Re: Monotonic clock with KVM pv-clock

[Nadav Har'El <nyh@math.technion.ac.il>]

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.

> > 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