clocksource: hyper-v: make sure Invariant-TSC is used if it is available

If Hyper-V TSC page is unavailable and Invariant-TSC is available,
currently hyperv_cs_msr (rather than Invariant-TSC) is used by default.

Use Invariant-TSC by default by downgrading hyperv_cs_msr.rating in
hv_init_tsc_clocksource(), if Invariant-TSC is available.
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/20230408210339.15085-1-decui@microsoft.comSigned-off-by: Wei Liu <wei.liu@kernel.org>

drivers/clocksource/hyperv_timer.c

@@ -517,9 +517,6 @@ static bool __init hv_init_tsc_clocksource(void)
 {
 	union hv_reference_tsc_msr tsc_msr;
 
-	if (!(ms_hyperv.features & HV_MSR_REFERENCE_TSC_AVAILABLE))
-		return false;
-
 	/*
 	 * If Hyper-V offers TSC_INVARIANT, then the virtualized TSC correctly
 	 * handles frequency and offset changes due to live migration,
@@ -536,6 +533,9 @@ static bool __init hv_init_tsc_clocksource(void)
 		hyperv_cs_msr.rating = 250;
 	}
 
+	if (!(ms_hyperv.features & HV_MSR_REFERENCE_TSC_AVAILABLE))
+		return false;
+
 	hv_read_reference_counter = read_hv_clock_tsc;
 
 	/*