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 F2EBDC433DF for ; Fri, 22 May 2020 11:00:38 +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 D0C3120679 for ; Fri, 22 May 2020 11:00:38 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org D0C3120679 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 1jc5PZ-0007HN-HP; Fri, 22 May 2020 11:00:21 +0000 Received: from us1-rack-iad1.inumbo.com ([172.99.69.81]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1jc5PX-0007HI-TQ for xen-devel@lists.xenproject.org; Fri, 22 May 2020 11:00:19 +0000 X-Inumbo-ID: 6c67d9a4-9c1b-11ea-ae69-bc764e2007e4 Received: from ppsw-31.csi.cam.ac.uk (unknown [131.111.8.131]) by us1-rack-iad1.inumbo.com (Halon) with ESMTPS id 6c67d9a4-9c1b-11ea-ae69-bc764e2007e4; Fri, 22 May 2020 11:00:19 +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]:39742 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 1jc5PS-000Dj6-LG (Exim 4.92.3) (return-path ); Fri, 22 May 2020 12:00:14 +0100 Subject: Re: [PATCH v3 1/5] x86: suppress XPTI-related TLB flushes when possible To: Jan Beulich , "xen-devel@lists.xenproject.org" References: <3ce4ab2c-8cb6-1482-6ce9-3d5b019e10c1@suse.com> From: Andrew Cooper Message-ID: <17f1b674-92f9-6ee9-8e10-0fc30f055fe8@citrix.com> Date: Fri, 22 May 2020 12:00:14 +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: George Dunlap , Wei Liu , =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= Errors-To: xen-devel-bounces@lists.xenproject.org Sender: "Xen-devel" On 25/09/2019 16:23, Jan Beulich wrote: > When there's no XPTI-enabled PV domain at all, there's no need to issue > respective TLB flushes. Hardwire opt_xpti_* to false when !PV, and > record the creation of PV domains by bumping opt_xpti_* accordingly. > > As to the sticky opt_xpti_domu vs increment/decrement of opt_xpti_hwdom, > this is done this way to avoid > (a) widening the former variable, > (b) any risk of a missed flush, which would result in an XSA if a DomU > was able to exercise it, and > (c) any races updating the variable. > Fundamentally the TLB flush done when context switching out the domain's > vCPU-s the last time before destroying the domain ought to be > sufficient, so in principle DomU handling could be made match hwdom's. > > Signed-off-by: Jan Beulich I am still concerned about the added complexity for no obvious use case. Under what circumstances do we expect to XPTI-ness come and go on a system, outside of custom dev-testing scenarios?