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=-4.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, TVD_SUBJ_WIPE_DEBT,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 0E17CC43461 for ; Thu, 10 Sep 2020 15:54:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BBE7D20BED for ; Thu, 10 Sep 2020 15:54:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731449AbgIJPym (ORCPT ); Thu, 10 Sep 2020 11:54:42 -0400 Received: from gate.crashing.org ([63.228.1.57]:45384 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731322AbgIJPws (ORCPT ); Thu, 10 Sep 2020 11:52:48 -0400 Received: from gate.crashing.org (localhost.localdomain [127.0.0.1]) by gate.crashing.org (8.14.1/8.14.1) with ESMTP id 08AFGK0M032611; Thu, 10 Sep 2020 10:16:20 -0500 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 08AFGJBX032610; Thu, 10 Sep 2020 10:16:19 -0500 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Thu, 10 Sep 2020 10:16:19 -0500 From: Segher Boessenkool To: David Laight Cc: "'Christophe Leroy'" , "'Linus Torvalds'" , linux-arch , Kees Cook , the arch/x86 maintainers , Nick Desaulniers , Linux Kernel Mailing List , Christoph Hellwig , Luis Chamberlain , Al Viro , linux-fsdevel , linuxppc-dev , Alexey Dobriyan Subject: Re: remove the last set_fs() in common code, and remove it for x86 and powerpc v3 Message-ID: <20200910151619.GI28786@gate.crashing.org> References: <20200903142242.925828-1-hch@lst.de> <20200903142803.GM1236603@ZenIV.linux.org.uk> <20200909184001.GB28786@gate.crashing.org> <3beb8b019e4a4f7b81fdb1bc68bd1e2d@AcuMS.aculab.com> <186a62fc-042c-d6ab-e7dc-e61b18945498@csgroup.eu> <59a64e9a210847b59f70f9bd2d02b5c3@AcuMS.aculab.com> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <59a64e9a210847b59f70f9bd2d02b5c3@AcuMS.aculab.com> User-Agent: Mutt/1.4.2.3i Sender: linux-arch-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-arch@vger.kernel.org On Thu, Sep 10, 2020 at 09:26:28AM +0000, David Laight wrote: > From: Christophe Leroy > > Sent: 10 September 2020 09:14 > > > > Le 10/09/2020 à 10:04, David Laight a écrit : > > > From: Linus Torvalds > > >> Sent: 09 September 2020 22:34 > > >> On Wed, Sep 9, 2020 at 11:42 AM Segher Boessenkool > > >> wrote: > > >>> > > >>> It will not work like this in GCC, no. The LLVM people know about that. > > >>> I do not know why they insist on pushing this, being incompatible and > > >>> everything. > > >> > > >> Umm. Since they'd be the ones supporting this, *gcc* would be the > > >> incompatible one, not clang. > > > > > > I had an 'interesting' idea. > > > > > > Can you use a local asm register variable as an input and output to > > > an 'asm volatile goto' statement? > > > > > > Well you can - but is it guaranteed to work :-) > > > > > > > With gcc at least it should work according to > > https://gcc.gnu.org/onlinedocs/gcc/Local-Register-Variables.html > > > > They even explicitely tell: "The only supported use for this feature is > > to specify registers for input and output operands when calling Extended > > asm " > > A quick test isn't good.... > > int bar(char *z) > { > __label__ label; > register int eax asm ("eax") = 6; > asm volatile goto (" mov $1, %%eax" ::: "eax" : label); > > label: > return eax; > } It is neither input nor output operand here! Only *then* is a local register asm guaranteed to be in the given reg: as input or output to an inline asm. Segher