[RFC 2/2] x86/tsc: mark tsc reliable for qualified platforms

[Feng Tang <feng.tang@intel.com>]

There are cases that tsc clocksource are wrongly judged as unstable by
clocksource watchdogs like hpet, acpi_pm or 'refined-jiffies'. While
there is hardly a general reliable way to check the validity of a
watchdog, and to protect the innocent tsc, Thomas Gleixner proposed [1]:

"I'm inclined to lift that requirement when the CPU has:

    1) X86_FEATURE_CONSTANT_TSC
    2) X86_FEATURE_NONSTOP_TSC
    3) X86_FEATURE_NONSTOP_TSC_S3
    4) X86_FEATURE_TSC_ADJUST
    5) At max. 4 sockets

 After two decades of horrors we're finally at a point where TSC seems
 to be halfway reliable and less abused by BIOS tinkerers. TSC_ADJUST
 was really key as we can now detect even small modifications reliably
 and the important point is that we can cure them as well (not pretty
 but better than all other options)."

As feature #3 X86_FEATURE_NONSTOP_TSC_S3 only exists on several generations
of Atom processor, and is always coupled with X86_FEATURE_CONSTANT_TSC
and X86_FEATURE_NONSTOP_TSC, skip checking it,. and also be more defensive
to use maxim of 2 sockets.

The check is done inside tsc_init() before registering 'tsc-early' and
'tsc' clocksources, as there were cases that both of them had been
wrongly judged as unreliable.

Another idea is to not lift the check here, but inside the
tsc_sync_check_timer, stop the clocksource watchdog timer if there is
no TSC_ADJUST overridden found in 10 minutes after boot.

[1]. https://lore.kernel.org/lkml/87eekfk8bd.fsf@nanos.tec.linutronix.de/
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Feng Tang <feng.tang@intel.com>
---
 arch/x86/kernel/tsc.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index f70dffc..3a451e3 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -1193,6 +1193,17 @@ static void __init check_system_tsc_reliable(void)
 #endif
 	if (boot_cpu_has(X86_FEATURE_TSC_RELIABLE))
 		tsc_clocksource_reliable = 1;
+
+	/*
+	 * Ideally the socket number should be checked, but this is called
+	 * by tsc_init() which is in early boot phase and the socket numbers
+	 * may not be available. Use 'nr_online_nodes' as a fallback solution
+	 */
+	if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC) &&
+	    boot_cpu_has(X86_FEATURE_NONSTOP_TSC) &&
+	    boot_cpu_has(X86_FEATURE_TSC_ADJUST) &&
+	    nr_online_nodes <= 2)
+		tsc_clocksource_reliable = 1;
 }
 
 /*
-- 
2.7.4