linux-kernel - [PATCH v3] kbuild: Use -fmin-function-alignment when available

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Message-Id: <20240222133500.16991-1-petr.pavlu@suse.com>
Date: Thu, 22 Feb 2024 14:35:00 +0100
From: Petr Pavlu <petr.pavlu@...e.com>
To: masahiroy@...nel.org,
	nathan@...nel.org,
	nicolas@...sle.eu,
	mark.rutland@....com
Cc: linux-kbuild@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Petr Pavlu <petr.pavlu@...e.com>
Subject: [PATCH v3] kbuild: Use -fmin-function-alignment when available

GCC recently added option -fmin-function-alignment, which should appear
in GCC 14. Unlike -falign-functions, this option causes all functions to
be aligned at the specified value, including the cold ones.

In particular, when an arm64 kernel is built with
DYNAMIC_FTRACE_WITH_CALL_OPS=y, the 8-byte function alignment is
required for correct functionality. This was done by -falign-functions=8
and having workarounds in the kernel to force the compiler to follow
this alignment. The new -fmin-function-alignment option directly
guarantees it.

Detect availability of -fmin-function-alignment and use it instead of
-falign-functions when present. Introduce CC_HAS_SANE_FUNCTION_ALIGNMENT
and enable __cold to work as expected when it is set.

Signed-off-by: Petr Pavlu <petr.pavlu@...e.com>
Reviewed-by: Nathan Chancellor <nathan@...nel.org>
Acked-by: Mark Rutland <mark.rutland@....com>
---

Changes since v2 [1]:
- Remove the conditional use of __function_aligned for abort() which resulted in
  less readable code.
- Mention in the commit message that a particular target for this change is an
  arm64 kernel with DYNAMIC_FTRACE_WITH_CALL_OPS=y.

Changes since v1 [2]:
- Check the availability of -fmin-function-alignment only in one place.

[1] https://lore.kernel.org/linux-kbuild/20240215151642.8970-1-petr.pavlu@suse.com/
[2] https://lore.kernel.org/linux-kbuild/20240212145355.1050-1-petr.pavlu@suse.com/

 Makefile                       |  7 +++++++
 arch/Kconfig                   | 12 ++++++++++++
 include/linux/compiler_types.h | 10 +++++-----
 3 files changed, 24 insertions(+), 5 deletions(-)

diff --git a/Makefile b/Makefile
index 7e0b2ad98905..6f20ab5e2e44 100644
--- a/Makefile
+++ b/Makefile
@@ -974,8 +974,15 @@ export CC_FLAGS_CFI
 endif
 
 ifneq ($(CONFIG_FUNCTION_ALIGNMENT),0)
+# Set the minimal function alignment. Use the newer GCC option
+# -fmin-function-alignment if it is available, or fall back to -falign-funtions.
+# See also CONFIG_CC_HAS_SANE_FUNCTION_ALIGNMENT.
+ifdef CONFIG_CC_HAS_MIN_FUNCTION_ALIGNMENT
+KBUILD_CFLAGS += -fmin-function-alignment=$(CONFIG_FUNCTION_ALIGNMENT)
+else
 KBUILD_CFLAGS += -falign-functions=$(CONFIG_FUNCTION_ALIGNMENT)
 endif
+endif
 
 # arch Makefile may override CC so keep this after arch Makefile is included
 NOSTDINC_FLAGS += -nostdinc
diff --git a/arch/Kconfig b/arch/Kconfig
index a5af0edd3eb8..bd6c6335efac 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -1507,4 +1507,16 @@ config FUNCTION_ALIGNMENT
 	default 4 if FUNCTION_ALIGNMENT_4B
 	default 0
 
+config CC_HAS_MIN_FUNCTION_ALIGNMENT
+	# Detect availability of the GCC option -fmin-function-alignment which
+	# guarantees minimal alignment for all functions, unlike
+	# -falign-functions which the compiler ignores for cold functions.
+	def_bool $(cc-option, -fmin-function-alignment=8)
+
+config CC_HAS_SANE_FUNCTION_ALIGNMENT
+	# Set if the guaranteed alignment with -fmin-function-alignment is
+	# available or extra care is required in the kernel. Clang provides
+	# strict alignment always, even with -falign-functions.
+	def_bool CC_HAS_MIN_FUNCTION_ALIGNMENT || CC_IS_CLANG
+
 endmenu
diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 663d8791c871..f0152165e83c 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -99,17 +99,17 @@ static inline void __chk_io_ptr(const volatile void __iomem *ptr) { }
  *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Label-Attributes.html#index-cold-label-attribute
  *
  * When -falign-functions=N is in use, we must avoid the cold attribute as
- * contemporary versions of GCC drop the alignment for cold functions. Worse,
- * GCC can implicitly mark callees of cold functions as cold themselves, so
- * it's not sufficient to add __function_aligned here as that will not ensure
- * that callees are correctly aligned.
+ * GCC drops the alignment for cold functions. Worse, GCC can implicitly mark
+ * callees of cold functions as cold themselves, so it's not sufficient to add
+ * __function_aligned here as that will not ensure that callees are correctly
+ * aligned.
  *
  * See:
  *
  *   https://lore.kernel.org/lkml/Y77%2FqVgvaJidFpYt@FVFF77S0Q05N
  *   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88345#c9
  */
-#if !defined(CONFIG_CC_IS_GCC) || (CONFIG_FUNCTION_ALIGNMENT == 0)
+#if defined(CONFIG_CC_HAS_SANE_FUNCTION_ALIGNMENT) || (CONFIG_FUNCTION_ALIGNMENT == 0)
 #define __cold				__attribute__((__cold__))
 #else
 #define __cold

base-commit: 841c35169323cd833294798e58b9bf63fa4fa1de
-- 
2.35.3