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

From: Zachary Amsden
Date: Tue Jun 15 2010 - 16:37:13 EST

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)) {
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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