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=-0.8 required=3.0 tests=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 BE63EC7618F for ; Mon, 22 Jul 2019 23:22:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 94CF121951 for ; Mon, 22 Jul 2019 23:22:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727123AbfGVXV7 (ORCPT ); Mon, 22 Jul 2019 19:21:59 -0400 Received: from ozlabs.org ([203.11.71.1]:58339 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726120AbfGVXV7 (ORCPT ); Mon, 22 Jul 2019 19:21:59 -0400 Received: from authenticated.ozlabs.org (localhost [127.0.0.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange ECDHE (P-256) server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by mail.ozlabs.org (Postfix) with ESMTPSA id 45syLF0b78z9s4Y; Tue, 23 Jul 2019 09:21:57 +1000 (AEST) From: Michael Ellerman To: Segher Boessenkool 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 In-Reply-To: <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> <20190722151801.GC20882@gate.crashing.org> Date: Tue, 23 Jul 2019 09:21:53 +1000 Message-ID: <875znt7izy.fsf@concordia.ellerman.id.au> MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Segher Boessenkool writes: > 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? AFAIK clang 8 is the first version that we could build with, without hacks. >> 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. Zero is small, but I guess some things are smaller? :P >> 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. Yeah fair enough, will wait and see what their diagnosis is. cheers