From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-9.1 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING, SPF_HELO_NONE,SPF_PASS autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B7B50C388F2 for ; Fri, 6 Nov 2020 16:45:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6334821556 for ; Fri, 6 Nov 2020 16:45:38 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1604681138; bh=0K0W2lRmQqU/CWXrnBp8f9sTiCbm+as4zM2nijcCQeQ=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=0DehoFcFaPHn0BFzAttetW+ozeTYmBWzTGkK757ABfp//r0bbEcNUYiN1eoCL7a5S 981MYCVQkYMMKaCVdyvER2IWU7ZiN6f2staxnnTyv84az7ZA2MQehT6SSVWIQuozyW y41qQPTFsErP7hhXuZQlgfEXAlhQ4Rmdtpn7vRSc= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726732AbgKFQpi (ORCPT ); Fri, 6 Nov 2020 11:45:38 -0500 Received: from mail-lj1-f193.google.com ([209.85.208.193]:38570 "EHLO mail-lj1-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725868AbgKFQph (ORCPT ); Fri, 6 Nov 2020 11:45:37 -0500 Received: by mail-lj1-f193.google.com with SMTP id v19so2039575lji.5 for ; Fri, 06 Nov 2020 08:45:36 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=8NXRtqHffFnsMIzZ6TAmIGSVk+62VdYhbYjy8LdH8vo=; b=sJXNJueJ5vYTgFMNfd69JDY9z3HAEAK6YW2IWIc5V7OKj4FB5R+Ugn8K54Pc5IYGgm J0wYebmZTXuBdVUZGEhp44A5v/W/wgwbAnh3wl5T7KlN3IAIwPKfGQL2sgURG2XQgFuc 8T1wzs6hTGKdwxmbuWeNVh9H3kgL/tl89CKbS3Ig0u5Ph8UuARALm+XAtFmtFEspUbpd kdFXESrtRow/rZ/KxOhvN8z+7SBUe9wKGSMEXG4Kiz7vyUR0U91PmblOAatv1r2S3QGF rx9twtKphhn8jzTm4MI5ptwr7aMSUrtBDpQtTRKXV6Lrio3gbAwXl0gwuppS1hDWmp+R EtTg== X-Gm-Message-State: AOAM531iJIUTXi47aaKeaEeInzu+BRLsKuK+/VRAsx43t4fNtePPkpl4 N9sjjahYtaMS75YavgMnTXU= X-Google-Smtp-Source: ABdhPJzQDc4igKg3ddTAUZAeM/D96jrzN/OvKFYB6yCypvurl1JtTkTAMCaE+gtMQy6S8MKMYrvbbA== X-Received: by 2002:a2e:5705:: with SMTP id l5mr1006042ljb.93.1604681135209; Fri, 06 Nov 2020 08:45:35 -0800 (PST) Received: from xi.terra (c-beaee455.07-184-6d6c6d4.bbcust.telenor.se. [85.228.174.190]) by smtp.gmail.com with ESMTPSA id q14sm220476lfb.281.2020.11.06.08.45.34 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 06 Nov 2020 08:45:34 -0800 (PST) Received: from johan by xi.terra with local (Exim 4.93.0.4) (envelope-from ) id 1kb4rp-0000Ae-NT; Fri, 06 Nov 2020 17:45:38 +0100 Date: Fri, 6 Nov 2020 17:45:37 +0100 From: Johan Hovold To: Jessica Yu Cc: Johan Hovold , linux-kernel@vger.kernel.org, Linus Torvalds , Rob Herring , Frank Rowand , Greg Kroah-Hartman , Nick Desaulniers , Arnd Bergmann , Geert Uytterhoeven , Dmitry Torokhov , David Miller , Jakub Jelinek , Peter Zijlstra , Thomas Gleixner , Steven Rostedt , Daniel Kurtz , linux-arch@vger.kernel.org, linux-m68k@lists.linux-m68k.org Subject: Re: [PATCH 0/8] linker-section array fix and clean ups Message-ID: <20201106164537.GD4085@localhost> References: <20201103175711.10731-1-johan@kernel.org> <20201106160344.GA12184@linux-8ccs.fritz.box> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201106160344.GA12184@linux-8ccs.fritz.box> Precedence: bulk List-ID: X-Mailing-List: linux-m68k@vger.kernel.org On Fri, Nov 06, 2020 at 05:03:45PM +0100, Jessica Yu wrote: > +++ Johan Hovold [03/11/20 18:57 +0100]: > >We rely on the linker to create arrays for a number of things including > >kernel parameters and device-tree-match entries. > > > >The stride of these linker-section arrays obviously needs to match the > >expectations of the code accessing them or bad things will happen. > > > >One thing to watch out for is that gcc is known to increase the > >alignment of larger objects with static extent as an optimisation (on > >x86), but this can be suppressed by using the aligned attribute when > >declaring entries. > > > >We've been relying on this behaviour for 16 years for kernel parameters > >(and other structures) and it indeed hasn't changed since the > >introduction of the aligned attribute in gcc 3.1 (see align_variable() > >in [1]). > > > >Occasionally this gcc optimisation do cause problems which have instead > >been worked around in various creative ways including using indirection > >through an array of pointers. This was originally done for tracepoints > >[2] after a number of failed attempts to create properly aligned arrays, > >and the approach was later reused for module-version attributes [3] and > >earlycon entries. > > >[2] https://lore.kernel.org/lkml/20110126222622.GA10794@Krystal/ > So unfortunately, I am not familiar enough with the semantics of gcc's > aligned attribute. AFAICT from the patch you linked in [2], the > original purpose of the pointer indirection workaround was to avoid > relying on (potentially inconsistent) compiler-specific behavior with > respect to the aligned attribute. The main concern was potential > up-alignment being done by gcc (or the linker) despite the desired > alignment being specified. Indeed, the gcc documentation also states > that the aligned attribute only specifies the *minimum* alignment, > although there's no guarantee that up-alignment wouldn't occur. > > So I guess my question is, is there some implicit guarantee that > specifying alignment by type via __alignof__ that's supposed to > prevent gcc from up-aligning? Or are we just assuming that gcc won't > increase the alignment? The gcc docs don't seem to clarify this > unfortunately. It's simply specifying alignment when declaring the variable that prevents this optimisation. The relevant code is in the function align_variable() in [1] where DATA_ALIGNMENT() is never called in case an alignment has been specified (!DECL_USER_ALIGN(decl)). There's no mention in the documentation of this that I'm aware of, but this is the way the aligned attribute has worked since its introduction judging from the commit history. As mentioned above, we've been relying on this for kernel parameters and other structures since 2003-2004 so if it ever were to change we'd find out soon enough. It's about to be used for scheduler classes as well. [2] Johan [1] https://github.com/gcc-mirror/gcc/blob/master/gcc/varasm.c [2] https://lore.kernel.org/r/160396870486.397.377616182428528939.tip-bot2@tip-bot2