[PATCH AUTOSEL 5.13 013/219] x86/hyperv: fix for unwanted manipulation of sched_clock when TSC marked unstable

From: Sasha Levin
Date: Thu Sep 09 2021 - 08:10:06 EST


From: Ani Sinha <ani@xxxxxxxxxxx>

[ Upstream commit c445535c3efbfb8cb42d098e624d46ab149664b7 ]

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@xxxxxxxxxxx>
Reviewed-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>
Tested-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20210713030522.1714803-1-ani@xxxxxxxxxxx
Signed-off-by: Wei Liu <wei.liu@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
arch/x86/kernel/cpu/mshyperv.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index 4fa0a4280895..ea87d9ed77e9 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 +430,13 @@ 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)
--
2.30.2