From: Jim Mattson <jmattson@google.com>
To: kvm@vger.kernel.org, pbonzini@redhat.com
Cc: Jim Mattson <jmattson@google.com>, Will Auld <will.auld.intel@gmail.com>
Subject: [kvm-unit-tests PATCH 1/3] x86: tsc_adjust: Remove flaky timing test
Date: Thu, 27 Jan 2022 13:55:46 -0800 [thread overview]
Message-ID: <20220127215548.2016946-1-jmattson@google.com> (raw)
The subtest labeled, "MSR_IA32_TSC_ADJUST msr adjustment on tsc
write," sometimes fails. The behavior tested is neither architected
nor guaranteed. Running under qemu/kvm, the 'est_delta_time' has been
observed to be as much as an order of magnitude greater than the
expression that is supposed to be its upper bound.
Remove the flaky subtest, and replace it with some invariants that
actually are architecturally guaranteed (as long as IA32_TSC doesn't
wrap around).
Fixes: 5fecf5d8cad1 ("Added tests for ia32_tsc_adjust funtionality.")
Cc: Will Auld <will.auld.intel@gmail.com>
Signed-off-by: Jim Mattson <jmattson@google.com>
---
x86/tsc_adjust.c | 10 +++-------
1 file changed, 3 insertions(+), 7 deletions(-)
diff --git a/x86/tsc_adjust.c b/x86/tsc_adjust.c
index 3636b5e082d5..b0d79c499edb 100644
--- a/x86/tsc_adjust.c
+++ b/x86/tsc_adjust.c
@@ -4,7 +4,6 @@
int main(void)
{
u64 t1, t2, t3, t4, t5;
- u64 est_delta_time;
if (this_cpu_has(X86_FEATURE_TSC_ADJUST)) { // MSR_IA32_TSC_ADJUST Feature is enabled?
report(rdmsr(MSR_IA32_TSC_ADJUST) == 0x0,
@@ -26,12 +25,9 @@ int main(void)
wrtsc(t4);
t2 = rdtsc();
t5 = rdmsr(MSR_IA32_TSC_ADJUST);
- // est of time between reading tsc and writing tsc,
- // (based on MSR_IA32_TSC_ADJUST msr value) should be small
- est_delta_time = t4 - t5 - t1;
- // arbitray 2x latency (wrtsc->rdtsc) threshold
- report(est_delta_time <= (2 * (t2 - t4)),
- "MSR_IA32_TSC_ADJUST msr adjustment on tsc write");
+ report(t1 <= t4 - t5,
+ "Internal TSC advances across write to IA32_TSC");
+ report(t2 >= t4, "IA32_TSC advances after write to IA32_TSC");
}
else {
report_pass("MSR_IA32_TSC_ADJUST feature not enabled");
--
2.35.0.rc2.247.g8bbb082509-goog
next reply other threads:[~2022-01-27 21:56 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-01-27 21:55 Jim Mattson [this message]
2022-01-27 21:55 ` [kvm-unit-tests PATCH 2/3] x86: tsc_adjust: Use report_skip when skipping test Jim Mattson
2022-01-27 21:55 ` [kvm-unit-tests PATCH 3/3] x86: Define wrtsc(tsc) as wrmsr(MSR_IA32_TSC, tsc) Jim Mattson
2022-01-28 15:17 ` Paolo Bonzini
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