powerpc64/ftrace: Support .text larger than 32MB with out-of-line stubs

We are restricted to a .text size of ~32MB when using out-of-line
function profile sequence. Allow this to be extended up to the previous
limit of ~64MB by reserving space in the middle of .text.

A new config option CONFIG_PPC_FTRACE_OUT_OF_LINE_NUM_RESERVE is
introduced to specify the number of function stubs that are reserved in
.text. On boot, ftrace utilizes stubs from this area first before using
the stub area at the end of .text.

A ppc64le defconfig has ~44k functions that can be traced. A more
conservative value of 32k functions is chosen as the default value of
PPC_FTRACE_OUT_OF_LINE_NUM_RESERVE so that we do not allot more space
than necessary by default. If building a kernel that only has 32k
trace-able functions, we won't allot any more space at the end of .text
during the pass on vmlinux.o. Otherwise, only the remaining functions
get space for stubs at the end of .text. This default value should help
cover a .text size of ~48MB in total (including space reserved at the
end of .text which can cover up to 32MB), which should be sufficient for
most common builds. For a very small kernel build, this can be set to 0.
Or, this can be bumped up to a larger value to support vmlinux .text
size up to ~64MB.

Signed-off-by: Naveen N Rao <naveen@kernel.org>
Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://patch.msgid.link/20241030070850.1361304-14-hbathini@linux.ibm.com

2 files changed, 17 insertions, 7 deletions

diff --git a/arch/powerpc/tools/Makefile b/arch/powerpc/tools/Makefile
index d2e7ecd5f46f..e1f7afcd9fdf 100644
--- a/arch/powerpc/tools/Makefile
+++ b/arch/powerpc/tools/Makefile
@@ -1,7 +1,8 @@
 # SPDX-License-Identifier: GPL-2.0-or-later
 
 quiet_cmd_gen_ftrace_ool_stubs = GEN     $@
-      cmd_gen_ftrace_ool_stubs = $< "$(CONFIG_64BIT)" "$(OBJDUMP)" vmlinux.o $@
+	cmd_gen_ftrace_ool_stubs = $< "$(CONFIG_PPC_FTRACE_OUT_OF_LINE_NUM_RESERVE)" "$(CONFIG_64BIT)" \
+				   "$(OBJDUMP)" vmlinux.o $@
 
 $(obj)/vmlinux.arch.S: $(src)/ftrace-gen-ool-stubs.sh vmlinux.o FORCE
 	$(call if_changed,gen_ftrace_ool_stubs)
diff --git a/arch/powerpc/tools/ftrace-gen-ool-stubs.sh b/arch/powerpc/tools/ftrace-gen-ool-stubs.sh
index 96e1ca5803e4..6a201df83524 100755
--- a/arch/powerpc/tools/ftrace-gen-ool-stubs.sh
+++ b/arch/powerpc/tools/ftrace-gen-ool-stubs.sh
@@ -4,10 +4,11 @@
 # Error out on error
 set -e
 
-is_64bit="$1"
-objdump="$2"
-vmlinux_o="$3"
-arch_vmlinux_S="$4"
+num_ool_stubs_text_builtin="$1"
+is_64bit="$2"
+objdump="$3"
+vmlinux_o="$4"
+arch_vmlinux_S="$5"
 
 RELOCATION=R_PPC64_ADDR64
 if [ -z "$is_64bit" ]; then
@@ -19,15 +20,23 @@ num_ool_stubs_text=$($objdump -r -j __patchable_function_entries "$vmlinux_o" |
 num_ool_stubs_inittext=$($objdump -r -j __patchable_function_entries "$vmlinux_o" |
 			 grep ".init.text" | grep -c "$RELOCATION")
 
+if [ "$num_ool_stubs_text" -gt "$num_ool_stubs_text_builtin" ]; then
+	num_ool_stubs_text_end=$((num_ool_stubs_text - num_ool_stubs_text_builtin))
+else
+	num_ool_stubs_text_end=0
+fi
+
 cat > "$arch_vmlinux_S" <<EOF
 #include <asm/asm-offsets.h>
 #include <linux/linkage.h>
 
 .pushsection .tramp.ftrace.text,"aw"
-SYM_DATA(ftrace_ool_stub_text_end_count, .long $num_ool_stubs_text)
+SYM_DATA(ftrace_ool_stub_text_end_count, .long $num_ool_stubs_text_end)
 
 SYM_CODE_START(ftrace_ool_stub_text_end)
-	.space $num_ool_stubs_text * FTRACE_OOL_STUB_SIZE
+#if $num_ool_stubs_text_end
+	.space $num_ool_stubs_text_end * FTRACE_OOL_STUB_SIZE
+#endif
 SYM_CODE_END(ftrace_ool_stub_text_end)
 .popsection