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[23.128.96.18]) by mx.google.com with ESMTP id jr6si258921ejb.259.2020.06.25.02.40.03; Thu, 25 Jun 2020 02:41:09 -0700 (PDT) Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18; Authentication-Results: mx.google.com; dkim=fail header.i=@infradead.org header.s=merlin.20170209 header.b="ibvgrw/6"; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2390546AbgFYIY5 (ORCPT + 99 others); Thu, 25 Jun 2020 04:24:57 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46954 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2389522AbgFYIY4 (ORCPT ); Thu, 25 Jun 2020 04:24:56 -0400 Received: from merlin.infradead.org (unknown [IPv6:2001:8b0:10b:1231::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 89C13C061573; Thu, 25 Jun 2020 01:24:56 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=merlin.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=3AMdYg8fQNUtXAfj2fZZ8NPZ9U5EFgepOOFBuun2GQY=; b=ibvgrw/6VgAeXKGL/ZvFOV5Ile k/DG2hTUgCNpBFh0SVRZtiOzJ0tqIzRrnNRTZQdCWgPu3f3vKwwqumEO3VAaSPjMg/ls3R3fwVjKM Nhs12sSgDR+t8SDyMh/h3ITlxN3ZcgcTvbI7CAd5D8YCcq8qCphd0KnXhjA+ZVPLtaGvU/eak1k++ 6iVJ/9DrfMDdhwuZ5/zvsYALWHu77JxkNZKUQ+1riHrSFoU2dIT3hgk0fU4PGeIKpR0kVxQwAc5hC H0eBkD5up1mt7ynRtI/herLh3Yr5SxMM5rpkTFOeHKnOBtnv2OaV3fJ7KHA9qL8uVglbMrLJEM+Gs wUku04xw==; 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 Cc: Sami Tolvanen , Masahiro Yamada , Will Deacon , Greg Kroah-Hartman , "Paul E. McKenney" , Kees Cook , clang-built-linux , Kernel Hardening , linux-arch , Linux ARM , Linux Kbuild mailing list , LKML , linux-pci@vger.kernel.org, "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 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-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200625080313.GY4817@hirez.programming.kicks-ass.net> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.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.