Re: KVM clock synchronization

[Marcelo Tosatti <mtosatti@redhat.com>]

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.