On Mon, 17 Sep 2018, John Stultz wrote: > On Mon, Sep 17, 2018 at 12:25 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote: > > Also, I'm not entirely convinced that this "last" thing is needed at > > all. John, what's the scenario under which we need it? > > So my memory is probably a bit foggy, but I recall that as we > accelerated gettimeofday, we found that even on systems that claimed > to have synced TSCs, they were actually just slightly out of sync. > Enough that right after cycles_last had been updated, a read on > another cpu could come in just behind cycles_last, resulting in a > negative interval causing lots of havoc. > > So the sanity check is needed to avoid that case. Your memory serves you right. That's indeed observable on CPUs which lack TSC_ADJUST. @Andy: Welcome to the wonderful world of TSC. Thanks, tglx _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
