[RFC PATCH] powerpc/64/ftrace: mprofile-kernel patch out mflr

[Nicholas Piggin <npiggin@gmail.com>]

The new mprofile-kernel mcount sequence is

  mflr	r0
  bl	_mcount

Dynamic ftrace patches the branch instruction with a noop, but leaves
the mflr. mflr is executed by the branch unit that can only execute one
per cycle on POWER9 and shared with branches, so it would be nice to
avoid it where possible.

This patch is a hacky proof of concept to nop out the mflr. Can we do
this or are there races or other issues with it?
---
 arch/powerpc/kernel/trace/ftrace.c | 77 +++++++++++++++++++++++++++++-
 1 file changed, 75 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/kernel/trace/ftrace.c b/arch/powerpc/kernel/trace/ftrace.c
index 52ee24fd353f..ecc75baef23e 100644
--- a/arch/powerpc/kernel/trace/ftrace.c
+++ b/arch/powerpc/kernel/trace/ftrace.c
@@ -172,6 +172,19 @@ __ftrace_make_nop(struct module *mod,
 		pr_err("Unexpected instruction %08x around bl _mcount\n", op);
 		return -EINVAL;
 	}
+
+	if (patch_instruction((unsigned int *)ip, pop)) {
+		pr_err("Patching NOP failed.\n");
+		return -EPERM;
+	}
+
+	if (op == PPC_INST_MFLR) {
+		if (patch_instruction((unsigned int *)(ip - 4), pop)) {
+			pr_err("Patching NOP failed.\n");
+			return -EPERM;
+		}
+	}
+
 #else
 	/*
 	 * Our original call site looks like:
@@ -202,13 +215,14 @@ __ftrace_make_nop(struct module *mod,
 		pr_err("Expected %08x found %08x\n", PPC_INST_LD_TOC, op);
 		return -EINVAL;
 	}
-#endif /* CONFIG_MPROFILE_KERNEL */
 
 	if (patch_instruction((unsigned int *)ip, pop)) {
 		pr_err("Patching NOP failed.\n");
 		return -EPERM;
 	}
 
+#endif /* CONFIG_MPROFILE_KERNEL */
+
 	return 0;
 }
 
@@ -421,6 +435,20 @@ static int __ftrace_make_nop_kernel(struct dyn_ftrace *rec, unsigned long addr)
 		return -EPERM;
 	}
 
+#ifdef CONFIG_MPROFILE_KERNEL
+	if (probe_kernel_read(&op, (void *)(ip - 4), 4)) {
+		pr_err("Fetching instruction at %lx failed.\n", ip - 4);
+		return -EFAULT;
+	}
+
+	if (op == PPC_INST_MFLR) {
+		if (patch_instruction((unsigned int *)(ip - 4), PPC_INST_NOP)) {
+			pr_err("Patching NOP failed.\n");
+			return -EPERM;
+		}
+	}
+#endif
+
 	return 0;
 }
 
@@ -437,9 +465,20 @@ int ftrace_make_nop(struct module *mod,
 	 */
 	if (test_24bit_addr(ip, addr)) {
 		/* within range */
+		int rc;
+
 		old = ftrace_call_replace(ip, addr, 1);
 		new = PPC_INST_NOP;
-		return ftrace_modify_code(ip, old, new);
+		rc = ftrace_modify_code(ip, old, new);
+		if (rc)
+			return rc;
+#ifdef CONFIG_MPROFILE_KERNEL
+		old = PPC_INST_MFLR;
+		new = PPC_INST_NOP;
+		ftrace_modify_code(ip - 4, old, new);
+		/* old mprofile kernel will error because no mflr */
+#endif
+		return rc;
 	} else if (core_kernel_text(ip))
 		return __ftrace_make_nop_kernel(rec, addr);
 
@@ -562,6 +601,20 @@ __ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
 		return -EINVAL;
 	}
 
+#ifdef CONFIG_MPROFILE_KERNEL
+	if (probe_kernel_read(op, (ip - 4), 4)) {
+		pr_err("Fetching instruction at %lx failed.\n", (unsigned long)(ip - 4));
+		return -EFAULT;
+	}
+
+	if (op[0] == PPC_INST_NOP) {
+		if (patch_instruction((ip - 4), PPC_INST_MFLR)) {
+			pr_err("Patching mflr failed.\n");
+			return -EINVAL;
+		}
+	}
+#endif
+
 	if (patch_branch(ip, tramp, BRANCH_SET_LINK)) {
 		pr_err("REL24 out of range!\n");
 		return -EINVAL;
@@ -650,6 +703,20 @@ static int __ftrace_make_call_kernel(struct dyn_ftrace *rec, unsigned long addr)
 		return -EINVAL;
 	}
 
+#ifdef CONFIG_MPROFILE_KERNEL
+	if (probe_kernel_read(&op, (ip - 4), 4)) {
+		pr_err("Fetching instruction at %lx failed.\n", (unsigned long)(ip - 4));
+		return -EFAULT;
+	}
+
+	if (op == PPC_INST_NOP) {
+		if (patch_instruction((ip - 4), PPC_INST_MFLR)) {
+			pr_err("Patching mflr failed.\n");
+			return -EINVAL;
+		}
+	}
+#endif
+
 	if (patch_branch(ip, tramp, BRANCH_SET_LINK)) {
 		pr_err("Error patching branch to ftrace tramp!\n");
 		return -EINVAL;
@@ -670,6 +737,12 @@ int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
 	 */
 	if (test_24bit_addr(ip, addr)) {
 		/* within range */
+#ifdef CONFIG_MPROFILE_KERNEL
+		old = PPC_INST_NOP;
+		new = PPC_INST_MFLR;
+		ftrace_modify_code(ip - 4, old, new);
+		/* old mprofile-kernel sequence will error because no nop */
+#endif
 		old = PPC_INST_NOP;
 		new = ftrace_call_replace(ip, addr, 1);
 		return ftrace_modify_code(ip, old, new);
-- 
2.20.1