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=-5.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,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 2686AC433DF for ; Fri, 22 May 2020 10:07:41 +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 E2B6B205CB for ; Fri, 22 May 2020 10:07:40 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org E2B6B205CB 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 1jc4aQ-00015m-Nc; Fri, 22 May 2020 10:07:30 +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 1jc4aQ-00015h-9p for xen-devel@lists.xenproject.org; Fri, 22 May 2020 10:07:30 +0000 X-Inumbo-ID: 0b65bbd2-9c14-11ea-aba9-12813bfff9fa Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-amaz-eas2.inumbo.com (Halon) with ESMTPS id 0b65bbd2-9c14-11ea-aba9-12813bfff9fa; Fri, 22 May 2020 10:07:29 +0000 (UTC) X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 7E252AC63; Fri, 22 May 2020 10:07:31 +0000 (UTC) Subject: Re: [PATCH v3] x86/PV: remove unnecessary toggle_guest_pt() overhead To: Andrew Cooper , "xen-devel@lists.xenproject.org" References: <24d8b606-f74b-9367-d67e-e952838c7048@suse.com> From: Jan Beulich Message-ID: Date: Fri, 22 May 2020 12:07:27 +0200 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.8.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit 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 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? Jan