Messages in this thread | | | Date | Wed, 16 Jun 2010 09:36:25 -1000 | From | Zachary Amsden <> | Subject | Re: [PATCH 11/17] Fix a possible backwards warp of kvmclock |
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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.
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