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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>
This commit is contained in:
Petr Pavlu 2024-02-22 14:35:00 +01:00 committed by Masahiro Yamada
parent c31f96a00f
commit 5270316c9f
3 changed files with 24 additions and 5 deletions
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7
Makefile
View File

@ -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

12
arch/Kconfig
View File

@ -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

10
include/linux/compiler_types.h
View File

@ -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