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=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 9311BC2BB84 for ; Mon, 7 Sep 2020 08:49:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5997D20757 for ; Mon, 7 Sep 2020 08:49:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728304AbgIGIte convert rfc822-to-8bit (ORCPT ); Mon, 7 Sep 2020 04:49:34 -0400 Received: from eu-smtp-delivery-151.mimecast.com ([185.58.86.151]:26253 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728272AbgIGItc (ORCPT ); Mon, 7 Sep 2020 04:49:32 -0400 Received: from AcuMS.aculab.com (156.67.243.126 [156.67.243.126]) (Using TLS) by relay.mimecast.com with ESMTP id uk-mta-35-5zmwIGe8NUucU3bez4pxOg-1; Mon, 07 Sep 2020 09:49:28 +0100 X-MC-Unique: 5zmwIGe8NUucU3bez4pxOg-1 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) by AcuMS.aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) with Microsoft SMTP Server (TLS) id 15.0.1347.2; Mon, 7 Sep 2020 09:49:28 +0100 Received: from AcuMS.Aculab.com ([fe80::43c:695e:880f:8750]) by AcuMS.aculab.com ([fe80::43c:695e:880f:8750%12]) with mapi id 15.00.1347.000; Mon, 7 Sep 2020 09:49:28 +0100 From: David Laight To: 'Christoph Hellwig' , Paul Walmsley , Palmer Dabbelt , "Arnd Bergmann" , Alexander Viro CC: "linux-kernel@vger.kernel.org" , "linux-arch@vger.kernel.org" Subject: Using asm_volatile_goto for get_user() Thread-Topic: Using asm_volatile_goto for get_user() Thread-Index: AdaE8q1aQOesIAbfTka1rFFrCf3mnA== Date: Mon, 7 Sep 2020 08:49:27 +0000 Message-ID: <39a56b046b104566922dd0d0ce7f794a@AcuMS.aculab.com> Accept-Language: en-GB, en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=C51A453 smtp.mailfrom=david.laight@aculab.com X-Mimecast-Spam-Score: 0.001 X-Mimecast-Originator: aculab.com Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org I had an idea that might let 'asm_volatile_goto' be used for get_user() even though gcc doesn't allow outputs. What if (eg) 'register eax asm ("eax") is used for the output and (probably) given in the 'clobber' list. Such variables are usually used to get explicit registers used when there is no suitable constraint. I don't see why it shouldn't work for 'asm goto' as well as just 'asm'. While this forces the read value into a specific register that probably doesn't make much difference to the code. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)