[PATCH RFC] x86/tsc: Make recalibration default on for TSC_KNOWN_FREQ cases

[Feng Tang]

Commit a7ec817d5542 ("x86/tsc: Add option to force frequency
recalibration with HW timer") was added to handle cases that the
firmware has bug and provides a wrong TSC frequency number, and it
is optional given that this kind of firmware issue rarely happens
(Paul reported once [1]).

But Rui reported that some Sapphire Rapids platform met this issue
again recently, and as firmware is also a kind of 'software' which
can't be bug free, make the recalibration default on. When the
values from firmware and HW timer's calibration have big gap,
raise a warning and let vendor to check which side is broken.

One downside is, many VMs also has X86_FEATURE_TSC_KNOWN_FREQ set,
and they will also do this recalibration.

[1]. https://lore.kernel.org/lkml/20221117230910.GI4001@paulmck-ThinkPad-P17-Gen-1/
Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>
---
 Documentation/admin-guide/kernel-parameters.txt |  4 ----
 arch/x86/kernel/tsc.c                           | 14 ++++----------
 2 files changed, 4 insertions(+), 14 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 9e5bab29685f..a826ab3b5dfb 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -6451,10 +6451,6 @@
 			in situations with strict latency requirements (where
 			interruptions from clocksource watchdog are not
 			acceptable).
-			[x86] recalibrate: force recalibration against a HW timer
-			(HPET or PM timer) on systems whose TSC frequency was
-			obtained from HW or FW using either an MSR or CPUID(0x15).
-			Warn if the difference is more than 500 ppm.
 			[x86] watchdog: Use TSC as the watchdog clocksource with
 			which to check other HW timers (HPET or PM timer), but
 			only on systems where TSC has been deemed trustworthy.
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 344698852146..b77c5b1ad304 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -48,8 +48,6 @@ static DEFINE_STATIC_KEY_FALSE(__use_tsc);
 
 int tsc_clocksource_reliable;
 
-static int __read_mostly tsc_force_recalibrate;
-
 static u32 art_to_tsc_numerator;
 static u32 art_to_tsc_denominator;
 static u64 art_to_tsc_offset;
@@ -310,8 +308,6 @@ static int __init tsc_setup(char *str)
 				 __func__);
 		tsc_as_watchdog = 0;
 	}
-	if (!strcmp(str, "recalibrate"))
-		tsc_force_recalibrate = 1;
 	if (!strcmp(str, "watchdog")) {
 		if (no_tsc_watchdog)
 			pr_alert("%s: tsc=watchdog overridden by earlier tsc=nowatchdog\n",
@@ -1395,7 +1391,6 @@ static void tsc_refine_calibration_work(struct work_struct *work)
 	else
 		freq = calc_pmtimer_ref(delta, ref_start, ref_stop);
 
-	/* Will hit this only if tsc_force_recalibrate has been set */
 	if (boot_cpu_has(X86_FEATURE_TSC_KNOWN_FREQ)) {
 
 		/* Warn if the deviation exceeds 500 ppm */
@@ -1456,17 +1451,16 @@ static int __init init_tsc_clocksource(void)
 		clocksource_tsc.flags |= CLOCK_SOURCE_SUSPEND_NONSTOP;
 
 	/*
-	 * When TSC frequency is known (retrieved via MSR or CPUID), we skip
-	 * the refined calibration and directly register it as a clocksource.
+	 * When TSC frequency is known (retrieved via MSR or CPUID), we
+	 * directly register it as a clocksource. As the firmware could
+	 * be wrong (very unlikely) about the values, the recalibration
+	 * by hardware timer is kept just as a sanity check.
 	 */
 	if (boot_cpu_has(X86_FEATURE_TSC_KNOWN_FREQ)) {
 		if (boot_cpu_has(X86_FEATURE_ART))
 			art_related_clocksource = &clocksource_tsc;
 		clocksource_register_khz(&clocksource_tsc, tsc_khz);
 		clocksource_unregister(&clocksource_tsc_early);
-
-		if (!tsc_force_recalibrate)
-			return 0;
 	}
 
 	schedule_delayed_work(&tsc_irqwork, 0);
-- 
2.34.1