[PATCH] powerpc/bug: Add ARCH_WARN_ASM and refactor _EMIT_BUG_ENTRY for Rust support

[Mukesh Kumar Chaurasiya (IBM)]

The Rust kernel infrastructure generates inline asm for WARN() via
ARCH_WARN_ASM(file, line, flags, size), expanding it through a C
preprocessor pass (generated_arch_warn_asm.rs.S) to produce an
arch-specific asm template string for use in Rust's core::arch macros.

powerpc currently lacks ARCH_WARN_ASM and ARCH_WARN_REACHABLE, causing
Rust builds to fail on powerpc.

Refactor _EMIT_BUG_ENTRY to accept explicit (file, line, flags) string
arguments rather than relying on positional asm operand references
(%0, %1, %2, %3). This allows the macro to be composed as a plain
string concatenation, which is required for ARCH_WARN_ASM where no asm
operand context exists.

Move the .org and .previous directives out of _EMIT_BUG_ENTRY and into
the BUG_ENTRY() call site to preserve existing behaviour while enabling
ARCH_WARN_ASM to supply its own size operand independently.

Add ARCH_WARN_REACHABLE as an empty define, matching the arm64
convention, indicating that no additional reachability annotation is
needed after a WARN on powerpc.

This brings powerpc into line with x86, arm64, s390, and riscv, all of
which already define ARCH_WARN_ASM and ARCH_WARN_REACHABLE.

Suggested-by: FUJITA Tomonori <tomo at flapping.org>
Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras at gmail.com>
---
 arch/powerpc/include/asm/bug.h | 28 ++++++++++++++++------------
 1 file changed, 16 insertions(+), 12 deletions(-)

diff --git a/arch/powerpc/include/asm/bug.h b/arch/powerpc/include/asm/bug.h
index 0db48977c70c..8aba39e0cf26 100644
--- a/arch/powerpc/include/asm/bug.h
+++ b/arch/powerpc/include/asm/bug.h
@@ -32,34 +32,38 @@
 #endif /* verbose */
 
 #else /* !__ASSEMBLER__ */
-/* _EMIT_BUG_ENTRY expects args %0,%1,%2,%3 to be FILE, LINE, flags and
-   sizeof(struct bug_entry), respectively */
 #ifdef CONFIG_DEBUG_BUGVERBOSE
-#define _EMIT_BUG_ENTRY				\
+#define _EMIT_BUG_ENTRY(file, line, flags)	\
 	".section __bug_table,\"aw\"\n"		\
 	"2:	.4byte 1b - .\n"		\
-	"	.4byte %0 - .\n"		\
-	"	.short %1, %2\n"		\
-	".org 2b+%3\n"				\
-	".previous\n"
+	"	.4byte " file " - .\n"		\
+	"	.short " line ", " flags "\n"
 #else
-#define _EMIT_BUG_ENTRY				\
+#define _EMIT_BUG_ENTRY(file, line, flags)	\
 	".section __bug_table,\"aw\"\n"		\
 	"2:	.4byte 1b - .\n"		\
-	"	.short %2\n"			\
-	".org 2b+%3\n"				\
-	".previous\n"
+	"	.short " flags "\n"
 #endif
 
 #define BUG_ENTRY(cond_str, insn, flags, ...)		\
 	__asm__ __volatile__(				\
 		"1:	" insn "\n"			\
-		_EMIT_BUG_ENTRY				\
+		_EMIT_BUG_ENTRY("%0", "%1", "%2")	\
+		".org 2b+%3\n"				\
+		".previous\n"				\
 		: : "i" (WARN_CONDITION_STR(cond_str) __FILE__), "i" (__LINE__),	\
 		  "i" (flags),				\
 		  "i" (sizeof(struct bug_entry)),	\
 		  ##__VA_ARGS__)
 
+#define ARCH_WARN_ASM(file, line, flags, size)		\
+		"1:	twi 31, 0, 0\n"			\
+		_EMIT_BUG_ENTRY(file, line, flags)	\
+		".org 2b+" size "\n"			\
+		".previous\n"
+
+#define ARCH_WARN_REACHABLE
+
 /*
  * BUG_ON() and WARN_ON() do their best to cooperate with compile-time
  * optimisations. However depending on the complexity of the condition
-- 
2.55.0