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Subject[PATCH] Hyper-V: fix for unwanted manipulation of sched_clock when TSC marked unstable
Date
Marking TSC as unstable has a side effect of marking sched_clock as
unstable when TSC is still being used as the sched_clock. This is not
desirable. Hyper-V ultimately uses a paravirtualized clock source that
provides a stable scheduler clock even on systems without TscInvariant
CPU capability. Hence, mark_tsc_unstable() call should be called _after_
scheduler clock has been changed to the paravirtualized clocksource. This
will prevent any unwanted manipulation of the sched_clock. Only TSC will
be correctly marked as unstable.

Signed-off-by: Ani Sinha <ani@anisinha.ca>
---
arch/x86/kernel/cpu/mshyperv.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index 22f13343b5da..715458b7729a 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -370,8 +370,6 @@ static void __init ms_hyperv_init_platform(void)
if (ms_hyperv.features & HV_ACCESS_TSC_INVARIANT) {
wrmsrl(HV_X64_MSR_TSC_INVARIANT_CONTROL, 0x1);
setup_force_cpu_cap(X86_FEATURE_TSC_RELIABLE);
- } else {
- mark_tsc_unstable("running on Hyper-V");
}

/*
@@ -432,6 +430,12 @@ static void __init ms_hyperv_init_platform(void)
/* Register Hyper-V specific clocksource */
hv_init_clocksource();
#endif
+ /* TSC should be marked as unstable only after Hyper-V
+ * clocksource has been initialized. This ensures that the
+ * stability of the sched_clock is not altered.
+ */
+ if (!(ms_hyperv.features & HV_ACCESS_TSC_INVARIANT))
+ mark_tsc_unstable("running on Hyper-V");
}

static bool __init ms_hyperv_x2apic_available(void)
--
2.25.1
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