objtool/loongarch: Add unwind hints in prepare_frametrace()

If 'regs' points to a local stack variable, prepare_frametrace() stores
all registers to the stack.  This confuses objtool as it expects them to
be restored from the stack later.

The stores don't affect stack tracing, so use unwind hints to hide them
from objtool.

Fixes the following warnings:

  arch/loongarch/kernel/traps.o: warning: objtool: show_stack+0xe0: stack state mismatch: reg1[22]=-1+0 reg2[22]=-2-160
  arch/loongarch/kernel/traps.o: warning: objtool: show_stack+0xe0: stack state mismatch: reg1[23]=-1+0 reg2[23]=-2-152

Fixes: cb8a2ef ("LoongArch: Add ORC stack unwinder support")
Reported-by: kernel test robot <lkp@intel.com>
Tested-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/270cadd8040dda74db2307f23497bb68e65db98d.1743481539.git.jpoimboe@kernel.org
Closes: https://lore.kernel.org/oe-kbuild-all/202503280703.OARM8SrY-lkp@intel.com/

Author: jpoimboe

arch/loongarch/include/asm/stacktrace.h

@@ -8,6 +8,7 @@
 #include <asm/asm.h>
 #include <asm/ptrace.h>
 #include <asm/loongarch.h>
+#include <asm/unwind_hints.h>
 #include <linux/stringify.h>
 
 enum stack_type {
@@ -43,6 +44,7 @@ int get_stack_info(unsigned long stack, struct task_struct *task, struct stack_i
 static __always_inline void prepare_frametrace(struct pt_regs *regs)
 {
 	__asm__ __volatile__(
+		UNWIND_HINT_SAVE
 		/* Save $ra */
 		STORE_ONE_REG(1)
 		/* Use $ra to save PC */
@@ -80,6 +82,7 @@ static __always_inline void prepare_frametrace(struct pt_regs *regs)
 		STORE_ONE_REG(29)
 		STORE_ONE_REG(30)
 		STORE_ONE_REG(31)
+		UNWIND_HINT_RESTORE
 		: "=m" (regs->csr_era)
 		: "r" (regs->regs)
 		: "memory");

arch/loongarch/include/asm/unwind_hints.h

@@ -23,6 +23,14 @@
 	UNWIND_HINT sp_reg=ORC_REG_SP type=UNWIND_HINT_TYPE_CALL
 .endm
 
-#endif /* __ASSEMBLY__ */
+#else /* !__ASSEMBLY__ */
+
+#define UNWIND_HINT_SAVE \
+	UNWIND_HINT(UNWIND_HINT_TYPE_SAVE, 0, 0, 0)
+
+#define UNWIND_HINT_RESTORE \
+	UNWIND_HINT(UNWIND_HINT_TYPE_RESTORE, 0, 0, 0)
+
+#endif /* !__ASSEMBLY__ */
 
 #endif /* _ASM_LOONGARCH_UNWIND_HINTS_H */