[PATCH 26/41] x86/hyperv: fix for unwanted manipulation of sched_clock when TSC marked unstable

[Tim Gardner]

From: Ani Sinha <ani at anisinha.ca>

BugLink: https://bugs.launchpad.net/bugs/1949770

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 at anisinha.ca>
Reviewed-by: Michael Kelley <mikelley at microsoft.com>
Tested-by: Michael Kelley <mikelley at microsoft.com>
Link: https://lore.kernel.org/r/20210713030522.1714803-1-ani@anisinha.ca
Signed-off-by: Wei Liu <wei.liu at kernel.org>
(cherry picked from commit c445535c3efbfb8cb42d098e624d46ab149664b7 linux-next)
Signed-off-by: Tim Gardner <tim.gardner at canonical.com>
---
 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 77947bd68c08..654ec4412161 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -335,8 +335,6 @@ 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");
 	}
 
 	/*
@@ -395,6 +393,13 @@ 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)
-- 
2.33.1