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    Subject[PATCH 11/17] Fix a possible backwards warp of kvmclock
    Date
    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@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 -
    + 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();
    --
    1.7.1


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