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

From: Zachary Amsden
Date: Tue Jun 15 2010 - 20:21:32 EST

On 06/15/2010 01:47 PM, Marcelo Tosatti wrote:
On Mon, Jun 14, 2010 at 09:34:13PM -1000, 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.

Signed-off-by: Zachary Amsden<zamsden@xxxxxxxxxx>
---
arch/x86/include/asm/kvm_host.h | 4 ++
arch/x86/kvm/x86.c | 79 +++++++++++++++++++++++++++++++++------
2 files changed, 71 insertions(+), 12 deletions(-)

+ /*
+ * 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);
+ 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);
+ }
+ kernel_ns = max_kernel_ns;
+ }

/* With all the info we got, fill in the values */
-
- vcpu->hv_clock.system_time = ts.tv_nsec +
- (NSEC_PER_SEC * (u64)ts.tv_sec) + v->kvm->arch.kvmclock_offset;
+ vcpu->hv_clock.tsc_timestamp = tsc_timestamp;
+ vcpu->hv_clock.system_time = kernel_ns + v->kvm->arch.kvmclock_offset;
+ vcpu->last_kernel_ns = kernel_ns;

vcpu->hv_clock.flags = 0;

@@ -4836,6 +4889,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
if (hw_breakpoint_active())
hw_breakpoint_restore();

+ kvm_get_msr(vcpu, MSR_IA32_TSC,&vcpu->arch.last_guest_tsc);
+
atomic_set(&vcpu->guest_mode, 0);
smp_wmb();
local_irq_enable();
Is this still needed with the guest side global counter fix?

It's debatable. Instrumentation showed this happen 100% of the time when measuring TSC in the compensation sequence. When measuring TSC in the hot-path exit from hardware virt, before interrupts are enabled, the compensation rate drops to 0%.

That's with an HPET clocksource for kernel time. Kernels with less accurate and more granular clocksources would have worse problems, of course.

If we're ever going to turn on the "kvmclock is reliable" bit, though, I think at least paying attention to the potential need for compensation is required - it technically is a backwards warp of time, and even if we spend so long getting out of and back into hardware virtualization that the guest can't notice it today, that might not be true on a faster processor.

Zach
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