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@xxxxxxxx> Reviewed-by: Nathan Chancellor <nathan@xxxxxxxxxx> Acked-by: Mark Rutland <mark.rutland@xxxxxxx> --- 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@xxxxxxxx/ [2] https://lore.kernel.org/linux-kbuild/20240212145355.1050-1-petr.pavlu@xxxxxxxx/ 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
