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 <zamsden@xxxxxxxxxx>
> 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 <zamsden@xxxxxxxxxx>
>
> 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
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