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=-5.8 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 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 0C1F6C433DB for ; Mon, 1 Mar 2021 17:43:32 +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 B7D16650C5 for ; Mon, 1 Mar 2021 17:43:31 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org B7D16650C5 Authentication-Results: mail.kernel.org; dmarc=pass (p=none dis=none) header.from=xenproject.org 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.91868.173410 (Exim 4.92) (envelope-from ) id 1lGmZm-0003K4-57; Mon, 01 Mar 2021 17:43:22 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 91868.173410; Mon, 01 Mar 2021 17:43:22 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lGmZm-0003Jx-27; Mon, 01 Mar 2021 17:43:22 +0000 Received: by outflank-mailman (input) for mailman id 91868; Mon, 01 Mar 2021 17:43:21 +0000 Received: from mail.xenproject.org ([104.130.215.37]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lGmZl-0003Js-0r for xen-devel@lists.xenproject.org; Mon, 01 Mar 2021 17:43:21 +0000 Received: from xenbits.xenproject.org ([104.239.192.120]) by mail.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lGmZl-0005Og-00 for xen-devel@lists.xenproject.org; Mon, 01 Mar 2021 17:43:21 +0000 Received: from iwj (helo=mariner.uk.xensource.com) by xenbits.xenproject.org with local-bsmtp (Exim 4.92) (envelope-from ) id 1lGmZk-0004uH-UT for xen-devel@lists.xenproject.org; Mon, 01 Mar 2021 17:43:20 +0000 Received: from iwj by mariner.uk.xensource.com with local (Exim 4.89) (envelope-from ) id 1lGmZd-00058Q-6i; Mon, 01 Mar 2021 17:43:13 +0000 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" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=xenproject.org; s=20200302mail; h=References:In-Reply-To:Subject:Cc:To:Date :Message-ID:Content-Transfer-Encoding:Content-Type:MIME-Version:From; bh=U0CreRcQ5HyG0Zvfrs3/RntpCAxbuwUTGhco0LWVEUw=; b=kLTDbOqGLa4gKlD20i826701tH RVoCNycZ8I47InaFY3Z1pwDTQlA17aKX8N/KUy1ftCqExDBvE45nYVNd+/vKjwZvMSN5sBucw3jJ6 gzEKZSADrquI3zdvLYif2lEostmN6TQY2C3Z8ro49tmBP6aol3yF5oklVIH/o2XIAI4Q=; From: Ian Jackson MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: 8bit Message-ID: <24637.10160.977640.808417@mariner.uk.xensource.com> Date: Mon, 1 Mar 2021 17:43:12 +0000 To: Andrew Cooper Cc: Jan Beulich , Tim Deegan , "xen-devel\@lists.xenproject.org" , George Dunlap , Wei Liu , Roger Pau =?iso-8859-1?Q?Monn=E9?= Subject: Re: [PATCH][4.15] x86/shadow: suppress "fast fault path" optimization without reserved bits In-Reply-To: <99db39fd-e479-f71d-9cb9-5903ff6c12f4@citrix.com> References: <72c3c863-7465-ce26-1f57-b71227bb2b19@suse.com> <24637.9408.839033.439930@mariner.uk.xensource.com> <99db39fd-e479-f71d-9cb9-5903ff6c12f4@citrix.com> X-Mailer: VM 8.2.0b under 24.5.1 (i686-pc-linux-gnu) 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: > > Jan Beulich writes ("Re: [PATCH][4.15] x86/shadow: suppress "fast fault path" optimization without reserved bits"): > >> On 26.02.2021 18:07, Tim Deegan wrote: > >>> Yes, I think it could be reduced to use just one reserved address bit. > >>> IIRC we just used such a large mask so the magic entries would be > >>> really obvious in debugging, and there was no need to support arbitrary > >>> address widths for emulated devices. > >> Will cook a patch, albeit I guess I'll keep as many of the bits set > >> as possible, while still being able to encode a full-40-bit GFN. > >> > >> Ian - I don't suppose you'd consider this a reasonable thing to do > >> for 4.15? It would allow limiting the negative (performance) effect > >> the change here has. > > 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. > > Intel IceLake CPUs (imminently coming out) have no reserved bits in > pagetable entries, so these "optimisations" malfunction.  It is > definitely an issue for HVM Shadow guests, and possibly migration of PV > guests (I can never remember whether we use the GNP fastpath on PV or not). > > It is arguably wrong that we ever depended on reserved behaviour. > > I've got a (not-yet-upsteamed) XTF test which can comprehensively test > this.  I'll find some time to give them a spin and give a T-by. > > Without this fix, some combinations of "normal" VM settings will > malfunction. Thanks for that explanation. I don't quite follow how that relates to Jan's comment >> Will cook a patch, albeit I guess I'll keep as many of the bits set >> as possible, while still being able to encode a full-40-bit GFN. >> >> Ian - I don't suppose you'd consider this a reasonable thing to do >> for 4.15? It would allow limiting the negative (performance) effect >> the change here has. I already gave a release-ack for the original patch. I think Jan is asking for a release-ack for a different way of fixing the problem. Ian.