Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965343AbaGPOzs (ORCPT ); Wed, 16 Jul 2014 10:55:48 -0400 Received: from mx1.redhat.com ([209.132.183.28]:62621 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934066AbaGPOzr (ORCPT ); Wed, 16 Jul 2014 10:55:47 -0400 Message-ID: <53C69269.3080204@redhat.com> Date: Wed, 16 Jul 2014 16:55:37 +0200 From: Paolo Bonzini User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.6.0 MIME-Version: 1.0 To: Igor Mammedov CC: Marcelo Tosatti , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, x86@kernel.org Subject: Re: [PATCH] ensure guest's kvmclock never goes backwards when TSC jumps backward References: <1405504368-5581-1-git-send-email-imammedo@redhat.com> <53C6517D.9090600@redhat.com> <20140716114100.GA7394@amt.cnet> <20140716155523.174a6b0e@igors-macbook-pro.local> <53C68931.7000308@redhat.com> <20140716165153.02d403c1@igors-macbook-pro.local> In-Reply-To: <20140716165153.02d403c1@igors-macbook-pro.local> X-Enigmail-Version: 1.6 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Il 16/07/2014 16:51, Igor Mammedov ha scritto: >>>> I'm not sure that a per-CPU value is enough; your patch can make >>>> the problem much less frequent of course, but I'm not sure neither >>>> detection nor correction are 100% reliable. Your addition is >>>> basically a faster but less reliable version of the last_value >>>> logic. >>> >>> How is it less reliable than last_value logic? >> >> Suppose CPU 1 is behind by 3 nanoseconds >> >> CPU 0 CPU 1 >> time = 100 (at time 100) >> time = 99 (at time 102) >> time = 104 (at time 104) >> time = 105 (at time 108) >> >> Your patch will not detect this. > Is it possible for each cpu to have it's own time? Yes, that's one of the reasons for TSC not to be stable (it could also happen simply because the value of TSC_ADJUST MSR is bogus). > tsc_wrap_test starts to fail almost imediately, > > I'll check how much tries it takes to fail for the first time, if it is > not too much I guess we could add check to check_system_tsc_reliable(). Thanks! Paolo -- 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/