Attachment 3498 Details for Bug 23258 – Reset vcpu->hv_clock.tsc_timestamp on TSC write

[patch] Reset vcpu->hv_clock.tsc_timestamp on TSC write

23258_kvm-clock-reset.diff (text/plain), 2.82 KB, created by Philipp Hahn on 2011-09-05 15:38:08 CEST

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Creator: Philipp Hahn

Created: 2011-09-05 15:38:08 CEST

Size: 2.82 KB

patch

obsolete

>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           â
>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         â
>
>[9336.910995]
>vcpu->last_guest_tsc        =9_438_510_584             â
>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_nsn              =6_148_296_831_006_663_830 â
>
>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@univention.de>
>--- 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;
> 	}

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