From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932291AbeBSXh6 (ORCPT ); Mon, 19 Feb 2018 18:37:58 -0500 Received: from edison.jonmasters.org ([173.255.233.168]:59916 "EHLO edison.jonmasters.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932222AbeBSXh5 (ORCPT ); Mon, 19 Feb 2018 18:37:57 -0500 To: David Woodhouse , Paolo Bonzini , tglx@linutronix.de, x86@kernel.org, kvm@vger.kernel.org, torvalds@linux-foundation.org, linux-kernel@vger.kernel.org, arjan.van.de.ven@intel.com, dave.hansen@intel.com Cc: Ingo Molnar References: <1518449255-2182-1-git-send-email-dwmw@amazon.co.uk> <1518449255-2182-2-git-send-email-dwmw@amazon.co.uk> <7e2e5ad1-49b6-1fdb-4a62-8ad6aefc30a0@redhat.com> <1518509708.12890.33.camel@infradead.org> <27c85759-e662-d281-f8a0-0a80ca8ee18f@redhat.com> <1518517262.12890.43.camel@infradead.org> <1518518198.12890.48.camel@infradead.org> <02bd3fdd-1b73-6cab-fb09-38ba933396bd@redhat.com> <1518775136.7876.13.camel@infradead.org> <75287047-77a0-e0ff-c2e8-61c81641251e@redhat.com> <1518776517.7876.21.camel@infradead.org> <1518783021.7876.34.camel@infradead.org> From: Jon Masters Organization: World Organi{s,z}ation Of Broken Dreams Message-ID: <4882860e-23c5-75b3-ac02-c700f615156e@jonmasters.org> Date: Mon, 19 Feb 2018 18:37:49 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0 MIME-Version: 1.0 In-Reply-To: <1518783021.7876.34.camel@infradead.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-SA-Exim-Connect-IP: 82.111.139.164 X-SA-Exim-Mail-From: jcm@jonmasters.org Subject: Re: [PATCH 2/2] x86/speculation: Support "Enhanced IBRS" on future CPUs X-SA-Exim-Version: 4.2.1 (built Sun, 08 Nov 2009 07:31:22 +0000) X-SA-Exim-Scanned: Yes (on edison.jonmasters.org) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 02/16/2018 07:10 AM, David Woodhouse wrote: > On Fri, 2018-02-16 at 12:04 +0100, Paolo Bonzini wrote: >> On 16/02/2018 11:21, David Woodhouse wrote: >>> Even if the guest doesn't have/support IBRS_ALL, and is frobbing the >>> (now emulated) MSR on every kernel entry/exit, that's *still* going to >>> be a metric shitload faster than what it *thought* it was doing. Is there any indication/log to the admin that VM doesn't know about IBRS_ALL and is constantly uselessly writing to an emulated MSR? While it's probably true that the overhead in time is similar to (or better than) an actual IBRS MSR write, if the admin/user knows the VM needs updating, then there's a fighting chance that they might do so. Jon.