| Message ID | 20240212145355.1050-1-petr.pavlu@suse.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | kbuild: Use -fmin-function-alignment when available | expand |
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@suse.com> > --- > 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. > 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 > diff --git a/kernel/exit.c b/kernel/exit.c > index dfb963d2f862..5a6fed4ad3df 100644 > --- a/kernel/exit.c > +++ b/kernel/exit.c > @@ -1920,7 +1920,10 @@ EXPORT_SYMBOL(thread_group_exited); > * > * See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88345#c11 > */ > -__weak __function_aligned void abort(void) > +#ifndef CONFIG_CC_HAS_SANE_FUNCTION_ALIGNMENT > +__function_aligned > +#endif > +__weak void abort(void) > { > BUG(); > > > base-commit: 841c35169323cd833294798e58b9bf63fa4fa1de > -- > 2.35.3 >
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@suse.com> >> --- >> 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=$(CONFIG_FUNCTION_ALIGNMENT) > endif > > It is wasteful to call cc-option twice in my opinion, especially if we > are checking it in Kconfig. Makes sense, thanks. -- Petr
diff --git a/Makefile b/Makefile index 7e0b2ad98905..9516e43f6e45 100644 --- a/Makefile +++ b/Makefile @@ -974,7 +974,12 @@ export CC_FLAGS_CFI endif ifneq ($(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 + 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 diff --git a/kernel/exit.c b/kernel/exit.c index dfb963d2f862..5a6fed4ad3df 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -1920,7 +1920,10 @@ EXPORT_SYMBOL(thread_group_exited); * * See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88345#c11 */ -__weak __function_aligned void abort(void) +#ifndef CONFIG_CC_HAS_SANE_FUNCTION_ALIGNMENT +__function_aligned +#endif +__weak void abort(void) { BUG();
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@suse.com> --- Makefile | 7 ++++++- arch/Kconfig | 8 ++++++++ include/linux/compiler_types.h | 10 +++++----- kernel/exit.c | 5 ++++- 4 files changed, 23 insertions(+), 7 deletions(-) base-commit: 841c35169323cd833294798e58b9bf63fa4fa1de