+static inline int kvm_tsc_changes_freq(void)
+{
+ int cpu = get_cpu();
+ int ret = !boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
+ cpufreq_quick_get(cpu) != 0;
+ put_cpu();
+ return ret;
+}
+
+static inline u64 nsec_to_cycles(u64 nsec)
+{
+ u64 ret;
+
+ WARN_ON(preemptible());
+ if (kvm_tsc_changes_freq())
+ printk_once(KERN_WARNING
+ "kvm: unreliable cycle conversion on adjustable rate TSC\n");
+ ret = nsec * __get_cpu_var(cpu_tsc_khz);
+ do_div(ret, USEC_PER_SEC);
+ return ret;
+}
+
+void kvm_write_tsc(struct kvm_vcpu *vcpu, u64 data)
+{
+ struct kvm *kvm = vcpu->kvm;
+ u64 offset, ns, elapsed;
+ unsigned long flags;
+ s64 sdiff;
+
+ spin_lock_irqsave(&kvm->arch.tsc_write_lock, flags);
+ offset = data - native_read_tsc();
+ ns = get_kernel_ns();
+ elapsed = ns - kvm->arch.last_tsc_nsec;
+ sdiff = data - kvm->arch.last_tsc_write;
+ if (sdiff < 0)
+ sdiff = -sdiff;
+
+ /*
+ * Special case: close write to TSC within 5 seconds of
+ * another CPU is interpreted as an attempt to synchronize
+ * The 5 seconds is to accomodate host load / swapping as
+ * well as any reset of TSC during the boot process.
+ *
+ * In that case, for a reliable TSC, we can match TSC offsets,
+ * or make a best guest using elapsed value.
+ */
+ if (sdiff < nsec_to_cycles(5ULL * NSEC_PER_SEC) &&
+ elapsed < 5ULL * NSEC_PER_SEC) {
+ if (!check_tsc_unstable()) {
+ offset = kvm->arch.last_tsc_offset;
+ pr_debug("kvm: matched tsc offset for %llu\n", data);
+ } else {
+ u64 delta = nsec_to_cycles(elapsed);
+ offset += delta;
+ pr_debug("kvm: adjusted tsc offset by %llu\n", delta);
+ }
+ ns = kvm->arch.last_tsc_nsec;
+ }
+ kvm->arch.last_tsc_nsec = ns;
+ kvm->arch.last_tsc_write = data;
+ kvm->arch.last_tsc_offset = offset;
+ kvm_x86_ops->write_tsc_offset(vcpu, offset);
+ spin_unlock_irqrestore(&kvm->arch.tsc_write_lock, flags);
+
+ /* Reset of TSC must disable overshoot protection below */
+ vcpu->arch.hv_clock.tsc_timestamp = 0;
+}
+EXPORT_SYMBOL_GPL(kvm_write_tsc);
+
+static int kvm_guest_time_update(struct kvm_vcpu *v)