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=-7.1 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,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 E3B88C43461 for ; Mon, 7 Sep 2020 10:15:00 +0000 (UTC) Received: from lists.xenproject.org (lists.xenproject.org [192.237.175.120]) (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 96EB620C09 for ; Mon, 7 Sep 2020 10:15:00 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 96EB620C09 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=suse.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=xen-devel-bounces@lists.xenproject.org Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kFEAe-0001om-Rm; Mon, 07 Sep 2020 10:14:44 +0000 Received: from all-amaz-eas1.inumbo.com ([34.197.232.57] helo=us1-amaz-eas2.inumbo.com) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kFEAd-0001oh-5t for xen-devel@lists.xenproject.org; Mon, 07 Sep 2020 10:14:43 +0000 X-Inumbo-ID: e63f33c3-d714-4805-bec7-643b03391f9a Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-amaz-eas2.inumbo.com (Halon) with ESMTPS id e63f33c3-d714-4805-bec7-643b03391f9a; Mon, 07 Sep 2020 10:14:41 +0000 (UTC) X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id BE19DAB0E; Mon, 7 Sep 2020 10:14:41 +0000 (UTC) Subject: Re: [PATCH] x86: guard against straight-line speculation past RET From: Jan Beulich To: Andrew Cooper Cc: "xen-devel@lists.xenproject.org" , Wei Liu , =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= References: <38aaf96b-1235-b205-71d6-16aee057c402@citrix.com> Message-ID: <3d3dec3b-5f5b-3a8b-9172-22f726b0fab4@suse.com> Date: Mon, 7 Sep 2020 12:14:39 +0200 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.12.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit X-BeenThere: xen-devel@lists.xenproject.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Errors-To: xen-devel-bounces@lists.xenproject.org Sender: "Xen-devel" On 07.09.2020 11:25, Jan Beulich wrote: > On 04.09.2020 20:18, Andrew Cooper wrote: >> Clang doesn't actually expand the macro for ret instructions, so a Clang >> build of Xen only ends up getting protected in the assembly files. >> >> The following experiment demonstrates the issue: >> >> $ cat ret.c >> asm (".macro ret\n\t" >>      ".error \"foo\"\n\t" >>      ".endm\n\t"); >> void foo(void) {} >> >> $ gcc -O3 -c ret.c -o ret.o && objdump -d ret.o >> /tmp/ccf8hkyN.s: Assembler messages: >> /tmp/ccf8hkyN.s:16: Error: foo >> >> $ clang-10 -O3 -c ret.c -o ret.o && objdump -d ret.o >> >> ret.o:     file format elf64-x86-64 >> >> >> Disassembly of section .text: >> >> 0000000000000000 : >>    0:    c3                       retq >> >> >> Worse, -no-integrated-as doesn't immediately help, even though it >> invokes $(AS). >> >> I tracked that down to the difference between ret and retq, which >> highlights an assumption about GCC which may not remain true in the future. >> >> Adding a second macro covering retq fixes the scenario in combination >> with -no-integrated-as. > > Ah, yes, I should of course have thought of retq. Albeit as per > above - generated code looks fine here when using clang 5. I'm sorry, I can indeed see this part of the issue. I did look at the wrong build tree when putting together the earlier reply. Jan