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).
