linux-kernel - [PATCH v4 03/12] riscv: ftrace: align patchable functions to 4 Byte boundary

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Message-Id: <20250407180838.42877-3-andybnac@gmail.com>
Date: Tue,  8 Apr 2025 02:08:27 +0800
From: Andy Chiu <andybnac@...il.com>
To: linux-riscv@...ts.infradead.org,
	alexghiti@...osinc.com,
	palmer@...belt.com
Cc: Andy Chiu <andy.chiu@...ive.com>,
	Evgenii Shatokhin <e.shatokhin@...ro.com>,
	Björn Töpel <bjorn@...osinc.com>,
	linux-kernel@...r.kernel.org,
	Alexandre Ghiti <alex@...ti.fr>,
	puranjay12@...il.com,
	paul.walmsley@...ive.com,
	greentime.hu@...ive.com,
	nick.hu@...ive.com,
	nylon.chen@...ive.com,
	eric.lin@...ive.com,
	vicent.chen@...ive.com,
	zong.li@...ive.com,
	yongxuan.wang@...ive.com,
	samuel.holland@...ive.com,
	olivia.chu@...ive.com,
	c2232430@...il.com
Subject: [PATCH v4 03/12] riscv: ftrace: align patchable functions to 4 Byte boundary

From: Andy Chiu <andy.chiu@...ive.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@...ro.com>
Signed-off-by: Andy Chiu <andy.chiu@...ive.com>
Reviewed-by: Björn Töpel <bjorn@...osinc.com>
---
 arch/riscv/Kconfig | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index bbec87b79309..7dbed10843d2 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -150,6 +150,7 @@ config RISCV
 	select HAVE_DEBUG_KMEMLEAK
 	select HAVE_DMA_CONTIGUOUS if MMU
 	select HAVE_DYNAMIC_FTRACE if !XIP_KERNEL && MMU && (CLANG_SUPPORTS_DYNAMIC_FTRACE || GCC_SUPPORTS_DYNAMIC_FTRACE)
+	select FUNCTION_ALIGNMENT_4B if HAVE_DYNAMIC_FTRACE && RISCV_ISA_C
 	select HAVE_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
 	select HAVE_DYNAMIC_FTRACE_WITH_ARGS if HAVE_DYNAMIC_FTRACE
 	select HAVE_FTRACE_GRAPH_FUNC
@@ -236,6 +237,7 @@ 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_HAS_MIN_FUNCTION_ALIGNMENT || !RISCV_ISA_C
 
 config HAVE_SHADOW_CALL_STACK
 	def_bool $(cc-option,-fsanitize=shadow-call-stack)
-- 
2.39.3 (Apple Git-145)