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=-5.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,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 AB8EFC433E6 for ; Tue, 9 Feb 2021 14:01:45 +0000 (UTC) Received: from lists.ozlabs.org (lists.ozlabs.org [203.11.71.2]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 9326064D99 for ; Tue, 9 Feb 2021 14:01:42 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 9326064D99 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=kernel.crashing.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Received: from bilbo.ozlabs.org (lists.ozlabs.org [IPv6:2401:3900:2:1::3]) by lists.ozlabs.org (Postfix) with ESMTP id 4DZl1S3bfVzDrcP for ; Wed, 10 Feb 2021 01:01:32 +1100 (AEDT) Authentication-Results: lists.ozlabs.org; spf=permerror (SPF Permanent Error: Unknown mechanism found: ip:192.40.192.88/32) smtp.mailfrom=kernel.crashing.org (client-ip=63.228.1.57; helo=gate.crashing.org; envelope-from=segher@kernel.crashing.org; receiver=) Received: from gate.crashing.org (gate.crashing.org [63.228.1.57]) by lists.ozlabs.org (Postfix) with ESMTP id 4DZksB2YnvzDspH for ; Wed, 10 Feb 2021 00:54:21 +1100 (AEDT) Received: from gate.crashing.org (localhost.localdomain [127.0.0.1]) by gate.crashing.org (8.14.1/8.14.1) with ESMTP id 119Dot0b023626; Tue, 9 Feb 2021 07:50:55 -0600 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 119DorgE023625; Tue, 9 Feb 2021 07:50:53 -0600 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Tue, 9 Feb 2021 07:50:53 -0600 From: Segher Boessenkool To: Nicholas Piggin Subject: Re: [PATCH v5 20/22] powerpc/syscall: Avoid storing 'current' in another pointer Message-ID: <20210209135053.GD27854@gate.crashing.org> References: <24804747098369ebcdac38970b8f7a1260bdd248.1612796617.git.christophe.leroy@csgroup.eu> <1612838134.rvncv9kzls.astroid@bobo.none> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1612838134.rvncv9kzls.astroid@bobo.none> User-Agent: Mutt/1.4.2.3i X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-kernel@vger.kernel.org, Paul Mackerras , msuchanek@suse.de, linuxppc-dev@lists.ozlabs.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Tue, Feb 09, 2021 at 12:36:20PM +1000, Nicholas Piggin wrote: > What if you did this? > +static inline struct task_struct *get_current(void) > +{ > + register struct task_struct *task asm ("r2"); > + > + return task; > +} Local register asm variables are *only* guaranteed to live in that register as operands to an asm. See https://gcc.gnu.org/onlinedocs/gcc/Local-Register-Variables.html#Local-Register-Variables ("The only supported use" etc.) You can do something like static inline struct task_struct *get_current(void) { register struct task_struct *task asm ("r2"); asm("" : "+r"(task)); return task; } which makes sure that "task" actually is in r2 at the point of that asm. Segher