Re: Monotonic clock with KVM pv-clock

Marcelo Tosatti
2014-01-21T12:01:29Z

On Mon, Jan 20, 2014 at 11:59:39PM +0900, Fernando Luis Vazquez Cao wrote:
> (2014/01/20 22:33), Marcelo Tosatti wrote:
> >On Mon, Jan 20, 2014 at 11:56:56AM +0200, Nadav Har'El wrote:
> >>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().
>
> I would like to mention that Linux guests usually do not use sched_clock()
> directly. The reason being that the kvm_clock based sched_clock() is not
> marked stable (sched_clock_stable is 0), which means that the pair of
> wrappers sched_clock_local()/sched_clock_remote() is used instead.

Should verify the requirements of sched_clock_cpu() and enable
sched_clock_stable in case it fulfills requirements (kvmclock_read can
be nondecreasing due to TSC->nanosecond scaling, and not increase for a
longer duration with global accumulator, due to cmpxchg).