On Fri, May 14, 2010 at 04:07:43PM -1000, Zachary Amsden wrote: > I believe this fixes the root cause of the kvmclock warp. It's > quite a plausible phenomenon, and explains why it was so easy to > produce. > You mean this is the case for both SMP and UP, or just UP as we talked before?
I don't get the role of upscale in your patch. Frequency changes are already handled by the cpufreq notifier. > Currently it depends on some other patches; I can send a whole > patchset, but with all the patch activity, it isn't clear what has > been applied and to what trees. Where have Glauber's recent patches > been applied? > > I am looking for comments if this is a reasonably good explanation > and fix for the problem. > > I realize I messed up the overshoot calculation, it is not converted > to nsec, but the debug stats are just for debugging. > > Thanks, > > Zach > commit 24e1f31a4cdb43a8e5cab6cfb95d710c7c7bf18a > Author: Zachary Amsden <[email protected]> > Date: Fri Feb 26 15:13:31 2010 -1000 > > Fix a possible backwards warp of kvmclock > > 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 <[email protected]> > > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h > index 83df4db..ba765fa 100644 > --- a/arch/x86/include/asm/kvm_host.h > +++ b/arch/x86/include/asm/kvm_host.h > @@ -453,6 +453,8 @@ struct kvm_vcpu_stat { > u32 hypercalls; > u32 irq_injections; > u32 nmi_injections; > + u32 tsc_overshoot; > + u32 tsc_ahead; > }; > > struct kvm_x86_ops { > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index bb44f9e..2bf7e86 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -134,6 +134,8 @@ struct kvm_stats_debugfs_item debugfs_entries[] = { > { "insn_emulation_fail", VCPU_STAT(insn_emulation_fail) }, > { "irq_injections", VCPU_STAT(irq_injections) }, > { "nmi_injections", VCPU_STAT(nmi_injections) }, > + { "tsc_overshoot", VCPU_STAT(tsc_overshoot) }, > + { "tsc_ahead", VCPU_STAT(tsc_ahead) }, > { "mmu_shadow_zapped", VM_STAT(mmu_shadow_zapped) }, > { "mmu_pte_write", VM_STAT(mmu_pte_write) }, > { "mmu_pte_updated", VM_STAT(mmu_pte_updated) }, > @@ -849,35 +851,80 @@ static int kvm_recompute_guest_time(struct kvm_vcpu *v) > struct kvm_vcpu_arch *vcpu = &v->arch; > void *shared_kaddr; > unsigned long this_tsc_khz; > + s64 kernel_ns, delta; > + u64 tsc_timestamp; > + bool upscale; > > 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); > + 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. > + */ > + delta = native_read_tsc() - vcpu->hv_clock.tsc_timestamp; > + delta = pvclock_scale_delta(delta, vcpu->hv_clock.tsc_to_system_mul, > + vcpu->hv_clock.tsc_shift); > + delta += vcpu->hv_clock.system_time; > + > if (unlikely(vcpu->hw_tsc_khz != this_tsc_khz)) { > + upscale = this_tsc_khz > vcpu->hw_tsc_khz; > 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 (delta > kernel_ns) { > + s64 overshoot = delta - kernel_ns; > + ++v->stat.tsc_ahead; > + if (upscale) > + overshoot = overshoot * 9 / 10; > + if (overshoot > 1000ULL * this_tsc_khz / HZ) { > + ++v->stat.tsc_overshoot; > + } > + kernel_ns = delta; > + } > > /* 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; > > /* > * The interface expects us to write an even number signaling that the -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
