Re: Monotonic clock with KVM pv-clock

Marcelo Tosatti
2014-01-21T16:33:34Z

On Tue, Jan 21, 2014 at 03:24:43PM +0200, Nadav Har'El wrote:
> On Mon, Jan 20, 2014, Marcelo Tosatti wrote about "Re: Monotonic clock with KVM pv-clock":
> > On Mon, Jan 20, 2014 at 11:56:56AM +0200, Nadav Har'El wrote:
> > > Hi,
> > >
> > > I'm trying to figure out how a guest OS can get a monotonic clock using
> > > KVM's paravirtual clock.
> > >
> > > At first, I thought that the clock I get using KVM_SYSTEM_TIME is a
> > > monotonic clock, based on the host's monotonic clock.
> >
> > It is. However, it is used in conjunction with TSC delta, part
> > of the structure which is written at KVM_SYSTEM_TIME GPA. See
> > pvclock_clocksource_read at arch/x86/kernel/pvclock.c.
>
> Sure, I'm using the whole protocol for KVM_SYSTEM_TIME. I just wondered
> if it has a meaningful definition (namely, of being a monotonic clock) when
> being used not in conjunction with the KVM_WALL_CLOCK protocol.

If flags field contains PVCLOCK_TSC_STABLE_BIT, yes, otherwise, no.

Should use a global accumulator with cmpxchg if PVCLOCK_TSC_STABLE_BIT
is clear.

> > > 2. What happens when the wall-clock time is set on the host? I was
> > > hoping that only KVM_WALL_CLOCK changes and KVM_SYSTEM_TIME doesn't,
> > > but am no longer sure this is actually the case.
> >
> > Yes, it is the case. The host clock which backs system_timestamp field
> > of pvclock structure is
> >
> > CLOCK_MONOTONIC
> > Clock that cannot be set and represents monotonic time
> > since some unspecified starting point. This clock is not affected by
> > discontinuous jumps in the system time (e.g., if the system
> > administrator manually changes the clock), but is affected by the
> > incremental adjustments per???formed by adjtime(3) and NTP.
>
> Excellent.
>
> >
> > > If KVM_SYSTEM_TIME is not a correct way to get a monotonic paravirtual clock
> > > from KVM, is there a correct way?
> >
> > Inside a Linux guest? Can use sched_clock().
> >
> > If not a Linux guest, either implement kvmclock-like driver
> > (kvm-unit-test contains one).
>
> This is actually an OSv guest (http://osv.io/).
>
> Until now we were using kvmclock by adding up the wallclock and
> systemtime protocol, and this resulted with a good wall-time clock
> (CLOCK_REALTIME), but I really wanted to also have a monotonic clock
> (CLOCK_MONOTONIC) for the guest.
>
> I took a look and how a Linux guest uses the KVM pv clock, and thought
> that the "system time" part of the kvm clock protocol will be a good
> monotonic clock, but wasn't sure I was understanding this right.
>
> Thanks for the clarifications!
>
> Nadav.
>
> --
> Nadav Har'El | Tuesday, Jan 21 2014, 20 Shevat 5774
> nyh@math.technion.ac.il |-----------------------------------------
> Phone +972-523-790466, ICQ 13349191 |"The average person thinks he isn't." -
> http://nadav.harel.org.il |Father Larry Lorenzoni