On Wed, Mar 25, 2015 at 03:48:02PM -0700, Andy Lutomirski wrote: > On Wed, Mar 25, 2015 at 3:41 PM, Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote: > > On Wed, Mar 25, 2015 at 03:33:10PM -0700, Andy Lutomirski wrote: > >> On Mar 25, 2015 2:29 PM, "Marcelo Tosatti" <mtosatti@xxxxxxxxxx> wrote: > >> > > >> > On Wed, Mar 25, 2015 at 01:52:15PM +0100, Radim Krčmář wrote: > >> > > 2015-03-25 12:08+0100, Radim Krčmář: > >> > > > Reverting the patch protects us from any migration, but I don't think we > >> > > > need to care about changing VCPUs as long as we read a consistent data > >> > > > from kvmclock. (VCPU can change outside of this loop too, so it doesn't > >> > > > matter if we return a value not fit for this VCPU.) > >> > > > > >> > > > I think we could drop the second __getcpu if our kvmclock was being > >> > > > handled better; maybe with a patch like the one below: > >> > > > >> > > The second __getcpu is not neccessary, but I forgot about rdtsc. > >> > > We need to either use rtdscp, know the host has synchronized tsc, or > >> > > monitor VCPU migrations. Only the last one works everywhere. > >> > > >> > The vdso code is only used if host has synchronized tsc. > >> > > >> > But you have to handle the case where host goes from synchronized tsc to > >> > unsynchronized tsc (see the clocksource notifier in the host side). > >> > > >> > >> Can't we change the host to freeze all vcpus and clear the stable bit > >> on all of them if this happens? This would simplify and speed up > >> vclock_gettime. > >> > >> --Andy > > > > Seems interesting to do on 512-vcpus, but sure, could be done. > > > > If you have a 512-vcpu system that switches between stable and > unstable more than once per migration, then I expect that you have > serious problems and this is the least of your worries. > > Personally, I'd *much* rather we just made vcpu 0's pvti authoritative > if we're stable. If nothing else, I'm not even remotely convinced > that the current scheme gives monotonic timing due to skew between > when the updates happen on different vcpus. Can you write down the problem ? -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
