From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Marcelo Tosatti <mtosatti@redhat.com>,
Paolo Bonzini <pbonzini@redhat.com>
Cc: kvm@vger.kernel.org, Sean Christopherson <seanjc@google.com>,
Wanpeng Li <wanpengli@tencent.com>,
Jim Mattson <jmattson@google.com>
Subject: Re: [PATCH v2 6/4] selftests: kvm: Add basic Hyper-V clocksources tests
Date: Fri, 19 Mar 2021 10:35:29 +0100 [thread overview]
Message-ID: <87pmzv5vvi.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <20210318175515.GA40821@fuller.cnet>
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
prev parent reply other threads:[~2021-03-19 9:36 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-16 14:37 [PATCH v2 0/4] KVM: x86: hyper-v: TSC page fixes Vitaly Kuznetsov
2021-03-16 14:37 ` [PATCH v2 1/4] KVM: x86: hyper-v: Limit guest to writing zero to HV_X64_MSR_TSC_EMULATION_STATUS Vitaly Kuznetsov
2021-03-16 14:37 ` [PATCH v2 2/4] KVM: x86: hyper-v: Prevent using not-yet-updated TSC page by secondary CPUs Vitaly Kuznetsov
2021-03-18 17:02 ` Marcelo Tosatti
2021-03-18 18:04 ` Marcelo Tosatti
2021-03-18 18:05 ` Paolo Bonzini
2021-03-18 18:30 ` Marcelo Tosatti
2021-03-19 9:29 ` Vitaly Kuznetsov
2021-03-16 14:37 ` [PATCH v2 3/4] KVM: x86: hyper-v: Track Hyper-V TSC page status Vitaly Kuznetsov
2021-03-17 8:07 ` Paolo Bonzini
2021-03-17 11:19 ` [PATCH v2 5/4] KVM: x86: hyper-v: Briefly document enum hv_tsc_page_status Vitaly Kuznetsov
2021-03-16 14:37 ` [PATCH v2 4/4] KVM: x86: hyper-v: Don't touch TSC page values when guest opted for re-enlightenment Vitaly Kuznetsov
2021-03-18 14:09 ` [PATCH v2 6/4] selftests: kvm: Add basic Hyper-V clocksources tests Vitaly Kuznetsov
2021-03-18 14:26 ` Paolo Bonzini
2021-03-18 14:52 ` Vitaly Kuznetsov
2021-03-18 15:01 ` Paolo Bonzini
2021-03-18 15:23 ` Vitaly Kuznetsov
2021-03-18 15:27 ` Paolo Bonzini
2021-03-18 16:57 ` Marcelo Tosatti
2021-03-18 17:50 ` Paolo Bonzini
2021-03-18 17:55 ` Marcelo Tosatti
2021-03-19 9:35 ` Vitaly Kuznetsov [this message]
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