[PATCH v2] clocksource: tsc: respect tsc bootparam for clocksource_tsc_early

From: Michael Zhivich
Date: Thu Oct 24 2019 - 14:01:14 EST

Introduction of clocksource_tsc_early broke functionality of "tsc=reliable"
and "tsc=nowatchdog" bootparams, since clocksource_tsc_early is *always*
registered with CLOCK_SOURCE_MUST_VERIFY and thus put on the watchdog list.

This frequently causes TSC to be declared unstable during boot:

clocksource: timekeeping watchdog on CPU0: Marking clocksource
'tsc-early' as unstable because the skew is too large:
clocksource: 'refined-jiffies' wd_now: fffb7018 wd_last: fffb6e9d
mask: ffffffff
clocksource: 'tsc-early' cs_now: 68a6a7070f6a0 cs_last: 68a69ab6f74d6
mask: ffffffffffffffff
tsc: Marking TSC unstable due to clocksource watchdog

The corresponding elapsed times are cs_nsec=1224152026 and wd_nsec=378942392, so
watchdog differs from TSC by 0.84 seconds. Since jiffies is not guaranteed
to provide reliable timekeeping due to possibility of missing timer
interrupts, this is a false positive.

This issue affects machines running with HPET disabled; otherwise, HPET would
be the watchdog for tsc-early clocksource, and this situation would be avoided.

Restore previous behavior by respecting "tsc=reliable" and "tsc=nowatchdog"
boot params when registering clocksource_tsc_early.

Fixes: aa83c45762a24 ("x86/tsc: Introduce early tsc clocksource")
Signed-off-by: Michael Zhivich <mzhivich@xxxxxxxxxx>
arch/x86/kernel/tsc.c | 3 +++
1 file changed, 3 insertions(+)

Changes from v1: updated commit message, added "Fixes" tag.

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index c59454c382fd..7e322e2daaf5 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -1505,6 +1505,9 @@ void __init tsc_init(void)

+ if (tsc_clocksource_reliable || no_tsc_watchdog)
+ clocksource_tsc_early.flags &= ~CLOCK_SOURCE_MUST_VERIFY;
clocksource_register_khz(&clocksource_tsc_early, tsc_khz);