Re: [patch 1/2] KVM: x86: switch KVMCLOCK base to CLOCK_MONOTONIC_RAW

Re: [patch 1/2] KVM: x86: switch KVMCLOCK base to CLOCK_MONOTONIC_RAW

[Marcelo Tosatti]

On Mon, Aug 13, 2018 at 09:52:55AM -0300, Marcelo Tosatti wrote:
> On Fri, Aug 10, 2018 at 09:15:51AM +0200, Paolo Bonzini wrote:
> > On 09/08/2018 16:52, Marcelo Tosatti wrote:
> > > Commit 0bc48bea36d1 ("KVM: x86: update master clock before computing kvmclock_offset")
> > > switches the order of operations to avoid the conversion 
> > > 
> > > TSC (without frequency correction) ->
> > > system_timestamp (with frequency correction), 
> > > 
> > > which might cause a time jump.
> > > 
> > > However, it leaves any other masterclock update unsafe, which includes, 
> > > at the moment:
> > > 
> > >         * HV_X64_MSR_REFERENCE_TSC MSR write.
> > >         * TSC writes.
> > >         * Host suspend/resume.
> > > 
> > > Avoid the time jump issue by using frequency uncorrected
> > > CLOCK_MONOTONIC_RAW clock. 
> > > 
> > > Its the guests time keeping software responsability
> > > to track and correct a reference clock such as UTC.
> > > 
> > > Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> > 
> > What happens across migration?
> > 
> > Paolo
> 
> You mean between
> 
> frequency corrected host -> frequency uncorrected host
> 
> And vice-versa? 
> 
> I'll check NTP's/Chrony's behaviour and let you know.
> 
> Thanks

They will measure and adjust their frequency corrections
(which is necessary anyway due to temperature variations 
for example).

http://doc.ntp.org/4.1.0/ntpd.htm

Frequency Discipline

The ntpd behavior at startup depends on whether the frequency file,
usually ntp.drift, exists. This file contains the latest estimate of
clock frequency error. When the ntpd is started and the file does not
exist, the ntpd enters a special mode designed to quickly adapt to the
particular system clock oscillator time and frequency error. This takes
approximately 15 minutes, after which the time and frequency are set to
nominal values and the ntpd enters normal mode, where the time and
frequency are continuously tracked relative to the server. After one
hour the frequency file is created and the current frequency offset
written to it. When the ntpd is started and the file does exist, the
ntpd frequency is initialized from the file and enters normal mode
immediately. After that the current frequency offset is written to the
file at hourly intervals.