Re: [PATCH 11/17] Fix a possible backwards warp of kvmclock

Zachary Amsden
2010-06-15T20:37:07Z

On 06/14/2010 10:40 PM, Avi Kivity wrote:
> 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.

FWIW, I was using this to track how often this case would hit and by how
much. Originally, tsc_ahead was firing near 100% and tsc_overshoot near
0%, but moving the observation of last_guest_tsc into the exit path
decreased both number to near zero. Obviously it's a bit hardware
dependent, as it matters how high resolution the kernel clocksource is
(and how recent your kernel).

I'll rip the stats stuff for sure.