This is a note to let you know that I've just added the patch titled
Compiler attributes: GCC cold function alignment workarounds
to the 6.2-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
compiler-attributes-gcc-cold-function-alignment-work.patch
and it can be found in the queue-6.2 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 7545e05b9cc4aa8b0252334ffaeb62470ff93b98
Author: Mark Rutland <mark.rutland@xxxxxxx>
Date: Mon Jan 23 13:45:57 2023 +0000
Compiler attributes: GCC cold function alignment workarounds
[ Upstream commit c27cd083cfb9d392f304657ed00fcde1136704e7 ]
Contemporary versions of GCC (e.g. GCC 12.2.0) drop the alignment
specified by '-falign-functions=N' for functions marked with the
__cold__ attribute, and potentially for callees of __cold__ functions as
these may be implicitly marked as __cold__ by the compiler. LLVM appears
to respect '-falign-functions=N' in such cases.
This has been reported to GCC in bug 88345:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88345
... which also covers alignment being dropped when '-Os' is used, which
will be addressed in a separate patch.
Currently, use of '-falign-functions=N' is limited to
CONFIG_FUNCTION_ALIGNMENT, which is largely used for performance and/or
analysis reasons (e.g. with CONFIG_DEBUG_FORCE_FUNCTION_ALIGN_64B), but
isn't necessary for correct functionality. However, this dropped
alignment isn't great for the performance and/or analysis cases.
Subsequent patches will use CONFIG_FUNCTION_ALIGNMENT as part of arm64's
ftrace implementation, which will require all instrumented functions to
be aligned to at least 8-bytes.
This patch works around the dropped alignment by avoiding the use of the
__cold__ attribute when CONFIG_FUNCTION_ALIGNMENT is non-zero, and by
specifically aligning abort(), which GCC implicitly marks as __cold__.
As the __cold macro is now dependent upon config options (which is
against the policy described at the top of compiler_attributes.h), it is
moved into compiler_types.h.
I've tested this by building and booting a kernel configured with
defconfig + CONFIG_EXPERT=y + CONFIG_DEBUG_FORCE_FUNCTION_ALIGN_64B=y,
and looking for misaligned text symbols in /proc/kallsyms:
* arm64:
Before:
# uname -rm
6.2.0-rc3 aarch64
# grep ' [Tt] ' /proc/kallsyms | grep -iv '[048c]0 [Tt] ' | wc -l
5009
After:
# uname -rm
6.2.0-rc3-00001-g2a2bedf8bfa9 aarch64
# grep ' [Tt] ' /proc/kallsyms | grep -iv '[048c]0 [Tt] ' | wc -l
919
* x86_64:
Before:
# uname -rm
6.2.0-rc3 x86_64
# grep ' [Tt] ' /proc/kallsyms | grep -iv '[048c]0 [Tt] ' | wc -l
11537
After:
# uname -rm
6.2.0-rc3-00001-g2a2bedf8bfa9 x86_64
# grep ' [Tt] ' /proc/kallsyms | grep -iv '[048c]0 [Tt] ' | wc -l
2805
There's clearly a substantial reduction in the number of misaligned
symbols. From manual inspection, the remaining unaligned text labels are
a combination of ACPICA functions (due to the use of '-Os'), static call
trampolines, and non-function labels in assembly, which will be dealt
with in subsequent patches.
Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Florent Revest <revest@xxxxxxxxxxxx>
Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Miguel Ojeda <ojeda@xxxxxxxxxx>
Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20230123134603.1064407-3-mark.rutland@xxxxxxx
Signed-off-by: Catalin Marinas <catalin.marinas@xxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
index 898b3458b24a0..b83126452c651 100644
--- a/include/linux/compiler_attributes.h
+++ b/include/linux/compiler_attributes.h
@@ -75,12 +75,6 @@
# define __assume_aligned(a, ...)
#endif
-/*
- * gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-cold-function-attribute
- * gcc: https://gcc.gnu.org/onlinedocs/gcc/Label-Attributes.html#index-cold-label-attribute
- */
-#define __cold __attribute__((__cold__))
-
/*
* Note the long name.
*
diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 7c1afe0f4129c..aab34e30128e9 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -79,6 +79,33 @@ static inline void __chk_io_ptr(const volatile void __iomem *ptr) { }
/* Attributes */
#include <linux/compiler_attributes.h>
+#if CONFIG_FUNCTION_ALIGNMENT > 0
+#define __function_aligned __aligned(CONFIG_FUNCTION_ALIGNMENT)
+#else
+#define __function_aligned
+#endif
+
+/*
+ * gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-cold-function-attribute
+ * 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.
+ *
+ * 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)
+#define __cold __attribute__((__cold__))
+#else
+#define __cold
+#endif
+
/* Builtins */
/*
diff --git a/kernel/exit.c b/kernel/exit.c
index bccfa4218356e..f2afdb0add7c5 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1905,7 +1905,14 @@ bool thread_group_exited(struct pid *pid)
}
EXPORT_SYMBOL(thread_group_exited);
-__weak void abort(void)
+/*
+ * This needs to be __function_aligned as GCC implicitly makes any
+ * implementation of abort() cold and drops alignment specified by
+ * -falign-functions=N.
+ *
+ * See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88345#c11
+ */
+__weak __function_aligned void abort(void)
{
BUG();
