Fix flaky monotonic time.

This change ensures that even platforms with some TSC issues (e.g. KVM),
can get reliable monotonic time by applied a lower bound on each read.

PiperOrigin-RevId: 309773801

pkg/sentry/kernel/timekeeper.go

@@ -16,6 +16,7 @@ package kernel
 
 import (
 	"fmt"
+	"sync/atomic"
 	"time"
 
 	"gvisor.dev/gvisor/pkg/log"
@@ -48,6 +49,9 @@ type Timekeeper struct {
 	// It is set only once, by SetClocks.
 	monotonicOffset int64 `state:"nosave"`
 
+	// monotonicLowerBound is the lowerBound for monotonic time.
+	monotonicLowerBound int64 `state:"nosave"`
+
 	// restored, if non-nil, indicates that this Timekeeper was restored
 	// from a state file. The clocks are not set until restored is closed.
 	restored chan struct{} `state:"nosave"`
@@ -271,6 +275,21 @@ func (t *Timekeeper) GetTime(c sentrytime.ClockID) (int64, error) {
 	now, err := t.clocks.GetTime(c)
 	if err == nil && c == sentrytime.Monotonic {
 		now += t.monotonicOffset
+		for {
+			// It's possible that the clock is shaky. This may be due to
+			// platform issues, e.g. the KVM platform relies on the guest
+			// TSC and host TSC, which may not be perfectly in sync. To
+			// work around this issue, ensure that the monotonic time is
+			// always bounded by the last time read.
+			oldLowerBound := atomic.LoadInt64(&t.monotonicLowerBound)
+			if now < oldLowerBound {
+				now = oldLowerBound
+				break
+			}
+			if atomic.CompareAndSwapInt64(&t.monotonicLowerBound, oldLowerBound, now) {
+				break
+			}
+		}
 	}
 	return now, err
 }