From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [PATCH 04/17] Fix deep C-state TSC desynchronization Date: Tue, 15 Jun 2010 11:09:35 +0300 Message-ID: <4C17353F.5020006@redhat.com> References: <1276587259-32319-1-git-send-email-zamsden@redhat.com> <1276587259-32319-5-git-send-email-zamsden@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: mtosatti@redhat.com, glommer@redhat.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org To: Zachary Amsden Return-path: In-Reply-To: <1276587259-32319-5-git-send-email-zamsden@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 06/15/2010 10:34 AM, Zachary Amsden wrote: > When CPUs with unstable TSCs enter deep C-state, TSC may stop > running. This causes us to require resynchronization. Since > we can't tell when this may potentially happen, we assume the > worst by forcing re-compensation for it at every point the VCPU > task is descheduled. > > Signed-off-by: Zachary Amsden > --- > arch/x86/kvm/x86.c | 11 +++++++++++ > 1 files changed, 11 insertions(+), 0 deletions(-) > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index c8289d0..618c435 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -1822,7 +1822,18 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu) > { > kvm_x86_ops->vcpu_put(vcpu); > kvm_put_guest_fpu(vcpu); > + > vcpu->arch.last_host_tsc = native_read_tsc(); > + > + /* > + * When potentially leaving a CPU with unstable TSCs, we risk > + * that the CPU enters deep C-state. If it does, the TSC may > + * go out of sync but we will not recalibrate because the test > + * vcpu->cpu != cpu can not detect this condition. So set > + * vcpu->cpu = -1 to force the recalibration above. > + */ > + if (check_tsc_unstable()) > + vcpu->cpu = -1; > } > That will cause us to miss a vmclear later on. Also it invalidates an invariant that the per-cpu list vcpus_on_cpu has all the vcpus with vcpu->cpu == cpu on this cpu (try saything that fast). -- error compiling committee.c: too many arguments to function