From mboxrd@z Thu Jan  1 00:00:00 1970
From: Marcelo Tosatti <mtosatti@redhat.com>
Subject: Re: Monotonic clock with KVM pv-clock
Date: Mon, 20 Jan 2014 22:19:45 -0200
Message-ID: <20140121001945.GA32105@amt.cnet>
References: <20140120095656.GA1282@fermat.math.technion.ac.il>
 <20140120133317.GA7509@amt.cnet>
 <52DD39DB.3040109@lab.ntt.co.jp>
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Cc: "Nadav Har'El" <nyh@math.technion.ac.il>, kvm@vger.kernel.org
To: Fernando Luis Vazquez Cao <fernando_b1@lab.ntt.co.jp>
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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).