From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751833AbdA3KqY (ORCPT ); Mon, 30 Jan 2017 05:46:24 -0500 Received: from Galois.linutronix.de ([146.0.238.70]:58245 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751498AbdA3KqW (ORCPT ); Mon, 30 Jan 2017 05:46:22 -0500 Date: Mon, 30 Jan 2017 11:20:25 +0100 (CET) From: Thomas Gleixner To: Henning Schild cc: LKML , Ingo Molnar , Peter Zijlstra , Borislav Petkov , Yinghai Lu Subject: Re: [3/8] x86/tsc: Store and check TSC ADJUST MSR In-Reply-To: <20170127143632.4f56d927@md1em3qc> Message-ID: References: <20161119134017.655323776@linutronix.de> <20170127143632.4f56d927@md1em3qc> User-Agent: Alpine 2.20 (DEB 67 2015-01-07) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Henning, On Fri, 27 Jan 2017, Henning Schild wrote: > > did you by any chance look into TSC synchronization by adjusting the > absolute value (MSR_IA32_TSC) as well? As far as i have seen Linux did > that a long time ago and eventually it was stopped because it caused more > harm than good. I was involved in both developing the TSC sync patches and ripping them out again. The problem with writing TSC directly is that you really _CANNOT_ reliably handle run-time differences and SMI/NMI induced deltas. With the adjust MRS it's a halfways sane thing to do, except for the brokeness of that botch job vs. the TSC deadline timer. > The ADJUST MSR offers an easy way to synchronize, still taking care of > all the special cases resulted in an 8-patch series. Synching without > that using the absolute value is likely much harder, but that series > might be a good foundation. I'm not even thinking about bringing the pure TSC based sync back. > The big question is whether we can rely on all future CPUs to > support that MSR. Do "new MSRs" disappear again at some point? If we > can not rely on the ADJUST MSR, now might be a good time to revisit the > idea of synching the absolute values. There is nothing you can ever be sure about, but I doubt that the ADJUST MSR is going to vanish. > I remember having read somewhere that this series might get backported > to longterm kernels, what is the status on that? No idea. Thanks, tglx