powerpc/bug: Provide better flexibility to WARN_ON/__WARN_FLAGS() with asm goto

Using asm goto in __WARN_FLAGS() and WARN_ON() allows more
flexibility to GCC.

For that add an entry to the exception table so that
program_check_exception() knowns where to resume execution
after a WARNING.

Here are two exemples. The first one is done on PPC32 (which
benefits from the previous patch), the second is on PPC64.

	unsigned long test(struct pt_regs *regs)
	{
		int ret;

		WARN_ON(regs->msr & MSR_PR);

		return regs->gpr[3];
	}

	unsigned long test9w(unsigned long a, unsigned long b)
	{
		if (WARN_ON(!b))
			return 0;
		return a / b;
	}

Before the patch:

	000003a8 <test>:
	 3a8:	81 23 00 84 	lwz     r9,132(r3)
	 3ac:	71 29 40 00 	andi.   r9,r9,16384
	 3b0:	40 82 00 0c 	bne     3bc <test+0x14>
	 3b4:	80 63 00 0c 	lwz     r3,12(r3)
	 3b8:	4e 80 00 20 	blr

	 3bc:	0f e0 00 00 	twui    r0,0
	 3c0:	80 63 00 0c 	lwz     r3,12(r3)
	 3c4:	4e 80 00 20 	blr

	0000000000000bf0 <.test9w>:
	 bf0:	7c 89 00 74 	cntlzd  r9,r4
	 bf4:	79 29 d1 82 	rldicl  r9,r9,58,6
	 bf8:	0b 09 00 00 	tdnei   r9,0
	 bfc:	2c 24 00 00 	cmpdi   r4,0
	 c00:	41 82 00 0c 	beq     c0c <.test9w+0x1c>
	 c04:	7c 63 23 92 	divdu   r3,r3,r4
	 c08:	4e 80 00 20 	blr

	 c0c:	38 60 00 00 	li      r3,0
	 c10:	4e 80 00 20 	blr

After the patch:

	000003a8 <test>:
	 3a8:	81 23 00 84 	lwz     r9,132(r3)
	 3ac:	71 29 40 00 	andi.   r9,r9,16384
	 3b0:	40 82 00 0c 	bne     3bc <test+0x14>
	 3b4:	80 63 00 0c 	lwz     r3,12(r3)
	 3b8:	4e 80 00 20 	blr

	 3bc:	0f e0 00 00 	twui    r0,0

	0000000000000c50 <.test9w>:
	 c50:	7c 89 00 74 	cntlzd  r9,r4
	 c54:	79 29 d1 82 	rldicl  r9,r9,58,6
	 c58:	0b 09 00 00 	tdnei   r9,0
	 c5c:	7c 63 23 92 	divdu   r3,r3,r4
	 c60:	4e 80 00 20 	blr

	 c70:	38 60 00 00 	li      r3,0
	 c74:	4e 80 00 20 	blr

In the first exemple, we see GCC doesn't need to duplicate what
happens after the trap.

In the second exemple, we see that GCC doesn't need to emit a test
and a branch in the likely path in addition to the trap.

We've got some WARN_ON() in .softirqentry.text section so it needs
to be added in the OTHER_TEXT_SECTIONS in modpost.c

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/389962b1b702e3c78d169e59bcfac56282889173.1618331882.git.christophe.leroy@csgroup.eu

1 files changed, 45 insertions, 9 deletions

diff --git a/arch/powerpc/include/asm/bug.h b/arch/powerpc/include/asm/bug.h
index d844de5adfcb..1ee0f22313ee 100644
--- a/arch/powerpc/include/asm/bug.h
+++ b/arch/powerpc/include/asm/bug.h
@@ -4,6 +4,7 @@
 #ifdef __KERNEL__
 
 #include <asm/asm-compat.h>
+#include <asm/extable.h>
 
 #ifdef CONFIG_BUG
 
@@ -30,6 +31,11 @@
 .endm
 #endif /* verbose */
 
+.macro EMIT_WARN_ENTRY addr,file,line,flags
+	EX_TABLE(\addr,\addr+4)
+	EMIT_BUG_ENTRY \addr,\file,\line,\flags
+.endm
+
 #else /* !__ASSEMBLY__ */
 /* _EMIT_BUG_ENTRY expects args %0,%1,%2,%3 to be FILE, LINE, flags and
    sizeof(struct bug_entry), respectively */
@@ -58,6 +64,16 @@
 		  "i" (sizeof(struct bug_entry)),	\
 		  ##__VA_ARGS__)
 
+#define WARN_ENTRY(insn, flags, label, ...)		\
+	asm_volatile_goto(				\
+		"1:	" insn "\n"			\
+		EX_TABLE(1b, %l[label])			\
+		_EMIT_BUG_ENTRY				\
+		: : "i" (__FILE__), "i" (__LINE__),	\
+		  "i" (flags),				\
+		  "i" (sizeof(struct bug_entry)),	\
+		  ##__VA_ARGS__ : : label)
+
 /*
  * BUG_ON() and WARN_ON() do their best to cooperate with compile-time
  * optimisations. However depending on the complexity of the condition
@@ -70,7 +86,15 @@
 } while (0)
 #define HAVE_ARCH_BUG
 
-#define __WARN_FLAGS(flags) BUG_ENTRY("twi 31, 0, 0", BUGFLAG_WARNING | (flags))
+#define __WARN_FLAGS(flags) do {				\
+	__label__ __label_warn_on;				\
+								\
+	WARN_ENTRY("twi 31, 0, 0", BUGFLAG_WARNING | (flags), __label_warn_on); \
+	unreachable();						\
+								\
+__label_warn_on:						\
+	break;							\
+} while (0)
 
 #ifdef CONFIG_PPC64
 #define BUG_ON(x) do {						\
@@ -83,15 +107,24 @@
 } while (0)
 
 #define WARN_ON(x) ({						\
-	int __ret_warn_on = !!(x);				\
-	if (__builtin_constant_p(__ret_warn_on)) {		\
-		if (__ret_warn_on)				\
+	bool __ret_warn_on = false;				\
+	do {							\
+		if (__builtin_constant_p((x))) {		\
+			if (!(x)) 				\
+				break; 				\
 			__WARN();				\
-	} else {						\
-		BUG_ENTRY(PPC_TLNEI " %4, 0",			\
-			  BUGFLAG_WARNING | BUGFLAG_TAINT(TAINT_WARN),	\
-			  "r" (__ret_warn_on));	\
-	}							\
+			__ret_warn_on = true;			\
+		} else {					\
+			__label__ __label_warn_on;		\
+								\
+			WARN_ENTRY(PPC_TLNEI " %4, 0",		\
+				   BUGFLAG_WARNING | BUGFLAG_TAINT(TAINT_WARN),	\
+				   __label_warn_on, "r" (x));	\
+			break;					\
+__label_warn_on:						\
+			__ret_warn_on = true;			\
+		}						\
+	} while (0);						\
 	unlikely(__ret_warn_on);				\
 })
 
@@ -104,8 +137,11 @@
 #ifdef __ASSEMBLY__
 .macro EMIT_BUG_ENTRY addr,file,line,flags
 .endm
+.macro EMIT_WARN_ENTRY addr,file,line,flags
+.endm
 #else /* !__ASSEMBLY__ */
 #define _EMIT_BUG_ENTRY
+#define _EMIT_WARN_ENTRY
 #endif
 #endif /* CONFIG_BUG */