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    SubjectRe: [PATCH 11/17] Fix a possible backwards warp of kvmclock
    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.
    > + 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.
    > + 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.
    > +
    > atomic_set(&vcpu->guest_mode, 0);
    > smp_wmb();
    > local_irq_enable();
    >



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