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From: Andy Chiu <andy.chiu@sifive.com>
Date: Thu, 13 Jun 2024 15:11:09 +0800
Subject: [PATCH 4/8] riscv: ftrace: align patchable functions to 4 Byte
 boundary
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Message-Id: <20240613-dev-andyc-dyn-ftrace-v4-v1-4-1a538e12c01e@sifive.com>
References: <20240613-dev-andyc-dyn-ftrace-v4-v1-0-1a538e12c01e@sifive.com>
In-Reply-To: <20240613-dev-andyc-dyn-ftrace-v4-v1-0-1a538e12c01e@sifive.com>
To: Paul Walmsley <paul.walmsley@sifive.com>, 
 Palmer Dabbelt <palmer@dabbelt.com>, Albert Ou <aou@eecs.berkeley.edu>, 
 Alexandre Ghiti <alexghiti@rivosinc.com>, Zong Li <zong.li@sifive.com>, 
 Steven Rostedt <rostedt@goodmis.org>, 
 Masami Hiramatsu <mhiramat@kernel.org>, Mark Rutland <mark.rutland@arm.com>, 
 Nathan Chancellor <nathan@kernel.org>, 
 Nick Desaulniers <ndesaulniers@google.com>, 
 Bill Wendling <morbo@google.com>, Justin Stitt <justinstitt@google.com>, 
 Puranjay Mohan <puranjay@kernel.org>
Cc: Palmer Dabbelt <palmer@rivosinc.com>, linux-riscv@lists.infradead.org, 
 linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, 
 llvm@lists.linux.dev, Evgenii Shatokhin <e.shatokhin@yadro.com>, 
 Andy Chiu <andy.chiu@sifive.com>

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@yadro.com>
Signed-off-by: Andy Chiu <andy.chiu@sifive.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