From patchwork Mon Jan 9 13:58:21 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Mark Rutland X-Patchwork-Id: 13093613 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 533DDC54EBD for ; Mon, 9 Jan 2023 13:58:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230513AbjAIN6x (ORCPT ); Mon, 9 Jan 2023 08:58:53 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37994 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237241AbjAIN6l (ORCPT ); Mon, 9 Jan 2023 08:58:41 -0500 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id DE339FCA; Mon, 9 Jan 2023 05:58:40 -0800 (PST) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 9B4601515; Mon, 9 Jan 2023 05:59:22 -0800 (PST) Received: from lakrids.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPA id C43293F23F; Mon, 9 Jan 2023 05:58:38 -0800 (PST) From: Mark Rutland To: linux-arm-kernel@lists.infradead.org Cc: catalin.marinas@arm.com, lenb@kernel.org, linux-acpi@vger.kernel.org, linux-kernel@vger.kernel.org, mark.rutland@arm.com, mhiramat@kernel.org, ndesaulniers@google.com, ojeda@kernel.org, peterz@infradead.org, rafael.j.wysocki@intel.com, revest@chromium.org, robert.moore@intel.com, rostedt@goodmis.org, will@kernel.org Subject: [PATCH 1/8] Compiler attributes: GCC function alignment workarounds Date: Mon, 9 Jan 2023 13:58:21 +0000 Message-Id: <20230109135828.879136-2-mark.rutland@arm.com> X-Mailer: git-send-email 2.30.2 In-Reply-To: <20230109135828.879136-1-mark.rutland@arm.com> References: <20230109135828.879136-1-mark.rutland@arm.com> MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: linux-acpi@vger.kernel.org From local testing, contemporary versions of of GCC (e.g. GCC 12.2.0) don't respect '-falign-functions=N' in all cases. This is unfortunate, as (for non-zero values of N) CONFIG_FUNCTION_ALIGNMENT=N will set '-falign-functions=N', but this won't take effect for all functions. LLVM appears to respect '-falign-functions=N' in call cases. Today, for x86 this turns out to be functionally benign, though it does somewhat undermine the CONFIG_FUNCTION_ALIGNMENT option, and it means that CONFIG_DEBUG_FORCE_FUNCTION_ALIGN_64B is not as robust as we'd like. On arm64 we'd like to use CONFIG_FUNCTION_ALIGNMENT to implement ftrace functionality, and we'll require function alignment to be respected for functional reasons. As far as I can tell, GCC doesn't respect '-falign-functions=N': * When the __weak__ attribute is used GCC seems to forget the alignment specified by '-falign-functions=N', but will respect the '__aligned__(N)' function attribute. Thus, we can work around this by explciitly setting the alignment for weak functions. * When the __cold__ attribute is used GCC seems to forget the alignment specified by '-falign-functions=N', and also doesn't seem to respect the '__aligned__(N)' function attribute. The only way to work around this is to not use the __cold__ attibute. This patch implements workarounds for these two cases, using a function attribute to set the alignment of __weak__ functions, and preventing the use of the __cold__ attribute when CONFIG_FUNCTION_ALIGNMENT is non-zero. I've tested this by selecting CONFIG_DEBUG_FORCE_FUNCTION_ALIGN_64B=y, building and booting a kernel, and looking for misaligned text symbols: * arm64: Before: # grep ' [Tt] ' /proc/kallsyms | grep -iv '[048c]0 [Tt] ' | wc -l 4939 After: # grep ' [Tt] ' /proc/kallsyms | grep -iv '[048c]0 [Tt] ' | wc -l 908 * x86_64: Before: # grep ' [Tt] ' /proc/kallsyms | grep -iv '[048c]0 [Tt] ' | wc -l 7969 After: # grep ' [Tt] ' /proc/kallsyms | grep -iv '[048c]0 [Tt] ' | wc -l 2057 With the patch applied, the remaining unaligned text labels are a combination of static call trampolines, non-function labels in assembly, and ACPICA functions, which will be dealt with in subsequent patches. Signed-off-by: Mark Rutland Cc: Catalin Marinas Cc: Florent Revest Cc: Masami Hiramatsu Cc: Peter Zijlstra Cc: Steven Rostedt Cc: Will Deacon Cc: Miguel Ojeda Cc: Nick Desaulniers --- include/linux/compiler_attributes.h | 23 ++++++++++++++++++++++- 1 file changed, 22 insertions(+), 1 deletion(-) diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h index 898b3458b24a0..dcb7ac67b764f 100644 --- a/include/linux/compiler_attributes.h +++ b/include/linux/compiler_attributes.h @@ -33,6 +33,17 @@ #define __aligned(x) __attribute__((__aligned__(x))) #define __aligned_largest __attribute__((__aligned__)) +/* + * Contemporary versions of GCC (e.g. 12.2.0) don't always respect + * '-falign-functions=N', and require alignment to be specificed via a function + * attribute in some cases. + */ +#if CONFIG_FUNCTION_ALIGNMENT > 0 +#define __function_aligned __aligned(CONFIG_FUNCTION_ALIGNMENT) +#else +#define __function_aligned +#endif + /* * Note: do not use this directly. Instead, use __alloc_size() since it is conditionally * available and includes other attributes. For GCC < 9.1, __alloc_size__ gets undefined @@ -78,8 +89,15 @@ /* * 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 + * + * GCC drops function alignment when the __cold__ attribute is used. Avoid the + * __cold__ attribute if function alignment is required. */ +#if !defined(CONFIG_CC_IS_GCC) || (CONFIG_FUNCTION_ALIGNMENT == 0) #define __cold __attribute__((__cold__)) +#else +#define __cold +#endif /* * Note the long name. @@ -369,8 +387,11 @@ /* * gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-weak-function-attribute * gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-weak-variable-attribute + * + * GCC drops function alignment when the __weak__ attribute is used. This can + * be restored with function attributes. */ -#define __weak __attribute__((__weak__)) +#define __weak __attribute__((__weak__)) __function_aligned /* * Used by functions that use '__builtin_return_address'. These function