lkml.org 
[lkml]   [2010]   [Jun]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH 11/17] Fix a possible backwards warp of kvmclock
    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



    \
     
     \ /
      Last update: 2010-06-15 10:43    [W:0.050 / U:60.044 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site