1 files changed, 24 insertions, 0 deletions

diff --git a/arch/x86/kernel/pvclock.c b/arch/x86/kernel/pvclock.c
index f7fdd56bc0ab..f5bc40e1697e 100644
--- a/arch/x86/kernel/pvclock.c
+++ b/arch/x86/kernel/pvclock.c
@@ -118,11 +118,14 @@ unsigned long pvclock_tsc_khz(struct pvclock_vcpu_time_info *src)
         return pv_tsc_khz;
 }
 
+static atomic64_t last_value = ATOMIC64_INIT(0);
+
 cycle_t pvclock_clocksource_read(struct pvclock_vcpu_time_info *src)
 {
         struct pvclock_shadow_time shadow;
         unsigned version;
         cycle_t ret, offset;
+        u64 last;
 
         do {
                 version = pvclock_get_time_values(&shadow, src);
@@ -132,6 +135,27 @@ cycle_t pvclock_clocksource_read(struct pvclock_vcpu_time_info *src)
                 barrier();
         } while (version != src->version);
 
+        /*
+         * Assumption here is that last_value, a global accumulator, always goes
+         * forward. If we are less than that, we should not be much smaller.
+         * We assume there is an error marging we're inside, and then the correction
+         * does not sacrifice accuracy.
+         *
+         * For reads: global may have changed between test and return,
+         * but this means someone else updated poked the clock at a later time.
+         * We just need to make sure we are not seeing a backwards event.
+         *
+         * For updates: last_value = ret is not enough, since two vcpus could be
+         * updating at the same time, and one of them could be slightly behind,
+         * making the assumption that last_value always go forward fail to hold.
+         */
+        last = atomic64_read(&last_value);
+        do {
+                if (ret < last)
+                        return last;
+                last = atomic64_cmpxchg(&last_value, last, ret);
+        } while (unlikely(last != ret));
+
         return ret;
 }