Commit 5270316c authored by Petr Pavlu's avatar Petr Pavlu Committed by Masahiro Yamada
Browse files

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: default avatarPetr Pavlu <petr.pavlu@suse.com>
Reviewed-by: default avatarNathan Chancellor <nathan@kernel.org>
Acked-by: default avatarMark Rutland <mark.rutland@arm.com>
Signed-off-by: default avatarMasahiro Yamada <masahiroy@kernel.org>
parent c31f96a0

Makefile

+7 −0
Original line number Diff line number Diff line
@@ -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

arch/Kconfig

+12 −0
Original line number Diff line number Diff line
@@ -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

include/linux/compiler_types.h

+5 −5
Original line number Diff line number Diff line
@@ -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