static int kvm_guest_time_update(struct kvm_vcpu *v)
{
	unsigned long flags;
	struct kvm_vcpu_arch *vcpu = &v->arch;
	void *shared_kaddr;
	unsigned long this_tsc_khz;
	s64 kernel_ns, max_kernel_ns;
	u64 tsc_timestamp;

	/* Keep irq disabled to prevent changes to the clock */
	local_irq_save(flags);
	kvm_get_msr(v, MSR_IA32_TSC, &tsc_timestamp);
	kernel_ns = get_kernel_ns();
	this_tsc_khz = __get_cpu_var(cpu_tsc_khz);

	if (unlikely(this_tsc_khz == 0)) {
		local_irq_restore(flags);
		kvm_make_request(KVM_REQ_CLOCK_UPDATE, v);
		return 1;
	}

	/*
	 * We may have to catch up the TSC to match elapsed wall clock
	 * time for two reasons, even if kvmclock is used.
	 *   1) CPU could have been running below the maximum TSC rate
	 *   2) Broken TSC compensation resets the base at each VCPU
	 *      entry to avoid unknown leaps of TSC even when running
	 *      again on the same CPU.  This may cause apparent elapsed
	 *      time to disappear, and the guest to stand still or run
	 *	very slowly.
	 */
	if (vcpu->tsc_catchup) {
		u64 tsc = compute_guest_tsc(v, kernel_ns);
		if (tsc > tsc_timestamp) {
			kvm_x86_ops->adjust_tsc_offset(v, tsc - tsc_timestamp);
			tsc_timestamp = tsc;
		}
	}
	
	local_irq_restore(flags);

	if (!vcpu->time_page)
		return 0;

	/*
	 * 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_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;
	}

	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->last_guest_tsc = tsc_timestamp;
	vcpu->hv_clock.flags = 0;

	/*
	 * The interface expects us to write an even number signaling that the
	 * update is finished. Since the guest won't see the intermediate
	 * state, we just increase by 2 at the end.
	 */
	vcpu->hv_clock.version += 2;

	shared_kaddr = kmap_atomic(vcpu->time_page, KM_USER0);

	memcpy(shared_kaddr + vcpu->time_offset, &vcpu->hv_clock,
	       sizeof(vcpu->hv_clock));

	kunmap_atomic(shared_kaddr, KM_USER0);

	mark_page_dirty(v->kvm, vcpu->time >> PAGE_SHIFT);
	return 0;
}