Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752668AbZJOFYj (ORCPT ); Thu, 15 Oct 2009 01:24:39 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752557AbZJOFYj (ORCPT ); Thu, 15 Oct 2009 01:24:39 -0400 Received: from claw.goop.org ([74.207.240.146]:47797 "EHLO claw.goop.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752543AbZJOFYi (ORCPT ); Thu, 15 Oct 2009 01:24:38 -0400 Message-ID: <4AD6B1F0.6030904@goop.org> Date: Wed, 14 Oct 2009 22:24:00 -0700 From: Jeremy Fitzhardinge User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.4pre) Gecko/20090922 Fedora/3.0-2.7.b4.fc11 Lightning/1.0pre Thunderbird/3.0b4 MIME-Version: 1.0 To: Dan Magenheimer CC: Jeremy Fitzhardinge , Linux Kernel Mailing List , Xen-devel , kurt.hackel@oracle.com, arch/x86 maintainers , Glauber de Oliveira Costa , Avi Kivity , chris.mason@oracle.com Subject: Re: [Xen-devel] [PATCH 05/12] xen/pvclock: add monotonicity check References: <0be3e26b-76b6-47c9-b7ab-51ed72b72b69@default> In-Reply-To: <0be3e26b-76b6-47c9-b7ab-51ed72b72b69@default> X-Enigmail-Version: 0.97a Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1019 Lines: 25 On 10/14/09 20:26, Dan Magenheimer wrote: > As long as we are going through the trouble of making > this monotonic, shouldn't it be monotonically increasing > (rather than just monotonically non-decreasing)? The > rdtsc instruction and any suitably high-precision > hardware timer will never return the same value > on subsequent uses so this might be a reasonable > precedent to obey. E.g. > > + return ret > xen_clocksource.cycle_last ? > + ret : ++xen_clocksource.cycle_last; > No, cycle_last isn't updated on every read, only on timer ticks. This test doesn't seem to be intended to make sure that every clocksource_read is globally monotonic, but just to avoid some boundary conditions in the timer interrupt. I just copied it directly from read_tsc(). J -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/