On 2/13/24 01:27, Nathan Chancellor wrote: > Hi Petr, > > On Mon, Feb 12, 2024 at 03:53:55PM +0100, Petr Pavlu wrote: >> 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. >> >> Detect availability of -fmin-function-alignment and use it instead of >> -falign-functions when present. Introduce CC_HAS_SANE_FUNCTION_ALIGNMENT >> and make the workarounds for the broken function alignment conditional >> on this setting. >> >> Signed-off-by: Petr Pavlu <petr.pavlu@xxxxxxxx> >> --- >> Makefile | 7 ++++++- >> arch/Kconfig | 8 ++++++++ >> include/linux/compiler_types.h | 10 +++++----- >> kernel/exit.c | 5 ++++- >> 4 files changed, 23 insertions(+), 7 deletions(-) >> >> diff --git a/Makefile b/Makefile >> index 7e0b2ad98905..9516e43f6e45 100644 >> --- a/Makefile >> +++ b/Makefile >> @@ -974,7 +974,12 @@ export CC_FLAGS_CFI >> endif >> >> ifneq ($(CONFIG_FUNCTION_ALIGNMENT),0) >> -KBUILD_CFLAGS += -falign-functions=$(CONFIG_FUNCTION_ALIGNMENT) >> +# Set the minimal function alignment. Try to use the newer GCC option >> +# -fmin-function-alignment, or fall back to -falign-funtions. See also >> +# CONFIG_CC_HAS_SANE_FUNCTION_ALIGNMENT. >> +KBUILD_CFLAGS += $(call cc-option, \ >> + -fmin-function-alignment=$(CONFIG_FUNCTION_ALIGNMENT), \ >> + -falign-functions=$(CONFIG_FUNCTION_ALIGNMENT)) >> endif >> >> # arch Makefile may override CC so keep this after arch Makefile is included >> diff --git a/arch/Kconfig b/arch/Kconfig >> index a5af0edd3eb8..e2448f927fae 100644 >> --- a/arch/Kconfig >> +++ b/arch/Kconfig >> @@ -1507,4 +1507,12 @@ config FUNCTION_ALIGNMENT >> default 4 if FUNCTION_ALIGNMENT_4B >> default 0 >> >> +config CC_HAS_SANE_FUNCTION_ALIGNMENT >> + # Detect availability of the GCC option -fmin-function-alignment which >> + # guarantees minimal alignment for all functions. GCC 13 and older have >> + # only -falign-functions which the compiler ignores for cold functions >> + # and this hence requires extra care in the kernel. Clang provides >> + # strict alignment always when using -falign-functions. >> + def_bool $(cc-option, -fmin-function-alignment=8) || CC_IS_CLANG >> + > > I think this configuration should be split into something like > CONFIG_CC_HAS_MIN_FUNCTION_ALIGNMENT that has the cc-option check then > CONFIG_CC_HAS_SANE_FUNCTION_ALIGNMENT can depend on that configuration > or Clang as you have it here, so that we can drop the cc-option check in > the main Makefile and have it be: > > ifdef CONFIG_CC_HAS_MIN_FUNCTION_ALIGNMENT > KBUILD_CFLAGS += -fmin-function-alignment=$(CONFIG_FUNCTION_ALIGNMENT) > else > KBUILD_CFLAGS += -falign-functions=$(CONFIG_FUNCTION_ALIGNMENT) > endif > > It is wasteful to call cc-option twice in my opinion, especially if we > are checking it in Kconfig. Makes sense, thanks. -- Petr
