Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754874Ab3ILQjj (ORCPT ); Thu, 12 Sep 2013 12:39:39 -0400 Received: from service87.mimecast.com ([91.220.42.44]:49556 "EHLO service87.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753195Ab3ILQjh convert rfc822-to-8bit (ORCPT ); Thu, 12 Sep 2013 12:39:37 -0400 Message-ID: <5231EE43.5090900@arm.com> Date: Thu, 12 Sep 2013 17:39:31 +0100 From: Marc Zyngier User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130330 Thunderbird/17.0.5 MIME-Version: 1.0 To: cinifr CC: Mark Rutland , "coosty@163.com" , "maxime.ripard@free-electrons.com" , "daniel.lezcano@linaro.org" , "linux@arm.linux.org.uk" , "tglx@linutronix.de" , "linux-arm-kernel@lists.infradead.org" , "linux-kernel@vger.kernel.org" , "pawel.moll@arm.co" , "rob.herring@calxeda.com" , "linux-sunxi@googlegroups.com" Subject: Re: [PATCH 3/4] Add physical count arch timer support for clocksource in ARMv7. References: <1378968687-8200-1-git-send-email-cinifr@gmail.com> <1378968687-8200-4-git-send-email-cinifr@gmail.com> <20130912112452.GA22013@e106331-lin.cambridge.arm.com> In-Reply-To: X-Enigmail-Version: 1.4.6 X-OriginalArrivalTime: 12 Sep 2013 16:39:32.0723 (UTC) FILETIME=[A89FCC30:01CEAFD6] X-MC-Unique: 113091217393402101 Content-Type: text/plain; charset=WINDOWS-1252 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1499 Lines: 36 On 12/09/13 17:07, cinifr wrote: >> This cannot be a compile-time option as above in a multiplatform build. >> Other paltforms (e.g. KVM guests) *must* use the virtual counters to get >> any semblance of a consistent view of time. > Yes I accept compile-time option is not perfect in my pre email, > But,Why Ohter paltforms *must* use the virtual counters? I think KVM > should not limit how to use arch timer in its guest OS. Of cause, if > KVM guest use vct can be more efficiency then that use pct. but KVM > should and must support guest OS to access pct. The virtual counter is there for a good reason: it allows a virtual machine to: - see its time starting at zero - be migrated to another host without seeing time shifting one way or another. So using the physical counter in a VM is a recipe for disaster if you're doing any kind of time tracking. The counter being used for sched_clock(), we cannot afford to see it being shifted one way or another. If you have issues with the use of the virtual counter, I suggest you fix your firmware to have a consistent CNTVOFF across CPUs. And/or even better, boot your kernel in HYP mode, as it will take care of setting CNTVOFF to zero. Cheers, M. -- Jazz is not dead. It just smells funny... -- 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/