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

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Marcelo Tosatti <mtosatti@redhat.com> writes:

> 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).

This particular place tests that Reference TSC
(HV_X64_MSR_TIME_REF_COUNT) is a 1Ghz clock. We test it against raw
TSC. TSC frequency is known to us (HV_X64_MSR_TSC_FREQUENCY) so we can
compare the delta after nop_loop().

We can't directly compare r1 and t1 (and r2 and t2) here because we
don't know the base precisely. We could've probably reset TSC to 0 and
kvmclock (which converts to Reference TSC) to 0 and compare after. For
now, we just check that Reference TSC is ticking as it should.

-- 
Vitaly