[PATCH v6 42/51] timekeeping: Resume clocksources before reading persistent clock

When resuming timekeeping after suspend, restore clocksources prior to
reading the persistent clock.  Paravirt clocks, e.g. kvmclock, tie the
validity of a PV persistent clock to a clocksource, i.e. reading the PV
persistent clock will return garbage if the underlying PV clocksource
hasn't been enabled.  The flaw has gone unnoticed because kvmclock is a
mess and uses its own suspend/resume hooks instead of the clocksource
suspend/resume hooks, which happens to work by sheer dumb luck (the
kvmclock resume hook runs before timekeeping_resume()).

Note, there is no evidence that any clocksource supported by the kernel
depends on a persistent clock.

Reviewed-by: Thomas Gleixner <redacted>
Reviewed-by: David Woodhouse <redacted>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 kernel/time/timekeeping.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 97db2e9393f9..585525f5c1f6 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c

@@ -2226,11 +2226,16 @@ void timekeeping_resume(void)
 	u64 cycle_now, nsec;
 	unsigned long flags;
 
-	read_persistent_clock64(&ts_new);
-
 	clockevents_resume();
 	clocksource_resume();
 
+	/*
+	 * Read persistent time after clocksources have been resumed.  Paravirt
+	 * clocks have a nasty habit of piggybacking a persistent clock on a
+	 * system clock, and may return garbage if the system clock is suspended.
+	 */
+	read_persistent_clock64(&ts_new);
+
 	raw_spin_lock_irqsave(&tk_core.lock, flags);
 
 	/*

-- 
2.55.0.679.g6767b8d81c-goog