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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



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