[PATCH v4 2/2] x86/tsc: Disable clocksource watchdog checking on recent and future UV platforms.

[Dimitri Sivanich <sivanich@hpe.com>]

HPE UV hardware and firmware is designed to ensure a reliable and
synchronized TSC mechanism.  Comparing the TSC against secondary
clocksources can result in false positives due to variable access
latency caused by system traffic.  The best course of action against
these false positives has been found to simply disable watchdog
checking of the TSC.

Commits [1] and [2] were introduced to avoid an issue where the TSC
is falsely declared unstable by exempting qualified platforms of up
to 4-sockets from TSC clocksource watchdog checking.  Extend that
exemption to include recent and future UV platforms.

[1] 'commit b50db7095fe0 ("x86/tsc: Disable clocksource watchdog for TSC on qualified platorms")'
[2] 'commit 233756a640be ("x86/tsc: Extend watchdog check exemption to 4-Sockets platform")'

Signed-off-by: Dimitri Sivanich <sivanich@hpe.com>
---
 arch/x86/kernel/tsc.c | 27 +++++++++++++++++++++------
 1 file changed, 21 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index c5110eb554bc..08e8e5511749 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -32,6 +32,7 @@
 #include <asm/i8259.h>
 #include <asm/msr.h>
 #include <asm/topology.h>
+#include <asm/uv/uv_hub.h>
 #include <asm/uv/uv.h>
 #include <asm/sev.h>
 
@@ -1228,6 +1229,20 @@ static void __init tsc_disable_clocksource_watchdog(void)
 	clocksource_tsc.flags &= ~CLOCK_SOURCE_MUST_VERIFY;
 }
 
+static bool __init platform_is_exempt_from_watchdog(void)
+{
+	/* Platforms with no more than 4 packages are exempt */
+	if (topology_max_packages() <= 4)
+		return true;
+
+#ifdef CONFIG_X86_64
+	/* Recent UV systems are exempt */
+	if (is_uvy_hub())
+		return true;
+#endif
+	return false;
+}
+
 static void __init check_system_tsc_reliable(void)
 {
 #if defined(CONFIG_MGEODEGX1) || defined(CONFIG_MGEODE_LX) || defined(CONFIG_X86_GENERIC)
@@ -1246,17 +1261,17 @@ static void __init check_system_tsc_reliable(void)
 		tsc_clocksource_reliable = 1;
 
 	/*
-	 * Disable the clocksource watchdog when the system has:
-	 *  - TSC running at constant frequency
-	 *  - TSC which does not stop in C-States
-	 *  - the TSC_ADJUST register which allows to detect even minimal
+	 * Disable the clocksource watchdog when the system:
+	 *  - has TSC running at constant frequency
+	 *  - has TSC which does not stop in C-States
+	 *  - has the TSC_ADJUST register which allows to detect even minimal
 	 *    modifications
-	 *  - not more than four packages
+	 *  - is exempt from running the clocksource watchdog
 	 */
 	if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
 	    boot_cpu_has(X86_FEATURE_NONSTOP_TSC) &&
 	    boot_cpu_has(X86_FEATURE_TSC_ADJUST) &&
-	    topology_max_packages() <= 4)
+	    platform_is_exempt_from_watchdog())
 		tsc_disable_clocksource_watchdog();
 }
 
-- 
2.43.0