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

[Vitaly Kuznetsov <vkuznets@redhat.com>]

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Paolo Bonzini <pbonzini@redhat.com> writes:

> On 18/03/21 15:52, Vitaly Kuznetsov wrote:
>> Paolo Bonzini <pbonzini@redhat.com> writes:
>> 
>>> On 18/03/21 15:09, Vitaly Kuznetsov wrote:
>>>> +static inline void check_tsc_msr_tsc_page(struct ms_hyperv_tsc_page *tsc_page)
>>>> +{
>>>> +	u64 r1, r2, t1, t2;
>>>> +	s64 delta_ns;
>>>> +
>>>> +	/* Compare TSC page clocksource with HV_X64_MSR_TIME_REF_COUNT */
>>>> +	t1 = mul_u64_u64_shr64(rdtsc(), tsc_page->tsc_scale) + tsc_page->tsc_offset;
>>>> +	r1 = rdmsr(HV_X64_MSR_TIME_REF_COUNT);
>>>> +	nop_loop();
>>>> +	t2 = mul_u64_u64_shr64(rdtsc(), tsc_page->tsc_scale) + tsc_page->tsc_offset;
>>>> +	r2 = rdmsr(HV_X64_MSR_TIME_REF_COUNT);
>>>> +
>>>> +	delta_ns = ((r2 - r1) - (t2 - t1)) * 100;
>>>> +	if (delta_ns < 0)
>>>> +		delta_ns = -delta_ns;
>>>> +
>>>> +	/* 1% tolerance */
>>>> +	GUEST_ASSERT(delta_ns * 100 < (t2 - t1) * 100);
>>>> +}
>>>> +
>>>
>>> I think you should also be able to check r1 and r2 individually, not
>>> just r1 and r2.  Is that correct?
>> 
>> Right, we could've checked r1 == t1 and r2 == t2 actually (with some
>> tiny margin of course). Let me try that.
>
> np, I can do that too.  Just checking my recollection of the TLFS.
>

I already hacked it, patch attached :-) Feel free to squash it or put
your own version in.

-- 
Vitaly


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From dae56d5d5c5fc7442d4dabf96a9c322acb42f458 Mon Sep 17 00:00:00 2001
From: Vitaly Kuznetsov <vkuznets@redhat.com>
Date: Thu, 18 Mar 2021 16:21:18 +0100
Subject: [PATCH] selftests: KVM: hyper-v: check_tsc_msr_tsc_page() fix.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
---
 .../selftests/kvm/x86_64/hyperv_clock.c       | 25 ++++++++++---------
 1 file changed, 13 insertions(+), 12 deletions(-)

diff --git a/tools/testing/selftests/kvm/x86_64/hyperv_clock.c b/tools/testing/selftests/kvm/x86_64/hyperv_clock.c
index 39d6491d8458..aacb6d2697da 100644
--- a/tools/testing/selftests/kvm/x86_64/hyperv_clock.c
+++ b/tools/testing/selftests/kvm/x86_64/hyperv_clock.c
@@ -82,22 +82,23 @@ static inline void check_tsc_msr_rdtsc(void)
 
 static inline void check_tsc_msr_tsc_page(struct ms_hyperv_tsc_page *tsc_page)
 {
-	u64 r1, r2, t1, t2;
-	s64 delta_ns;
+	u64 t1, t2;
 
-	/* Compare TSC page clocksource with HV_X64_MSR_TIME_REF_COUNT */
-	t1 = mul_u64_u64_shr64(rdtsc(), tsc_page->tsc_scale) + tsc_page->tsc_offset;
-	r1 = rdmsr(HV_X64_MSR_TIME_REF_COUNT);
+	/*
+	 * Make TSC run for a while to make sure clocksources don't diverge
+	 * over time.
+	 */
 	nop_loop();
-	t2 = mul_u64_u64_shr64(rdtsc(), tsc_page->tsc_scale) + tsc_page->tsc_offset;
-	r2 = rdmsr(HV_X64_MSR_TIME_REF_COUNT);
 
-	delta_ns = ((r2 - r1) - (t2 - t1)) * 100;
-	if (delta_ns < 0)
-		delta_ns = -delta_ns;
+	/*
+	 * Get readings from TSC page and Reference TSC clocksources and make
+	 * sure they match.
+	 */
+	t1 = mul_u64_u64_shr64(rdtsc(), tsc_page->tsc_scale) + tsc_page->tsc_offset;
+	t2 = rdmsr(HV_X64_MSR_TIME_REF_COUNT);
 
-	/* 1% tolerance */
-	GUEST_ASSERT(delta_ns * 100 < (t2 - t1) * 100);
+	/* 10us tolerance */
+	GUEST_ASSERT(t2 - t1 <= 100);
 }
 
 static void guest_main(struct ms_hyperv_tsc_page *tsc_page, vm_paddr_t tsc_page_gpa)
-- 
2.30.2