From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jason Wang Subject: Re: [PATCH 03/17] Unify vendor TSC logic Date: Thu, 17 Jun 2010 16:03:33 +0800 Message-ID: <4C19D6D5.7060501@redhat.com> References: <1276587259-32319-1-git-send-email-zamsden@redhat.com> <1276587259-32319-4-git-send-email-zamsden@redhat.com> <4C1886E2.2060509@redhat.com> <20100616132223.GI19104@mothafucka.localdomain> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Cc: Zachary Amsden , avi@redhat.com, mtosatti@redhat.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org To: Glauber Costa Return-path: In-Reply-To: <20100616132223.GI19104@mothafucka.localdomain> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org Glauber Costa wrote: > On Wed, Jun 16, 2010 at 04:10:10PM +0800, Jason Wang wrote: > >> Zachary Amsden wrote: >> >>> >>> void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu) >>> { >>> + kvm_x86_ops->vcpu_load(vcpu, cpu); >>> if (unlikely(vcpu->cpu != cpu)) { >>> + /* Make sure TSC doesn't go backwards */ >>> + s64 tsc_delta = !vcpu->arch.last_host_tsc ? 0 : >>> + native_read_tsc() - vcpu->arch.last_host_tsc; >>> + if (tsc_delta < 0 || check_tsc_unstable()) >>> >>> >> It's better to do the adjustment also when tsc_delta > 0 >> > And why do you think so? Doing it on tsc_delta > 0 would force us to adjust > at every entry but the first. And I guess we want to adjust as few times as > we can. > > This is not strange and is what current SVM code does. If we do not do this, guest may see a jump in the value of TSC when tsc_delta > 0. > For example, we would adjust on every cpu bounce even for machines that has > a perfectly sync tsc. This could introduce an error not present before. > > -- > To unsubscribe from this list: send the line "unsubscribe kvm" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html >