int kvm_arch_hardware_enable(void)
{
	struct kvm *kvm;
	struct kvm_vcpu *vcpu;
	unsigned long i;
	int ret;
	u64 local_tsc;
	u64 max_tsc = 0;
	bool stable, backwards_tsc = false;

	kvm_user_return_msr_cpu_online();
	ret = static_call(kvm_x86_hardware_enable)();
	if (ret != 0)
		return ret;

	local_tsc = rdtsc();
	stable = !kvm_check_tsc_unstable();
	list_for_each_entry(kvm, &vm_list, vm_list) {
		kvm_for_each_vcpu(i, vcpu, kvm) {
			if (!stable && vcpu->cpu == smp_processor_id())
				kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
			if (stable && vcpu->arch.last_host_tsc > local_tsc) {
				backwards_tsc = true;
				if (vcpu->arch.last_host_tsc > max_tsc)
					max_tsc = vcpu->arch.last_host_tsc;
			}
		}
	}

	/*
	 * Sometimes, even reliable TSCs go backwards.  This happens on
	 * platforms that reset TSC during suspend or hibernate actions, but
	 * maintain synchronization.  We must compensate.  Fortunately, we can
	 * detect that condition here, which happens early in CPU bringup,
	 * before any KVM threads can be running.  Unfortunately, we can't
	 * bring the TSCs fully up to date with real time, as we aren't yet far
	 * enough into CPU bringup that we know how much real time has actually
	 * elapsed; our helper function, ktime_get_boottime_ns() will be using boot
	 * variables that haven't been updated yet.
	 *
	 * So we simply find the maximum observed TSC above, then record the
	 * adjustment to TSC in each VCPU.  When the VCPU later gets loaded,
	 * the adjustment will be applied.  Note that we accumulate
	 * adjustments, in case multiple suspend cycles happen before some VCPU
	 * gets a chance to run again.  In the event that no KVM threads get a
	 * chance to run, we will miss the entire elapsed period, as we'll have
	 * reset last_host_tsc, so VCPUs will not have the TSC adjusted and may
	 * loose cycle time.  This isn't too big a deal, since the loss will be
	 * uniform across all VCPUs (not to mention the scenario is extremely
	 * unlikely). It is possible that a second hibernate recovery happens
	 * much faster than a first, causing the observed TSC here to be
	 * smaller; this would require additional padding adjustment, which is
	 * why we set last_host_tsc to the local tsc observed here.
	 *
	 * N.B. - this code below runs only on platforms with reliable TSC,
	 * as that is the only way backwards_tsc is set above.  Also note
	 * that this runs for ALL vcpus, which is not a bug; all VCPUs should
	 * have the same delta_cyc adjustment applied if backwards_tsc
	 * is detected.  Note further, this adjustment is only done once,
	 * as we reset last_host_tsc on all VCPUs to stop this from being
	 * called multiple times (one for each physical CPU bringup).
	 *
	 * Platforms with unreliable TSCs don't have to deal with this, they
	 * will be compensated by the logic in vcpu_load, which sets the TSC to
	 * catchup mode.  This will catchup all VCPUs to real time, but cannot
	 * guarantee that they stay in perfect synchronization.
	 */
	if (backwards_tsc) {
		u64 delta_cyc = max_tsc - local_tsc;
		list_for_each_entry(kvm, &vm_list, vm_list) {
			kvm->arch.backwards_tsc_observed = true;
			kvm_for_each_vcpu(i, vcpu, kvm) {
				vcpu->arch.tsc_offset_adjustment += delta_cyc;
				vcpu->arch.last_host_tsc = local_tsc;
				kvm_make_request(KVM_REQ_MASTERCLOCK_UPDATE, vcpu);
			}

			/*
			 * We have to disable TSC offset matching.. if you were
			 * booting a VM while issuing an S4 host suspend....
			 * you may have some problem.  Solving this issue is
			 * left as an exercise to the reader.
			 */
			kvm->arch.last_tsc_nsec = 0;
			kvm->arch.last_tsc_write = 0;
		}

	}
	return 0;
}