x86/hyperv: fix for unwanted manipulation of sched_clock when TSC marked unstable

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>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Tested-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/20210713030522.1714803-1-ani@anisinha.ca
Signed-off-by: Wei Liu <wei.liu@kernel.org>

diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index cc8f177..8bd1c01 100644 (file)
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -375,8 +375,6 @@ 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");
        }
 
        /*
@@ -437,6 +435,13 @@ 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)