On Tue, Jun 15, 2010 at 02:21:17PM -1000, Zachary Amsden wrote: > 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<[email protected]> > >>--- > >> 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
What is worrying is that if this keeps happening the guest clock will advance faster then it should. The solution you had before with "if (kernel_ns <= last_ns) compensate()" was simpler and more resistant in that respect, if i'm not missing anything. -- 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
