Messages in this thread | | | Date | Tue, 15 Jun 2010 11:40:14 +0300 | From | Avi Kivity <> | Subject | Re: [PATCH 11/17] Fix a possible backwards warp of kvmclock |
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On 06/15/2010 10:34 AM, Zachary Amsden wrote: > Kernel time, which advances in discrete steps may progress much slower > than TSC. As a result, when kvmclock is adjusted to a new base, the > apparent time to the guest, which runs at a much higher, nsec scaled > rate based on the current TSC, may have already been observed to have > a larger value (kernel_ns + scaled tsc) than the value to which we are > setting it (kernel_ns + 0). > > We must instead compute the clock as potentially observed by the guest > for kernel_ns to make sure it does not go backwards. > > @@ -455,6 +457,8 @@ struct kvm_vcpu_stat { > u32 hypercalls; > u32 irq_injections; > u32 nmi_injections; > + u32 tsc_overshoot; > + u32 tsc_ahead; > }; >
Please don't add new stats, instead add tracepoints which can also be observed as stats.
But does this really merit exposing? What would one do with this information?
> struct kvm_vcpu_arch *vcpu =&v->arch; > void *shared_kaddr; > unsigned long this_tsc_khz; > + s64 kernel_ns, max_kernel_ns; > + u64 tsc_timestamp; > > if ((!vcpu->time_page)) > return 0; > > - this_tsc_khz = get_cpu_var(cpu_tsc_khz); > - put_cpu_var(cpu_tsc_khz); > + /* > + * The protection we require is simple: we must not be preempted from > + * the CPU between our read of the TSC khz and our read of the TSC. > + * Interrupt protection is not strictly required, but it does result in > + * greater accuracy for the TSC / kernel_ns measurement. > + */ > + local_irq_save(flags); > + this_tsc_khz = __get_cpu_var(cpu_tsc_khz); > + kvm_get_msr(v, MSR_IA32_TSC,&tsc_timestamp); >
That's a slow path, since it has to go through kvm_get_msr()'s if tree. Could use its own accessor.
But this isn't introduced by this patch, so it can be fixed by another.
> + ktime_get_ts(&ts); > + monotonic_to_bootbased(&ts); > + kernel_ns = timespec_to_ns(&ts); > + local_irq_restore(flags); > + > if (unlikely(this_tsc_khz == 0)) { > kvm_request_guest_time_update(v); > return 1; > } > > + /* > + * Time as measured by the TSC may go backwards when resetting the base > + * tsc_timestamp. The reason for this is that the TSC resolution is > + * higher than the resolution of the other clock scales. Thus, many > + * possible measurments of the TSC correspond to one measurement of any > + * other clock, and so a spread of values is possible. This is not a > + * problem for the computation of the nanosecond clock; with TSC rates > + * around 1GHZ, there can only be a few cycles which correspond to one > + * nanosecond value, and any path through this code will inevitably > + * take longer than that. However, with the kernel_ns value itself, > + * the precision may be much lower, down to HZ granularity. If the > + * first sampling of TSC against kernel_ns ends in the low part of the > + * range, and the second in the high end of the range, we can get: > + * > + * (TSC - offset_low) * S + kns_old> (TSC - offset_high) * S + kns_new > + * > + * As the sampling errors potentially range in the thousands of cycles, > + * it is possible such a time value has already been observed by the > + * guest. To protect against this, we must compute the system time as > + * observed by the guest and ensure the new system time is greater. > + */ > + max_kernel_ns = 0; > + if (vcpu->hv_clock.tsc_timestamp) { > + max_kernel_ns = vcpu->last_guest_tsc - > + vcpu->hv_clock.tsc_timestamp; > + max_kernel_ns = pvclock_scale_delta(max_kernel_ns, > + vcpu->hv_clock.tsc_to_system_mul, > + vcpu->hv_clock.tsc_shift); > + max_kernel_ns += vcpu->last_kernel_ns; > + } > + > if (unlikely(vcpu->hw_tsc_khz != this_tsc_khz)) { > - kvm_set_time_scale(this_tsc_khz,&vcpu->hv_clock); > + kvm_get_time_scale(NSEC_PER_SEC / 1000, this_tsc_khz, > + &vcpu->hv_clock.tsc_shift, > + &vcpu->hv_clock.tsc_to_system_mul); > vcpu->hw_tsc_khz = this_tsc_khz; > } > > - /* Keep irq disabled to prevent changes to the clock */ > - local_irq_save(flags); > - kvm_get_msr(v, MSR_IA32_TSC,&vcpu->hv_clock.tsc_timestamp); > - ktime_get_ts(&ts); > - monotonic_to_bootbased(&ts); > - local_irq_restore(flags); > + if (max_kernel_ns> kernel_ns) { > + s64 overshoot = max_kernel_ns - kernel_ns; > + ++v->stat.tsc_ahead; > + if (overshoot> NSEC_PER_SEC / HZ) { > + ++v->stat.tsc_overshoot; > + if (printk_ratelimit()) > + pr_debug("ns overshoot: %lld\n", overshoot); > + } >
A tracepoint here would allow recording both the number of overshoots and the value of the overshoot. But I don't think this is of much use day-to-day.
-- error compiling committee.c: too many arguments to function
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