Re: [PATCH v2 6/4] selftests: kvm: Add basic Hyper-V clocksources tests

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Mar 18, 2021 at 06:50:35PM +0100, Paolo Bonzini wrote:
> On 18/03/21 17:57, Marcelo Tosatti wrote:
> > I think this should be monotonically increasing:
> > 
> > 1.	r1 = rdtsc();
> > 2.	t1 = rdmsr(HV_X64_MSR_TIME_REF_COUNT);
> > 3.	nop_loop();
> > 4.	r2 = rdtsc();
> > 5.	t2 = rdmsr(HV_X64_MSR_TIME_REF_COUNT);	
> > 
> > > 
> > > +	/* 1% tolerance */
> > > +	GUEST_ASSERT(delta_ns * 100 < (t2 - t1) * 100);
> > > +}
> > 
> > Doesnt an unbounded schedule-out/schedule-in (which resembles
> > overloaded host) of the qemu-kvm vcpu in any of the points 1,2,3,4,5
> > break the assertion above?
> 
> 
> Yes, there's a window of a handful of instructions (at least on
> non-preemptible kernels).  If anyone ever hits it, we can run the test 100
> times and check that it passes at least 95 or 99 of them.
> 
> Paolo

Yep, sounds like a good solution.

However this makes me wonder on the validity of the test: what its
trying to verify, again? (i would check the monotonicity that 
is r1 <= t1 <= r2 <= t2 as well, without the nop_loop in between).