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=-1.0 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED 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 DE9D2C433E0 for ; Thu, 25 Jun 2020 08:26:16 +0000 (UTC) Received: from merlin.infradead.org (merlin.infradead.org [205.233.59.134]) (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 A7D46207E8 for ; Thu, 25 Jun 2020 08:26:16 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="Qjzr6uGz" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org A7D46207E8 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=merlin.20170209; h=Sender:Content-Transfer-Encoding: Content-Type:Cc:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References:Message-ID: Subject:To:From:Date:Reply-To:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=abFxoK6gXGE12/hkH5oRwNy/aEI8gsQg4LUeb3yu6Vw=; b=Qjzr6uGzR1xXpyToswTqHHxbH CBIo9cu6Oq/JHJnTHRkA2uInhvavRscRH+VovZA3Et2okgu0XNlwkWq/aD6TSrFcY7XgT1Rk2riML wXlq9LzRVIdpJ35eVJyvYsKbiEBPGSUgTstho7MSkMf+KNabu5B5xl5+6x+DwAuMnZNIVU9tJN1nR pf3VixZW/6ug9WnWKU/dlqGjhC315neDtjXfBJihs9hHe8DtwlYLP40EB5zRc3oQaoctREoFkT8cZ Ag+j2GqI2YV1kNDsUJPXTW4WMKOoxLImlG9VH2qnMUi7HajRsuufOVgGKbKdva/DcqoZa3j8vFASo miYFpcg8w==; Received: from localhost ([::1] helo=merlin.infradead.org) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1joNBV-00010W-Bx; Thu, 25 Jun 2020 08:24:37 +0000 Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1joNBS-0000yr-J3; Thu, 25 Jun 2020 08:24:34 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 2EE4E301A7A; Thu, 25 Jun 2020 10:24:33 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 234DB2215AD45; Thu, 25 Jun 2020 10:24:33 +0200 (CEST) Date: Thu, 25 Jun 2020 10:24:33 +0200 From: Peter Zijlstra To: Nick Desaulniers Subject: Re: [PATCH 00/22] add support for Clang LTO Message-ID: <20200625082433.GC117543@hirez.programming.kicks-ass.net> References: <20200624203200.78870-1-samitolvanen@google.com> <20200624211540.GS4817@hirez.programming.kicks-ass.net> <20200625080313.GY4817@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20200625080313.GY4817@hirez.programming.kicks-ass.net> X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-arch , "maintainer:X86 ARCHITECTURE \(32-BIT AND 64-BIT\)" , Kees Cook , "Paul E. McKenney" , Kernel Hardening , Greg Kroah-Hartman , Masahiro Yamada , Linux Kbuild mailing list , LKML , clang-built-linux , Sami Tolvanen , linux-pci@vger.kernel.org, Will Deacon , Linux ARM Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Thu, Jun 25, 2020 at 10:03:13AM +0200, Peter Zijlstra wrote: > On Wed, Jun 24, 2020 at 02:31:36PM -0700, Nick Desaulniers wrote: > > On Wed, Jun 24, 2020 at 2:15 PM Peter Zijlstra wrote: > > > > > > On Wed, Jun 24, 2020 at 01:31:38PM -0700, Sami Tolvanen wrote: > > > > This patch series adds support for building x86_64 and arm64 kernels > > > > with Clang's Link Time Optimization (LTO). > > > > > > > > In addition to performance, the primary motivation for LTO is to allow > > > > Clang's Control-Flow Integrity (CFI) to be used in the kernel. Google's > > > > Pixel devices have shipped with LTO+CFI kernels since 2018. > > > > > > > > Most of the patches are build system changes for handling LLVM bitcode, > > > > which Clang produces with LTO instead of ELF object files, postponing > > > > ELF processing until a later stage, and ensuring initcall ordering. > > > > > > > > Note that first objtool patch in the series is already in linux-next, > > > > but as it's needed with LTO, I'm including it also here to make testing > > > > easier. > > > > > > I'm very sad that yet again, memory ordering isn't addressed. LTO vastly > > > increases the range of the optimizer to wreck things. > > > > Hi Peter, could you expand on the issue for the folks on the thread? > > I'm happy to try to hack something up in LLVM if we check that X does > > or does not happen; maybe we can even come up with some concrete test > > cases that can be added to LLVM's codebase? > > I'm sure Will will respond, but the basic issue is the trainwreck C11 > made of dependent loads. > > Anyway, here's a link to the last time this came up: > > https://lore.kernel.org/linux-arm-kernel/20171116174830.GX3624@linux.vnet.ibm.com/ Another good read: https://lore.kernel.org/lkml/20150520005510.GA23559@linux.vnet.ibm.com/ and having (partially) re-read that, I now worry intensily about things like latch_tree_find(), cyc2ns_read_begin, __ktime_get_fast_ns(). It looks like kernel/time/sched_clock.c uses raw_read_seqcount() which deviates from the above patterns by, for some reason, using a primitive that includes an extra smp_rmb(). And this is just the few things I could remember off the top of my head, who knows what else is out there. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel