On Tue, Dec 28, 2010 at 07:38:16PM -1000, Zachary Amsden wrote: > A further set of improvements to KVM clock. Now it actually can > stay synchronized on SMP systems with a stable TSC, does not get > destabilized by host suspend, and is resistant to migration. > > I did look a bit into the second to last remaining migration issue; > that is, we read the TSCs for all VCPUs at different times, then > write them back at different times as well. We have compensation > for this already (see patch 1), but there is a possibility that > ordering issues create a backwards condition: > > read VTSC-0 > stop VCPU-0 > read VTSC-1 > stop VCPU-1 > > In that case, what the compensation code does is reset VTSC-1 > back to VTSC-0. With the above ordering, this is broken. > > However, in QEMU, the migration order is: > > stop VCPU-0 > stop VCPU-1 > read VTSC-0 > read VTSC-1 > > So even if a higher TSC value is read for VTSC-1, no backwards > condition can be generated, as VCPU-1 was not running at the time > and thus could not have observed the higher TSC. > > This brings us close to having a perfect KVM clock. > > Next steps will be testing to see if this in practice allows us > to drop the atomic backwards protection for KVM clock, and if so, > to implement vread so we have fast system calls for time. > > Zach Patchset looks good to me. -- 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
