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 C8F56C433DB for ; Tue, 2 Mar 2021 13:18:03 +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 706E560231 for ; Tue, 2 Mar 2021 13:18:03 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 706E560231 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.92337.174227 (Exim 4.92) (envelope-from ) id 1lH4u5-0003Gs-BB; Tue, 02 Mar 2021 13:17:33 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 92337.174227; Tue, 02 Mar 2021 13:17:33 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lH4u5-0003Gl-7v; Tue, 02 Mar 2021 13:17:33 +0000 Received: by outflank-mailman (input) for mailman id 92337; Tue, 02 Mar 2021 13:17:31 +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 1lH4u3-0003Gg-Cx for xen-devel@lists.xenproject.org; Tue, 02 Mar 2021 13:17:31 +0000 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-amaz-eas2.inumbo.com (Halon) with ESMTPS id 8661fcfe-ecfb-407e-b8c4-ebe1983c32f7; Tue, 02 Mar 2021 13:17:30 +0000 (UTC) Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 8886CABF4; Tue, 2 Mar 2021 13:17:29 +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: 8661fcfe-ecfb-407e-b8c4-ebe1983c32f7 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=1614691049; 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=q3YLJ/1TbX5vgLaUMxs+xbO9ahunRESEPXHHc3rxuY4=; b=ZFHMWdImzcYvH6ZhCyw6FgCv+N9PKRoaA33IFE+dX+4gjvaj2hvz1lQUC4mahkN0vKtJ5o gNkfIZGinu3wupfZMb/AvSgOom1I6uWY8X6E2OwuXpWP2dlLkvpYwac8J6gmuiUxJtqZWl 94VVLo0aFqnP67xkS+WgoJ9psapTyfw= Subject: Re: [PATCH][4.15] x86/shadow: suppress "fast fault path" optimization without reserved bits To: Ian Jackson Cc: Tim Deegan , "xen-devel@lists.xenproject.org" , George Dunlap , Wei Liu , =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= , Andrew Cooper References: <72c3c863-7465-ce26-1f57-b71227bb2b19@suse.com> <24637.9408.839033.439930@mariner.uk.xensource.com> <99db39fd-e479-f71d-9cb9-5903ff6c12f4@citrix.com> <24637.10160.977640.808417@mariner.uk.xensource.com> <24638.12380.286845.325678@mariner.uk.xensource.com> From: Jan Beulich Message-ID: <4cde5206-9c9e-c1fa-ad2e-bc1f16450aca@suse.com> Date: Tue, 2 Mar 2021 14:17:29 +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: <24638.12380.286845.325678@mariner.uk.xensource.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit On 02.03.2021 13:32, Ian Jackson wrote: > Jan Beulich writes ("Re: [PATCH][4.15] x86/shadow: suppress "fast fault path" optimization without reserved bits"): >> On 01.03.2021 18:43, Ian Jackson wrote: >>> Andrew Cooper writes ("Re: [PATCH][4.15] x86/shadow: suppress "fast fault path" optimization without reserved bits"): >>>> On 01/03/2021 17:30, Ian Jackson wrote: >>>>> I'm afraid I don't follow enough of the background here to have an >>>>> opinion right now. Can someone explain to me the risks (and, >>>>> correspondingly, upsides) of the options ? Sorry to be dim, I don't >>>>> seem to be firing on all cylinders today. >> >> I guess the risk from that patch is no different than that from the >> patch here. It would merely improve performance for guests using >> very large GFNs for memory areas needing emulation by qemu, which I >> suppose originally wasn't expected to be happening in the first place. >> In fact if I would have been certain there are no side effects of the >> too narrow GFN representation used so far, I would probably have >> submitted the patches in reverse order, or even folded them. > > I am still confused. You are saying that the existing patch, and your > proposal that you are wanting me to have an opinion on, have the same > risk. So, what aspect of the proposed other way of fixing it might > make me say no ? Yet another change / yet more code churn for merely improving what got fixed already. But, oh, there looks to be a misunderstanding nevertheless - I'm not proposing another way of addressing the same issue, but instead a performance improvement (for perhaps just an unlikely case) on top of the fix that you did give your ack for already. (As said - had I been certain of no aspect that I might be overlooking, I might have folded that. But I wasn't certain, and hence I did post the basic fix first / individually.) Jan