+
+ /* If we can't use the TSC, we read the time value written by the Host.
+ * Since it's in two parts (seconds and nanoseconds), we risk reading
+ * it just as it's changing from 99 & 0.999999999 to 100 and 0, and
+ * getting 99 and 0. As Linux tends to come apart under the stress of
+ * time travel, we must be careful: */
+ do {
+ /* First we read the seconds part. */
+ sec = lguest_data.time.tv_sec;
+ /* This read memory barrier tells the compiler and the CPU that
+ * this can't be reordered: we have to complete the above
+ * before going on. */
+ rmb();
+ /* Now we read the nanoseconds part. */
+ nsec = lguest_data.time.tv_nsec;
+ /* Make sure we've done that. */
+ rmb();
+ /* Now if the seconds part has changed, try again. */
+ } while (unlikely(lguest_data.time.tv_sec != sec));
+
+ /* Our non-TSC clock is in real nanoseconds. */
+ return sec*1000000000ULL + nsec;