powerpc/ftrace: Stop re-purposing linker generated long branches for ftrace

Commit 67361cf8 ("powerpc/ftrace: Handle large kernel configs")
added ftrace support for ppc64 kernel images with a text section larger
than 32MB. The patch did two things:
1. Add stubs at the end of .text to branch into ftrace_[regs_]caller for
   functions that were out of branch range.
2. Re-purpose linker-generated long branches to _mcount to instead branch
   to ftrace_[regs_]caller.

Before that, we only supported kernel .text up to ~32MB. With the above,
we now support up to ~96MB:
- The first 32MB of kernel text can branch directly into
  ftrace_[regs_]caller since that symbol is usually at the beginning.
- The modified long_branch from (2) above is used by the next 32MB of
  kernel text.
- The next 32MB of kernel text can use the stub at the end of text to
  branch back to ftrace_[regs_]caller.

While re-purposing the long branch works in practice, it still restricts
ftrace to kernel text up to ~96MB. The stub at the end of kernel text
from (1) already enables us to extend ftrace support for kernel text
up to 64MB, which fulfils the original requirement. Further, once we
switch to -fpatchable-function-entry, there will not be a long branch
that we can use.

Stop re-purposing the linker-generated long branches for ftrace to
simplify the code. If there are good reasons to support ftrace on
kernels beyond 64MB, we can consider adding support by using
-fpatchable-function-entry.
Signed-off-by: Naveen N Rao <naveen@kernel.org>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/33fa3be97f8e1f2171254ef2e1b0d5c8836c11fd.1687166935.git.naveen@kernel.org

arch/powerpc/kernel/trace/ftrace.c

@@ -28,13 +28,7 @@
 #include <asm/syscall.h>
 #include <asm/inst.h>
 
-/*
- * We generally only have a single long_branch tramp and at most 2 or 3 plt
- * tramps generated. But, we don't use the plt tramps currently. We also allot
- * 2 tramps after .text and .init.text. So, we only end up with around 3 usable
- * tramps in total. Set aside 8 just to be sure.
- */
-#define	NUM_FTRACE_TRAMPS	8
+#define	NUM_FTRACE_TRAMPS	2
 static unsigned long ftrace_tramps[NUM_FTRACE_TRAMPS];
 
 static ppc_inst_t
@@ -100,11 +94,6 @@ static int is_bl_op(ppc_inst_t op)
 	return (ppc_inst_val(op) & ~PPC_LI_MASK) == PPC_RAW_BL(0);
 }
 
-static int is_b_op(ppc_inst_t op)
-{
-	return (ppc_inst_val(op) & ~PPC_LI_MASK) == PPC_RAW_BRANCH(0);
-}
-
 static unsigned long find_bl_target(unsigned long ip, ppc_inst_t op)
 {
 	int offset;
@@ -227,11 +216,7 @@ static unsigned long find_ftrace_tramp(unsigned long ip)
 {
 	int i;
 
-	/*
-	 * We have the compiler generated long_branch tramps at the end
-	 * and we prefer those
-	 */
-	for (i = NUM_FTRACE_TRAMPS - 1; i >= 0; i--)
+	for (i = 0; i < NUM_FTRACE_TRAMPS; i++)
 		if (!ftrace_tramps[i])
 			continue;
 		else if (is_offset_in_branch_range(ftrace_tramps[i] - ip))
@@ -240,75 +225,6 @@ static unsigned long find_ftrace_tramp(unsigned long ip)
 	return 0;
 }
 
-static int add_ftrace_tramp(unsigned long tramp)
-{
-	int i;
-
-	for (i = 0; i < NUM_FTRACE_TRAMPS; i++)
-		if (!ftrace_tramps[i]) {
-			ftrace_tramps[i] = tramp;
-			return 0;
-		}
-
-	return -1;
-}
-
-/*
- * If this is a compiler generated long_branch trampoline (essentially, a
- * trampoline that has a branch to _mcount()), we re-write the branch to
- * instead go to ftrace_[regs_]caller() and note down the location of this
- * trampoline.
- */
-static int setup_mcount_compiler_tramp(unsigned long tramp)
-{
-	int i;
-	ppc_inst_t op;
-	unsigned long ptr;
-
-	/* Is this a known long jump tramp? */
-	for (i = 0; i < NUM_FTRACE_TRAMPS; i++)
-		if (ftrace_tramps[i] == tramp)
-			return 0;
-
-	/* New trampoline -- read where this goes */
-	if (copy_inst_from_kernel_nofault(&op, (void *)tramp)) {
-		pr_debug("Fetching opcode failed.\n");
-		return -1;
-	}
-
-	/* Is this a 24 bit branch? */
-	if (!is_b_op(op)) {
-		pr_debug("Trampoline is not a long branch tramp.\n");
-		return -1;
-	}
-
-	/* lets find where the pointer goes */
-	ptr = find_bl_target(tramp, op);
-
-	if (ptr != ppc_global_function_entry((void *)_mcount)) {
-		pr_debug("Trampoline target %p is not _mcount\n", (void *)ptr);
-		return -1;
-	}
-
-	/* Let's re-write the tramp to go to ftrace_[regs_]caller */
-	if (IS_ENABLED(CONFIG_DYNAMIC_FTRACE_WITH_REGS))
-		ptr = ppc_global_function_entry((void *)ftrace_regs_caller);
-	else
-		ptr = ppc_global_function_entry((void *)ftrace_caller);
-
-	if (patch_branch((u32 *)tramp, ptr, 0)) {
-		pr_debug("REL24 out of range!\n");
-		return -1;
-	}
-
-	if (add_ftrace_tramp(tramp)) {
-		pr_debug("No tramp locations left\n");
-		return -1;
-	}
-
-	return 0;
-}
-
 static int __ftrace_make_nop_kernel(struct dyn_ftrace *rec, unsigned long addr)
 {
 	unsigned long tramp, ip = rec->ip;
@@ -331,13 +247,10 @@ static int __ftrace_make_nop_kernel(struct dyn_ftrace *rec, unsigned long addr)
 
 	pr_devel("ip:%lx jumps to %lx", ip, tramp);
 
-	if (setup_mcount_compiler_tramp(tramp)) {
-		/* Are other trampolines reachable? */
-		if (!find_ftrace_tramp(ip)) {
-			pr_err("No ftrace trampolines reachable from %ps\n",
-					(void *)ip);
-			return -EINVAL;
-		}
+	/* Are ftrace trampolines reachable? */
+	if (!find_ftrace_tramp(ip)) {
+		pr_err("No ftrace trampolines reachable from %ps\n", (void *)ip);
+		return -EINVAL;
 	}
 
 	if (patch_instruction((u32 *)ip, ppc_inst(PPC_RAW_NOP()))) {
@@ -725,6 +638,17 @@ void ftrace_free_init_tramp(void)
 		}
 }
 
+static void __init add_ftrace_tramp(unsigned long tramp)
+{
+	int i;
+
+	for (i = 0; i < NUM_FTRACE_TRAMPS; i++)
+		if (!ftrace_tramps[i]) {
+			ftrace_tramps[i] = tramp;
+			return;
+		}
+}
+
 int __init ftrace_dyn_arch_init(void)
 {
 	unsigned int *tramp[] = { ftrace_tramp_text, ftrace_tramp_init };