[PATCH RFC v2 riscv/for-next 1/5] riscv: align ftrace to 4 Byte boundary and increase ftrace prologue size

[Andy Chiu]

We are introducing a new ftrace mechanism in order to phase out
stop_machine() and enable kernel preemption. The new mechanism requires
ftrace patchable function entries to be 24 bytes and aligned to 4 Byte
boundaries.

Before applying this patch, the size of the kernel code, with 122465 of
ftrace entries, was at 12.46 MB. Under the same configuration, the size
has increased to 12.99 MB after applying this patch set.

However, we found the -falign-functions alone was not strong enoungh to
make all functions align as required. In fact, cold functions are not
aligned after turning on optimizations. We consider this is a bug in GCC
and turn off guess-branch-probility as a workaround to align all
functions.

GCC bug id: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88345

Signed-off-by: Andy Chiu <andy.chiu at sifive.com>
Reviewed-by: Greentime Hu <greentime.hu at sifive.com>
---
 arch/riscv/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index 3fa8ef336822..fd8069f59a59 100644
--- a/arch/riscv/Makefile
+++ b/arch/riscv/Makefile
@@ -11,7 +11,7 @@ LDFLAGS_vmlinux :=
 ifeq ($(CONFIG_DYNAMIC_FTRACE),y)
 	LDFLAGS_vmlinux := --no-relax
 	KBUILD_CPPFLAGS += -DCC_USING_PATCHABLE_FUNCTION_ENTRY
-	CC_FLAGS_FTRACE := -fpatchable-function-entry=8
+	CC_FLAGS_FTRACE := -fpatchable-function-entry=12  -falign-functions=4 -fno-guess-branch-probability
 endif
 
 ifeq ($(CONFIG_CMODEL_MEDLOW),y)
-- 
2.36.0