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=-7.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,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 5145EC433E0 for ; Tue, 2 Mar 2021 10:02:30 +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 E354164F12 for ; Tue, 2 Mar 2021 10:02:29 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E354164F12 Authentication-Results: mail.kernel.org; dmarc=fail (p=quarantine dis=none) header.from=suse.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=xen-devel-bounces@lists.xenproject.org Received: from list by lists.xenproject.org with outflank-mailman.92235.174021 (Exim 4.92) (envelope-from ) id 1lH1rB-0007vB-9b; Tue, 02 Mar 2021 10:02:21 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 92235.174021; Tue, 02 Mar 2021 10:02:21 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lH1rB-0007v4-5Q; Tue, 02 Mar 2021 10:02:21 +0000 Received: by outflank-mailman (input) for mailman id 92235; Tue, 02 Mar 2021 10:02:19 +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 1lH1r9-0007uy-ML for xen-devel@lists.xenproject.org; Tue, 02 Mar 2021 10:02:19 +0000 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-amaz-eas2.inumbo.com (Halon) with ESMTPS id 2ba1421b-d65f-424b-bb0d-582ced201441; Tue, 02 Mar 2021 10:02:18 +0000 (UTC) Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id C1AD8AD74; Tue, 2 Mar 2021 10:02:17 +0000 (UTC) X-BeenThere: xen-devel@lists.xenproject.org List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Errors-To: xen-devel-bounces@lists.xenproject.org Precedence: list Sender: "Xen-devel" X-Inumbo-ID: 2ba1421b-d65f-424b-bb0d-582ced201441 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1614679337; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=a6g4Mjjf8sjhNeIrAXR0tJkEZNDVJRke9j41eMR8MCk=; b=TEHOQTBYl9UOLfd/tdAjm8OykW7MOM8OXBqBbf36B5zOUUa1tMty+qL2D8NK0CyHb0/I+4 KBGm3Zz5UTDaJRZpFbgSgE2mutwWXfp1UwZzU1l7TCTyN/jCNJgpLrSMPF5/IzfnFUr7pP p1wlmh/UUDvNYnffjlESh9z0HGAyDJw= Subject: Re: [PATCH 2/2][4.15?] x86: fix build when NR_CPUS == 1 To: Ian Jackson Cc: "xen-devel@lists.xenproject.org" , Andrew Cooper , Wei Liu , =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= , George Dunlap References: <1d8d5089-32a9-9c42-5949-8f9cd20f33e0@suse.com> <24637.4165.400955.400473@mariner.uk.xensource.com> From: Jan Beulich Message-ID: <7b4c0c51-5a3c-b9bb-7c9c-62285095183b@suse.com> Date: Tue, 2 Mar 2021 11:02:17 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.7.1 MIME-Version: 1.0 In-Reply-To: <24637.4165.400955.400473@mariner.uk.xensource.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit On 01.03.2021 17:03, Ian Jackson wrote: > Jan Beulich writes ("[PATCH 2/2][4.15?] x86: fix build when NR_CPUS == 1"): >> In this case the compiler is recognizing that no valid array indexes >> remain (in x2apic_cluster()'s access to per_cpu(cpu_2_logical_apicid, >> ...)), but oddly enough isn't really consistent about the checking it >> does (see the code comment). > ... >> - if (this_cpu == cpu || x2apic_cluster(this_cpu) != x2apic_cluster(cpu)) >> + if ( this_cpu == cpu ) >> + continue; >> + /* >> + * Guard in particular against the compiler suspecting out-of-bounds >> + * array accesses below when NR_CPUS=1 (oddly enough with gcc 10 it >> + * is the 1st of these alone which actually helps, not the 2nd, nor >> + * are both required together there). >> + */ >> + BUG_ON(this_cpu >= NR_CPUS); >> + BUG_ON(cpu >= NR_CPUS); >> + if ( x2apic_cluster(this_cpu) != x2apic_cluster(cpu) ) >> continue; > > Is there some particular reason for not putting the BUG_ON before the > if test ? That would avoid the refactoring. I want it to be as close as possible to the place where the issue is. I also view the refactoring as a good thing, since it allows a style correction as a side effect. > Of course putting it in next to the array indexing would address that > too. See my earlier reply to Roger's similar remark (which still was along the lines of what I've said above). Jan