_handler((t), &fn##_na);	\
+})
+
 int __register_nmi_handler(unsigned int, struct nmiaction *);
 
 void unregister_nmi_handler(unsigned int, const char *);

arch/x86/kernel/nmi_selftest.c

···
 static void __init init_nmi_testsuite(void)
 {
 	/* trap all the unknown NMIs we may generate */
-	register_nmi_handler(NMI_UNKNOWN, nmi_unk_cb, 0, "nmi_selftest_unk");
+	register_nmi_handler_initonly(NMI_UNKNOWN, nmi_unk_cb, 0, "nmi_selftest_unk");
 }
 
 static void __init cleanup_nmi_testsuite(void)
···
 {
 	unsigned long timeout;
 
-	if (register_nmi_handler(NMI_LOCAL, test_nmi_ipi_callback,
+	if (register_nmi_handler_initonly(NMI_LOCAL, test_nmi_ipi_callback,
 				 NMI_FLAG_FIRST, "nmi_selftest")) {
 		nmi_fail = FAILURE;
 		return;