Re: KVM: x86: limit difference between kvmclock updates

[Marcelo Tosatti <mtosatti@redhat.com>]

On Tue, May 14, 2013 at 12:05:13PM +0300, Gleb Natapov wrote:
> On Thu, May 09, 2013 at 08:21:41PM -0300, Marcelo Tosatti wrote:
> > 
> > kvmclock updates which are isolated to a given vcpu, such as vcpu->cpu
> > migration, should not allow system_timestamp from the rest of the vcpus
> > to remain static. Otherwise ntp frequency correction applies to one
> > vcpu's system_timestamp but not the others.
> > 
> > So in those cases, request a kvmclock update for all vcpus. The worst
> > case for a remote vcpu to update its kvmclock is then bounded by maximum
> > nohz sleep latency.
> > 
> Does this mean that when one vcpu is migrated all others are kicked out
> from a guest mode?

Yes, those which are in guest mode. For guests with large number of
vcpus this is a problem, but i can't see a simpler method to fix the bug
for now.

Yes, this aspect must be improved (however, the bug incurs on timers in
the guest taking tens of milliseconds with vcpu->pcpu pinning, which can
be unacceptable).