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? -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html