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=-5.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_1 autolearn=ham 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 850A2C433E0 for ; Fri, 29 May 2020 16:25:10 +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 5AB51207F9 for ; Fri, 29 May 2020 16:25:10 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 5AB51207F9 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=citrix.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 1jehoW-0006EL-2H; Fri, 29 May 2020 16:24:56 +0000 Received: from us1-rack-iad1.inumbo.com ([172.99.69.81]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1jehoU-0006EG-VX for xen-devel@lists.xenproject.org; Fri, 29 May 2020 16:24:55 +0000 X-Inumbo-ID: ed8a5be4-a1c8-11ea-9947-bc764e2007e4 Received: from ppsw-31.csi.cam.ac.uk (unknown [131.111.8.131]) by us1-rack-iad1.inumbo.com (Halon) with ESMTPS id ed8a5be4-a1c8-11ea-9947-bc764e2007e4; Fri, 29 May 2020 16:24:54 +0000 (UTC) X-Cam-AntiVirus: no malware found X-Cam-ScannerInfo: http://help.uis.cam.ac.uk/email-scanner-virus Received: from 88-109-182-220.dynamic.dsl.as9105.com ([88.109.182.220]:35382 helo=[192.168.1.219]) by ppsw-31.csi.cam.ac.uk (smtp.hermes.cam.ac.uk [131.111.8.157]:465) with esmtpsa (PLAIN:amc96) (TLSv1.2:ECDHE-RSA-AES128-GCM-SHA256:128) id 1jehoR-000ouI-Lu (Exim 4.92.3) (return-path ); Fri, 29 May 2020 17:24:51 +0100 Subject: Re: [PATCH v3] x86/PV: remove unnecessary toggle_guest_pt() overhead To: Jan Beulich , "xen-devel@lists.xenproject.org" References: <24d8b606-f74b-9367-d67e-e952838c7048@suse.com> From: Andrew Cooper Message-ID: <3dc314dd-3016-aa5e-c327-834b88e9eec2@citrix.com> Date: Fri, 29 May 2020 17:24:50 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.7.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Content-Language: en-GB 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: , Cc: Wei Liu , =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= Errors-To: xen-devel-bounces@lists.xenproject.org Sender: "Xen-devel" On 22/05/2020 11:07, Jan Beulich wrote: > On 21.05.2020 18:46, Andrew Cooper wrote: >> On 05/05/2020 07:16, Jan Beulich wrote: >>> While the mere updating of ->pv_cr3 and ->root_pgt_changed aren't overly >>> expensive (but still needed only for the toggle_guest_mode() path), the >>> effect of the latter on the exit-to-guest path is not insignificant. >>> Move the logic into toggle_guest_mode(), on the basis that >>> toggle_guest_pt() will always be invoked in pairs, yet we can't safely >>> undo the setting of root_pgt_changed during the second of these >>> invocations. >>> >>> While at it, add a comment ahead of toggle_guest_pt() to clarify its >>> intended usage. >>> >>> Signed-off-by: Jan Beulich >> I'm still of the opinion that the commit message wants rewriting to get >> the important points across clearly. >> >> And those are that toggle_guest_pt() is called in pairs specifically to >> read kernel data structures when emulating a userspace action, and that >> this doesn't modify cr3 from the guests point of view, and therefore >> doesn't need the resync on exit-to-guest path. > Is this > > "toggle_guest_pt() is called in pairs, to read guest kernel data > structures when emulating a guest userspace action. Hence this doesn't > modify cr3 from the guest's point of view, and therefore doesn't need > any resync on the exit-to-guest path. Therefore move the updating of > ->pv_cr3 and ->root_pgt_changed into toggle_guest_mode(), since undoing > the changes during the second of these invocations wouldn't be a safe > thing to do." > > any better? Yes - that will do. Acked-by: Andrew Cooper