Philipp, Thanks for debugging this issue. We'll be fowarding the fix for inclusion in the official 2.6.32 tree. On Mon, Sep 05, 2011 at 04:06:57PM +0200, Philipp Hahn wrote: > Hello, > > (cc:-ing lost of people who reported similar bugs on kvm-devel) > > Changing clock in KVM host may cause VM to hang > > 2.6.32 guest with paravirt clock enabled hangs on 2.6.37.6 host (w > qemu-kvm-0.13.0) > > I found a bug regarding PV-clock in the KVM-kernel module. The attached patch > solves the problem of the guest being very slow after a reboot. Can you > please have a look and give it a try to see if it solves your problem as > well. > > Since the fix is only relevant for the stable 2.6.32 tree, where the code is > quiet different, please have a look and forward to stable@ as appropriate. > > Sincerely > Philipp > > PS: This bug is tracked in our German Bugzilla at > <https://forge.univention.org/bugzilla/show_bug.cgi?id=23258> > -- > Philipp Hahn Open Source Software Engineer hahn@xxxxxxxxxxxxx > Univention GmbH Linux for Your Business fon: +49 421 22 232- 0 > Mary-Somerville-Str.1 D-28359 Bremen fax: +49 421 22 232-99 > http://www.univention.de/ > ---------------------------------------------------------------------------- > Treffen Sie Univention auf der IT&Business vom 20. bis 22. September 2011 > auf dem Gemeinschaftsstand der Open Source Business Alliance in Stuttgart in > Halle 3 Stand 3D27-7. > Bug #23257: Reset tsc_timestamp on TSC writes > > vcpu->last_guest_tsc is updated in vcpu_enter_guest() and kvm_arch_vcpu_put() > by getting the last value of the TSC from the guest. > On reset, the SeaBIOS resets the TSC to 0, which triggers a bug on the next > call to kvm_write_guest_time(): Since vcpu->hw_clock.tsc_timestamp still > contains the old value before the reset, "max_kernel_ns = vcpu->last_guest_tsc > - vcpu->hw_clock.tsc_timestamp" gets negative. Since the variable is u64, it > gets translated to a large positive value. > > [9333.197080] > vcpu->last_guest_tsc =209_328_760_015 ← > vcpu->hv_clock.tsc_timestamp=209_328_708_109 > vcpu->last_kernel_ns =9_333_179_830_643 > kernel_ns =9_333_197_073_429 > max_kernel_ns =9_333_179_847_943 ← > > [9336.910995] > vcpu->last_guest_tsc =9_438_510_584 ← > vcpu->hv_clock.tsc_timestamp=211_080_593_143 > vcpu->last_kernel_ns =9_333_763_732_907 > kernel_ns =9_336_910_990_771 > max_kernel_ns =6_148_296_831_006_663_830 ← > > For completeness, here are the values for my 3 GHz CPU: > vcpu->hv_clock.tsc_shift =-1 > vcpu->hv_clock.tsc_to_system_mul =2_863_019_502 > > This makes the guest kernel crawl very slowly when clocksource=kvmclock is > used: sleeps take way longer than expected and don't match wall clock any more. > The times printed with printk() don't match real time and the reboot often > stalls for long times. > > In linux-git this isn't a problem, since on every MSR_IA32_TSC write > vcpu->arch.hv_clock.tsc_timestamp is reset to 0, which disables above logic. > The code there is only in arch/x86/kvm/x86.c, since much of the kvm-clock > related code has been refactured for 2.6.37: > 99e3e30a arch/x86/kvm/x86.c (Zachary Amsden 2010-08-19 22:07:17 -1000 1084) vcpu->arch.hv_clock.tsc_timestamp = 0; > Since 1d5f066e0b63271b67eac6d3752f8aa96adcbddb from 2.6.37 was back-ported to > 2.6.32.40 as ad2088cabe0fd7f633f38ba106025d33ed9a2105, the following patch is > needed to add the needed reset logic to 2.6.32 as well. > > Signed-off-by: Philipp Hahn <hahn@xxxxxxxxxxxxx> > --- a/arch/x86/kvm/vmx.c 2011-09-05 14:17:54.000000000 +0200 > +++ b/arch/x86/kvm/vmx.c 2011-09-05 14:18:03.000000000 +0200 > @@ -1067,6 +1067,7 @@ static int vmx_set_msr(struct kvm_vcpu * > case MSR_IA32_TSC: > rdtscll(host_tsc); > guest_write_tsc(data, host_tsc); > + vcpu->arch.hv_clock.tsc_timestamp = 0; > break; > case MSR_IA32_CR_PAT: > if (vmcs_config.vmentry_ctrl & VM_ENTRY_LOAD_IA32_PAT) { > --- a/arch/x86/kvm/svm.c 2011-09-05 14:17:57.000000000 +0200 > +++ b/arch/x86/kvm/svm.c 2011-09-05 14:18:00.000000000 +0200 > @@ -2256,6 +2256,7 @@ static int svm_set_msr(struct kvm_vcpu * > } > > svm->vmcb->control.tsc_offset = tsc_offset + g_tsc_offset; > + vcpu->arch.hv_clock.tsc_timestamp = 0; > > break; > } -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
