powerpc/ftrace: Separate the heuristics for checking call sites

In __ftrace_make_nop() (the 64-bit version), we have code to deal with
two ftrace ABIs. There is the original ABI, which looks mostly like a
function call, and then the mprofile-kernel ABI which is just a branch.

The code tries to handle both cases, by looking for the presence of a
load to restore the TOC pointer (PPC_INST_LD_TOC). If we detect the TOC
load, we assume the call site is for an mcount() call using the old ABI.
That means we patch the mcount() call with a b +8, to branch over the
TOC load.

However if the kernel was built with mprofile-kernel, then there will
never be a call site using the original ftrace ABI. If for some reason
we do see a TOC load, then it's there for a good reason, and we should
not jump over it.

So split the code, using the existing CC_USING_MPROFILE_KERNEL. Kernels
built with mprofile-kernel will only look for, and expect, the new ABI,
and similarly for the original ABI.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/ftrace.c

@@ -144,6 +144,21 @@ __ftrace_make_nop(struct module *mod,
 		return -EINVAL;
 	}
 
+#ifdef CC_USING_MPROFILE_KERNEL
+	/* When using -mkernel_profile there is no load to jump over */
+	pop = PPC_INST_NOP;
+
+	if (probe_kernel_read(&op, (void *)(ip - 4), 4)) {
+		pr_err("Fetching instruction at %lx failed.\n", ip - 4);
+		return -EFAULT;
+	}
+
+	/* We expect either a mflr r0, or a std r0, LRSAVE(r1) */
+	if (op != PPC_INST_MFLR && op != PPC_INST_STD_LR) {
+		pr_err("Unexpected instruction %08x around bl _mcount\n", op);
+		return -EINVAL;
+	}
+#else
 	/*
 	 * Our original call site looks like:
 	 *
@@ -170,24 +185,10 @@ __ftrace_make_nop(struct module *mod,
 	}
 
 	if (op != PPC_INST_LD_TOC) {
-		unsigned int inst;
-
-		if (probe_kernel_read(&inst, (void *)(ip - 4), 4)) {
-			pr_err("Fetching instruction at %lx failed.\n", ip - 4);
-			return -EFAULT;
-		}
-
-		/* We expect either a mlfr r0, or a std r0, LRSAVE(r1) */
-		if (inst != PPC_INST_MFLR && inst != PPC_INST_STD_LR) {
-			pr_err("Unexpected instructions around bl _mcount\n"
-			       "when enabling dynamic ftrace!\t"
-			       "(%08x,bl,%08x)\n", inst, op);
-			return -EINVAL;
-		}
-
-		/* When using -mkernel_profile there is no load to jump over */
-		pop = PPC_INST_NOP;
+		pr_err("Expected %08x found %08x\n", PPC_INST_LD_TOC, op);
+		return -EINVAL;
 	}
+#endif /* CC_USING_MPROFILE_KERNEL */
 
 	if (patch_instruction((unsigned int *)ip, pop)) {
 		pr_err("Patching NOP failed.\n");