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

[Zachary Amsden]

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.
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