linux-kernel - [PATCH 4/8] riscv: ftrace: align patchable functions to 4 Byte boundary

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Date: Thu, 13 Jun 2024 15:11:09 +0800
From: Andy Chiu <andy.chiu@...ive.com>
To: Paul Walmsley <paul.walmsley@...ive.com>, 
 Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, 
 Alexandre Ghiti <alexghiti@...osinc.com>, Zong Li <zong.li@...ive.com>, 
 Steven Rostedt <rostedt@...dmis.org>, 
 Masami Hiramatsu <mhiramat@...nel.org>, Mark Rutland <mark.rutland@....com>, 
 Nathan Chancellor <nathan@...nel.org>, 
 Nick Desaulniers <ndesaulniers@...gle.com>, 
 Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>, 
 Puranjay Mohan <puranjay@...nel.org>
Cc: Palmer Dabbelt <palmer@...osinc.com>, linux-riscv@...ts.infradead.org, 
 linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, 
 llvm@...ts.linux.dev, Evgenii Shatokhin <e.shatokhin@...ro.com>, 
 Andy Chiu <andy.chiu@...ive.com>
Subject: [PATCH 4/8] riscv: ftrace: align patchable functions to 4 Byte
 boundary

We are changing ftrace code patching in order to remove dependency from
stop_machine() and enable kernel preemption. This requires us to align
functions entry at a 4-B align address.

However, -falign-functions on older versions of GCC alone was not strong
enoungh to align all functions. 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

The option -fmin-function-alignment is able to align all functions
properly on newer versions of gcc. So, we add a cc-option to test if
the toolchain supports it.

Suggested-by: Evgenii Shatokhin <e.shatokhin@...ro.com>
Signed-off-by: Andy Chiu <andy.chiu@...ive.com>
---
 arch/riscv/Kconfig  | 1 +
 arch/riscv/Makefile | 7 ++++++-
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index b94176e25be1..80b8d48e1e46 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -203,6 +203,7 @@ config CLANG_SUPPORTS_DYNAMIC_FTRACE
 config GCC_SUPPORTS_DYNAMIC_FTRACE
 	def_bool CC_IS_GCC
 	depends on $(cc-option,-fpatchable-function-entry=8)
+	depends on $(cc-option,-fmin-function-alignment=4) || !RISCV_ISA_C
 
 config HAVE_SHADOW_CALL_STACK
 	def_bool $(cc-option,-fsanitize=shadow-call-stack)
diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index 06de9d365088..74628ad8dcf8 100644
--- a/arch/riscv/Makefile
+++ b/arch/riscv/Makefile
@@ -14,8 +14,13 @@ endif
 ifeq ($(CONFIG_DYNAMIC_FTRACE),y)
 	LDFLAGS_vmlinux += --no-relax
 	KBUILD_CPPFLAGS += -DCC_USING_PATCHABLE_FUNCTION_ENTRY
+ifeq ($(CONFIG_CC_IS_CLANG),y)
+	cflags_ftrace_align := -falign-functions=4
+else
+	cflags_ftrace_align := -fmin-function-alignment=4
+endif
 ifeq ($(CONFIG_RISCV_ISA_C),y)
-	CC_FLAGS_FTRACE := -fpatchable-function-entry=4
+	CC_FLAGS_FTRACE := -fpatchable-function-entry=4 $(cflags_ftrace_align)
 else
 	CC_FLAGS_FTRACE := -fpatchable-function-entry=2
 endif

-- 
2.43.0