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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT 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 F37DAC71132 for ; Mon, 15 Oct 2018 14:42:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B8AB92083C for ; Mon, 15 Oct 2018 14:42:49 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org B8AB92083C Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com 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 S1726935AbeJOW2V (ORCPT ); Mon, 15 Oct 2018 18:28:21 -0400 Received: from mx1.redhat.com ([209.132.183.28]:50363 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726614AbeJOW2U (ORCPT ); Mon, 15 Oct 2018 18:28:20 -0400 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 0AE16307D943; Mon, 15 Oct 2018 14:42:47 +0000 (UTC) Received: from amt.cnet (ovpn-112-3.gru2.redhat.com [10.97.112.3]) by smtp.corp.redhat.com (Postfix) with ESMTP id 06E4C7A26E; Mon, 15 Oct 2018 14:42:44 +0000 (UTC) Received: from amt.cnet (localhost [127.0.0.1]) by amt.cnet (Postfix) with ESMTP id 619A01050ED; Mon, 15 Oct 2018 10:40:12 -0300 (BRT) Received: (from marcelo@localhost) by amt.cnet (8.14.7/8.14.7/Submit) id w9FDdvDr003285; Mon, 15 Oct 2018 10:39:57 -0300 Date: Mon, 15 Oct 2018 10:39:56 -0300 From: Marcelo Tosatti To: Andy Lutomirski Cc: Peter Zijlstra , Vitaly Kuznetsov , Thomas Gleixner , Paolo Bonzini , Radim Krcmar , Wanpeng Li , LKML , X86 ML , Matt Rickard , Stephen Boyd , John Stultz , Florian Weimer , KY Srinivasan , devel@linuxdriverproject.org, Linux Virtualization , Arnd Bergmann , Juergen Gross Subject: Re: [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK_TAI support Message-ID: <20181015133952.GA2932@amt.cnet> References: <499807AB-E779-40C3-AA3F-E8C77A7770EC@amacapital.net> <20181006202731.GC7129@amt.cnet> <20181008152650.GB27822@amt.cnet> <20181008193632.GA31729@amt.cnet> <20181011222744.GA17955@amt.cnet> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.48]); Mon, 15 Oct 2018 14:42:47 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Oct 11, 2018 at 04:00:29PM -0700, Andy Lutomirski wrote: > On Thu, Oct 11, 2018 at 3:28 PM Marcelo Tosatti wrote: > > > > On Tue, Oct 09, 2018 at 01:09:42PM -0700, Andy Lutomirski wrote: > > > On Tue, Oct 9, 2018 at 8:28 AM Marcelo Tosatti wrote: > > > > > > > > On Mon, Oct 08, 2018 at 10:38:22AM -0700, Andy Lutomirski wrote: > > > > > On Mon, Oct 8, 2018 at 8:27 AM Marcelo Tosatti wrote: > > > > > > > > I read the comment three more times and even dug through the git > > > > > history. It seems like what you're saying is that, under certain > > > > > conditions (which arguably would be bugs in the core Linux timing > > > > > code), > > > > > > > > I don't see that as a bug. Its just a side effect of reading two > > > > different clocks (one is CLOCK_MONOTONIC and the other is TSC), > > > > and using those two clocks to as a "base + offset". > > > > > > > > As the comment explains, if you do that, can't guarantee monotonicity. > > > > > > > > > actually calling ktime_get_boot_ns() could be non-monotonic > > > > > with respect to the kvmclock timing. But get_kvmclock_ns() isn't used > > > > > for VM timing as such -- it's used for the IOCTL interfaces for > > > > > updating the time offset. So can you explain how my patch is > > > > > incorrect? > > > > > > > > ktime_get_boot_ns() has frequency correction applied, while > > > > reading masterclock + TSC offset does not. > > > > > > > > So the clock reads differ. > > > > > > > > > > Ah, okay, I finally think I see what's going on. In the kvmclock data > > > exposed to the guest, tsc_shift and tsc_to_system_mul come from > > > tgt_tsc_khz, whereas master_kernel_ns and master_cycle_now come from > > > CLOCK_BOOTTIME. So the kvmclock and kernel clock drift apart at a > > > rate given by the frequency shift and then suddenly agree again every > > > time the pvclock data is updated. > > > > Yes. > > > > > Is there a reason to do it this way? > > > > Since pvclock updates which update system_timestamp are expensive (must stop all vcpus), > > they should be avoided. > > > > Fair enough. > > > So only HW TSC counts > > makes sense. > > >, and used as offset against vcpu's tsc_timestamp. > > > > Why don't you just expose CLOCK_MONTONIC_RAW or CLOCK_MONOTONIC_RAW > plus suspend time, though? Then you would actually be tracking a real > kernel timekeeping mode, and you wouldn't need all this complicated > offsetting work to avoid accidentally going backwards. Can you outline how that would work ?