[v3] kbuild: Use -fmin-function-alignment when available
Commit Message
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>
---
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@suse.com/
[2] https://lore.kernel.org/linux-kbuild/20240212145355.1050-1-petr.pavlu@suse.com/
Makefile | 7 +++++++
arch/Kconfig | 12 ++++++++++++
include/linux/compiler_types.h | 10 +++++-----
3 files changed, 24 insertions(+), 5 deletions(-)
base-commit: 841c35169323cd833294798e58b9bf63fa4fa1de
Comments
On Thu, Feb 22, 2024 at 10:35 PM Petr Pavlu <petr.pavlu@suse.com> 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.
>
> 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>
> ---
Applied to linux-kbuild.
Thanks.
@@ -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
@@ -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
@@ -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