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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS 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 7A9BEECE561 for ; Tue, 18 Sep 2018 10:42:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 1607221471 for ; Tue, 18 Sep 2018 10:42:10 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 1607221471 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=linutronix.de Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729110AbeIRQOI (ORCPT ); Tue, 18 Sep 2018 12:14:08 -0400 Received: from Galois.linutronix.de ([146.0.238.70]:57415 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727207AbeIRQOH (ORCPT ); Tue, 18 Sep 2018 12:14:07 -0400 Received: from hsi-kbw-5-158-153-55.hsi19.kabel-badenwuerttemberg.de ([5.158.153.55] helo=nanos) by Galois.linutronix.de with esmtpsa (TLS1.2:DHE_RSA_AES_256_CBC_SHA256:256) (Exim 4.80) (envelope-from ) id 1g2DSF-0007YN-KN; Tue, 18 Sep 2018 12:42:03 +0200 Date: Tue, 18 Sep 2018 12:41:57 +0200 (CEST) From: Thomas Gleixner To: Peter Zijlstra cc: John Stultz , Andy Lutomirski , LKML , X86 ML , Matt Rickard , Stephen Boyd , Florian Weimer , "K. Y. Srinivasan" , Vitaly Kuznetsov , devel@linuxdriverproject.org, Linux Virtualization , Paolo Bonzini , Arnd Bergmann , Juergen Gross Subject: Re: [patch 09/11] x86/vdso: Simplify the invalid vclock case In-Reply-To: Message-ID: References: <20180914125006.349747096@linutronix.de> <20180914125118.909646643@linutronix.de> <20180918083055.GJ24106@hirez.programming.kicks-ass.net> User-Agent: Alpine 2.21 (DEB 202 2017-01-01) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 18 Sep 2018, Thomas Gleixner wrote: > On Tue, 18 Sep 2018, Thomas Gleixner wrote: > > On Tue, 18 Sep 2018, Peter Zijlstra wrote: > > > > Your memory serves you right. That's indeed observable on CPUs which > > > > lack TSC_ADJUST. > > > > > > But, if the gtod code can observe this, then why doesn't the code that > > > checks the sync? > > > > Because it depends where the involved CPUs are in the topology. The sync > > code might just run on the same package an simply not see it. Yes, w/o > > TSC_ADJUST the TSC sync code can just fail to see the havoc. > > Even with TSC adjust the TSC can be slightly off by design on multi-socket > systems. Here are the gory details: https://lore.kernel.org/lkml/3c1737210708230408i7a8049a9m5db49e6c4d89ab62@mail.gmail.com/ The changelog has an explanation as well. d8bb6f4c1670 ("x86: tsc prevent time going backwards") I still have one of the machines which is affected by this. Thanks, tglx