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=-2.3 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 34827C76188 for ; Mon, 22 Jul 2019 15:18:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 152D1204FD for ; Mon, 22 Jul 2019 15:18:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728440AbfGVPS2 (ORCPT ); Mon, 22 Jul 2019 11:18:28 -0400 Received: from gate.crashing.org ([63.228.1.57]:35288 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726443AbfGVPS2 (ORCPT ); Mon, 22 Jul 2019 11:18:28 -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 x6MFI3ra026871; Mon, 22 Jul 2019 10:18:03 -0500 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id x6MFI1aA026870; Mon, 22 Jul 2019 10:18:01 -0500 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Mon, 22 Jul 2019 10:18:01 -0500 From: Segher Boessenkool To: Michael Ellerman Cc: Nathan Chancellor , Christophe Leroy , Benjamin Herrenschmidt , Paul Mackerras , linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com Subject: Re: [PATCH v2] powerpc: slightly improve cache helpers Message-ID: <20190722151801.GC20882@gate.crashing.org> References: <45hnfp6SlLz9sP0@ozlabs.org> <20190708191416.GA21442@archlinux-threadripper> <20190709064952.GA40851@archlinux-threadripper> <20190719032456.GA14108@archlinux-threadripper> <20190719152303.GA20882@gate.crashing.org> <20190719160455.GA12420@archlinux-threadripper> <20190721075846.GA97701@archlinux-threadripper> <20190721180150.GN20882@gate.crashing.org> <87imru74ul.fsf@concordia.ellerman.id.au> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87imru74ul.fsf@concordia.ellerman.id.au> User-Agent: Mutt/1.4.2.3i Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jul 22, 2019 at 08:15:14PM +1000, Michael Ellerman wrote: > Segher Boessenkool writes: > > On Sun, Jul 21, 2019 at 12:58:46AM -0700, Nathan Chancellor wrote: > >> 0000017c clear_user_page: > >> 17c: 94 21 ff f0 stwu 1, -16(1) > >> 180: 38 80 00 80 li 4, 128 > >> 184: 38 63 ff e0 addi 3, 3, -32 > >> 188: 7c 89 03 a6 mtctr 4 > >> 18c: 38 81 00 0f addi 4, 1, 15 > >> 190: 8c c3 00 20 lbzu 6, 32(3) > >> 194: 98 c1 00 0f stb 6, 15(1) > >> 198: 7c 00 27 ec dcbz 0, 4 > >> 19c: 42 00 ff f4 bdnz .+65524 > > > > Uh, yeah, well, I have no idea what clang tried here, but that won't > > work. It's copying a byte from each target cache line to the stack, > > and then does clears the cache line containing that byte on the stack. > > So it seems like this is a clang bug. > > None of the distros we support use clang, but we would still like to > keep it working if we can. Which version? Which versions *are* broken? > Looking at the original patch, the only upside is that the compiler > can use both RA and RB to compute the address, rather than us forcing RA > to 0. > > But at least with my compiler here (GCC 8 vintage) I don't actually see > GCC ever using both GPRs even with the patch. Or at least, there's no > difference before/after the patch as far as I can see. The benefit is small, certainly. > So my inclination is to revert the original patch. We can try again in a > few years :D > > Thoughts? I think you should give the clang people time to figure out what is going on. Segher