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    Subject[PATCH 14/18] 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 | 2 +
    arch/x86/kvm/x86.c | 43 +++++++++++++++++++++++++++++++++++++-
    2 files changed, 43 insertions(+), 2 deletions(-)

    diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
    index d9e0aa0..94dd16b 100644
    --- a/arch/x86/include/asm/kvm_host.h
    +++ b/arch/x86/include/asm/kvm_host.h
    @@ -340,6 +340,8 @@ 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;
    diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
    index 574cb69..7b9fdf3 100644
    --- a/arch/x86/kvm/x86.c
    +++ b/arch/x86/kvm/x86.c
    @@ -973,14 +973,15 @@ static int kvm_write_guest_time(struct kvm_vcpu *v)
    struct kvm_vcpu_arch *vcpu = &v->arch;
    void *shared_kaddr;
    unsigned long this_tsc_khz;
    - s64 kernel_ns;
    + s64 kernel_ns, max_kernel_ns;
    + u64 tsc_timestamp;

    if ((!vcpu->time_page))
    return 0;

    /* 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);
    + kvm_get_msr(v, MSR_IA32_TSC, &tsc_timestamp);
    kernel_ns = get_kernel_ns();
    this_tsc_khz = __get_cpu_var(cpu_tsc_khz);
    local_irq_restore(flags);
    @@ -990,13 +991,49 @@ static int kvm_write_guest_time(struct kvm_vcpu *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 && vcpu->last_guest_tsc) {
    + 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);
    vcpu->hw_tsc_khz = this_tsc_khz;
    }

    + if (max_kernel_ns > kernel_ns)
    + kernel_ns = max_kernel_ns;
    +
    /* With all the info we got, fill in the values */
    + 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;

    /*
    @@ -4917,6 +4954,8 @@ 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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