[PATCH v12 15/19] tsc: Upgrade TSC clocksource rating

[Nikunj A Dadhania <nikunj@amd.com>]

In virtualized environments running on modern CPUs, the underlying
platforms guarantees to have a stable, always running TSC, i.e. that the
TSC is a superior timesource as compared to other clock sources (such as
kvmclock, HPET, ACPI timer, APIC, etc.).

Upgrade the rating of the early and regular clock source to prefer TSC over
other clock sources when TSC is invariant, non-stop and stable.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
---
 arch/x86/kernel/tsc.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index c83f1091bb4f..8150f2104474 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -1264,6 +1264,21 @@ static void __init check_system_tsc_reliable(void)
 		tsc_disable_clocksource_watchdog();
 }
 
+static void __init upgrade_clock_rating(struct clocksource *tsc_early,
+					struct clocksource *tsc)
+{
+	/*
+	 * Upgrade the clock rating for TSC early and regular clocksource when
+	 * the underlying platform provides non-stop, invaraint and stable TSC.
+	 */
+	if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
+	    boot_cpu_has(X86_FEATURE_NONSTOP_TSC) &&
+	    !tsc_unstable) {
+		tsc_early->rating = 499;
+		tsc->rating = 500;
+	}
+}
+
 /*
  * Make an educated guess if the TSC is trustworthy and synchronized
  * over all CPUs.
@@ -1565,6 +1580,8 @@ void __init tsc_init(void)
 	if (tsc_clocksource_reliable || no_tsc_watchdog)
 		tsc_disable_clocksource_watchdog();
 
+	upgrade_clock_rating(&clocksource_tsc_early, &clocksource_tsc);
+
 	clocksource_register_khz(&clocksource_tsc_early, tsc_khz);
 	detect_art();
 }
-- 
2.34.1