From c27cd083cfb9d392f304657ed00fcde1136704e7 Mon Sep 17 00:00:00 2001 From: Mark Rutland Date: Mon, 23 Jan 2023 13:45:57 +0000 Subject: Compiler attributes: GCC cold function alignment workarounds 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 Cc: Florent Revest Cc: Masami Hiramatsu Cc: Peter Zijlstra Cc: Steven Rostedt Cc: Will Deacon Cc: Miguel Ojeda Cc: Nick Desaulniers Link: https://lore.kernel.org/r/20230123134603.1064407-3-mark.rutland@arm.com Signed-off-by: Catalin Marinas --- kernel/exit.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) (limited to 'kernel/exit.c') diff --git a/kernel/exit.c b/kernel/exit.c index 15dc2ec80c46..c8e0375705f4 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -1898,7 +1898,14 @@ 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(); -- cgit v1.2.3