From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932267AbeBSXm1 (ORCPT ); Mon, 19 Feb 2018 18:42:27 -0500 Received: from mga18.intel.com ([134.134.136.126]:27310 "EHLO mga18.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932232AbeBSXm0 (ORCPT ); Mon, 19 Feb 2018 18:42:26 -0500 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.46,537,1511856000"; d="scan'208";a="29202249" From: "Van De Ven, Arjan" To: Jon Masters , David Woodhouse , Paolo Bonzini , "tglx@linutronix.de" , "x86@kernel.org" , "kvm@vger.kernel.org" , "torvalds@linux-foundation.org" , "linux-kernel@vger.kernel.org" , "Hansen, Dave" CC: Ingo Molnar Subject: RE: [PATCH 2/2] x86/speculation: Support "Enhanced IBRS" on future CPUs Thread-Topic: [PATCH 2/2] x86/speculation: Support "Enhanced IBRS" on future CPUs Thread-Index: AQHTpBYQl8Dyc4mRm0iZ20KRHB9/vaOif+eAgAADjgCAABy9AIAABnAAgAAEXACAAAFigIAEqxQAgAACw4CAAAOrgIAAC+oAgAASX4CABXcSgP//eu2w Date: Mon, 19 Feb 2018 23:42:24 +0000 Message-ID: <0575AF4FD06DD142AD198903C74E1CC87A61923C@ORSMSX103.amr.corp.intel.com> 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> <4882860e-23c5-75b3-ac02-c700f615156e@jonmasters.org> In-Reply-To: <4882860e-23c5-75b3-ac02-c700f615156e@jonmasters.org> Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: dlp-product: dlpe-windows dlp-version: 11.0.0.116 dlp-reaction: no-action x-originating-ip: [10.22.254.138] Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: 8bit X-MIME-Autoconverted: from base64 to 8bit by mail.home.local id w1JNgeUx018397 > > >>> 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. the guest is not the problem; guests obviously will already honor if Enhanced IBRS is enumerated. The problem is mixed migration pools where the hypervisor may need to decide to not pass this enumeration through to the guest.