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

Zachary Amsden
2010-06-16T19:36:31Z

On 06/15/2010 10:11 PM, Jason Wang wrote:
> 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).
>>
>>
> This is one issue of kvmclock which tries to supply a clocksource whose
> precision may even higher than host.
>
>> 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@redhat.com>
>> ---
>> arch/x86/include/asm/kvm_host.h | 4 ++
>> arch/x86/kvm/x86.c | 79 +++++++++++++++++++++++++++++++++------
>> 2 files changed, 71 insertions(+), 12 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>> index 1afecd7..7ec2472 100644
>> --- a/arch/x86/include/asm/kvm_host.h
>> +++ b/arch/x86/include/asm/kvm_host.h
>> @@ -338,6 +338,8 @@ struct kvm_vcpu_arch {
>> struct page *time_page;
>> u64 last_host_tsc;
>> u64 last_host_ns;
>> + u64 last_guest_tsc;
>> + u64 last_kernel_ns;
>>
>> bool nmi_pending;
>> bool nmi_injected;
>> @@ -455,6 +457,8 @@ 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 52d7d34..703ea43 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -138,6 +138,8 @@ 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) },
>> @@ -927,33 +929,84 @@ 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, max_kernel_ns;
>> + u64 tsc_timestamp;
>>
>> 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.
>> + */
>> + max_kernel_ns = 0;
>> + if (vcpu->hv_clock.tsc_timestamp) {
>> + max_kernel_ns = vcpu->last_guest_tsc -
>>
>>
> Since you do the comparison with kernel_ns, so what you need here is
> tsc_timestamp which looks more like the 'last' tsc seen by guest.
>

What this is computing is the highest bootbased nanosecond time value
potentially seen by the guest:

last_guest_tsc - hv_clock.tsc_timestamp is the maximum cycle offset the
guest has seen against the last version of kvmclock.

Then it is scaled and added to the last_kernel_ns value used for
hv_clock. I chose to cache vcpu->last_kernel_ns separately from
hv_clock so that kvmclock_offset can not change in the meantime, so the
value deliberately discounts kvmclock_offset.

>> + 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) {
>>
>>
> Both max_kernel_ns and kernel_ns are not adjusted by kvmclock_offset, so
> this comparing is not safe after migration.
>

They are deliberately not adjusted by kvmclock_offset, so they are
simply scalar bootbased nanosecond values, not kvmclock_offset values.
Thus, we can add, subtract and take maximums of them without worrying
about kvmclock_offset at all.

>> + 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;
>> + }
>>
>>
> A tsc_behind or something like this would make the problem more clear,
> and tsc_ahead should be zero when host using tsc as its clocksource.
>
>>
>> /* 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);
>>
>>
> This could be dropped since it does not take the time of guest execution
> into account.
>

This is required is for the maximum cycle offset above.