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@suse.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

diff --git a/Makefile b/Makefile
index bc54a6c..d84c0fb 100644 (file)
--- 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 a5af0ed..bd6c633 100644 (file)
--- 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 0caf354..fb88886 100644 (file)
--- 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