[PATCH v3 2/7] riscv: ftrace: align patchable functions to 4 Byte boundary

From: Andy Chiu <andybnac@gmail.com>
To: Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>
Cc: linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
	bjorn@rivosinc.com, puranjay12@gmail.com, alexghiti@rivosinc.com,
	yongxuan.wang@sifive.com, greentime.hu@sifive.com,
	nick.hu@sifive.com, nylon.chen@sifive.com, tommy.wu@sifive.com,
	eric.lin@sifive.com, viccent.chen@sifive.com, zong.li@sifive.com,
	samuel.holland@sifive.com
Subject: [PATCH v3 2/7] riscv: ftrace: align patchable functions to 4 Byte boundary
Date: Thu, 28 Nov 2024 01:29:03 +0800	[thread overview]
Message-ID: <20241127172908.17149-3-andybnac@gmail.com> (raw)
In-Reply-To: <20241127172908.17149-1-andybnac@gmail.com>

From: 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>
Reviewed-by: Björn Töpel <bjorn@rivosinc.com>
---
 arch/riscv/Kconfig | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 5bdda86ada37..75a5ebde4427 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -142,6 +142,7 @@ 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_MCOUNT_RECORD if !XIP_KERNEL
@@ -226,6 +227,7 @@ 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)


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