From 2c986870e35f967c88ebc1b7df7b576aad2c46d4 Mon Sep 17 00:00:00 2001
From: Adin Scannell <ascannell@google.com>
Date: Mon, 4 May 2020 10:39:36 -0700
Subject: 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 | 19 +++++++++++++++++++
1 file changed, 19 insertions(+)
(limited to 'pkg/sentry/kernel')
diff --git a/pkg/sentry/kernel/timekeeper.go b/pkg/sentry/kernel/timekeeper.go
index dc99301de..da0ea7bb5 100644
--- a/pkg/sentry/kernel/timekeeper.go
+++ b/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
}
--
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