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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 9AF2BC433E0 for ; Thu, 28 May 2020 09:42:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 728EA20B80 for ; Thu, 28 May 2020 09:42:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728467AbgE1Jl7 (ORCPT ); Thu, 28 May 2020 05:41:59 -0400 Received: from foss.arm.com ([217.140.110.172]:50144 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728300AbgE1Jl7 (ORCPT ); Thu, 28 May 2020 05:41:59 -0400 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 B72C9D6E; Thu, 28 May 2020 02:41:58 -0700 (PDT) Received: from gaia (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 7DABC3F6C4; Thu, 28 May 2020 02:41:56 -0700 (PDT) Date: Thu, 28 May 2020 10:41:54 +0100 From: Catalin Marinas To: Peter Smith Cc: Nick Desaulniers , Robin Murphy , Will Deacon , Naohiro Aota , Stephen Boyd , Masahiro Yamada , LKML , Manoj Gupta , Luis Lozano , Nathan Chancellor , Vincenzo Frascino , Linux ARM , Kristof Beyls , Victor Campos , "david.spickett@linaro.org" , Arnd Bergmann Subject: Re: [PATCH] arm64: vdso32: force vdso32 to be compiled as -marm Message-ID: <20200528094154.GB2961@gaia> References: <20200526173117.155339-1-ndesaulniers@google.com> <2f58c2a4-0f37-d507-7767-00161c6b5d98@arm.com> <34f261f7-c4b5-a628-9a4c-eb97b75fba52@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, May 28, 2020 at 09:05:08AM +0100, Peter Smith wrote: > I suggest using Arm if you need a frame pointer, and disable the > frame pointer if you want/need to use Thumb. My understanding is that > runtime unwinding using the frame pointer in Thumb is already difficult > due to Arm and Thumb functions using different registers for the frame > pointer. IIRC from the Thumb-2 kernel porting days, even in the absence of ARM/Thumb interworking, the Thumb-2 frame pointer was pretty useless for unwinding since it points to the bottom of the current stack frame (the reason I think is that some LDR/STR instructions with negative indexing are not available). So finding the previous frame pointer was not possible and had to rely on the exception unwinding information. -- Catalin