ftrace: Introduce ftrace_need_init_nop()

Implementing live patching on s390 requires each function's prologue to
contain a very special kind of nop, which gcc and clang don't generate.
However, the current code assumes that if CC_USING_NOP_MCOUNT is
defined, then whatever the compiler generates is good enough.

Move the CC_USING_NOP_MCOUNT check into the new ftrace_need_init_nop()
macro, that the architectures can override.

An alternative solution is to disable using -mnop-mcount in the
Makefile, however, this makes the build logic (even) more complicated
and forces the arch-specific code to deal with the useless __fentry__
symbol.
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Link: https://lore.kernel.org/r/20210728212546.128248-2-iii@linux.ibm.comSigned-off-by: Heiko Carstens <hca@linux.ibm.com>

include/linux/ftrace.h

@@ -643,6 +643,22 @@ static inline int ftrace_disable_ftrace_graph_caller(void) { return 0; }
 extern int ftrace_make_nop(struct module *mod,
 			   struct dyn_ftrace *rec, unsigned long addr);
 
+/**
+ * ftrace_need_init_nop - return whether nop call sites should be initialized
+ *
+ * Normally the compiler's -mnop-mcount generates suitable nops, so we don't
+ * need to call ftrace_init_nop() if the code is built with that flag.
+ * Architectures where this is not always the case may define their own
+ * condition.
+ *
+ * Return must be:
+ *  0	    if ftrace_init_nop() should be called
+ *  Nonzero if ftrace_init_nop() should not be called
+ */
+
+#ifndef ftrace_need_init_nop
+#define ftrace_need_init_nop() (!__is_defined(CC_USING_NOP_MCOUNT))
+#endif
 
 /**
  * ftrace_init_nop - initialize a nop call site

kernel/trace/ftrace.c

@@ -3100,6 +3100,7 @@ ops_references_rec(struct ftrace_ops *ops, struct dyn_ftrace *rec)
 
 static int ftrace_update_code(struct module *mod, struct ftrace_page *new_pgs)
 {
+	bool init_nop = ftrace_need_init_nop();
 	struct ftrace_page *pg;
 	struct dyn_ftrace *p;
 	u64 start, stop;
@@ -3138,8 +3139,7 @@ static int ftrace_update_code(struct module *mod, struct ftrace_page *new_pgs)
 			 * Do the initial record conversion from mcount jump
 			 * to the NOP instructions.
 			 */
-			if (!__is_defined(CC_USING_NOP_MCOUNT) &&
-			    !ftrace_nop_initialize(mod, p))
+			if (init_nop && !ftrace_nop_initialize(mod, p))
 				break;
 
 			update_cnt++;