[Qemu-devel] [PATCH] kvmclock: fix clock_is_reliable on migration from QEMU < 2.9

[Michael Chapman <mike@very.puzzling.org>]

When migrating from a pre-2.9 QEMU, no clock_is_reliable flag is
transferred. We should assume that the source host has an unreliable
KVM_GET_CLOCK, rather than using whatever was determined locally, to
ensure that any drift from the TSC-based value calculated by the guest
is corrected.

Signed-off-by: Michael Chapman <mike@very.puzzling.org>
---
 hw/i386/kvm/clock.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/hw/i386/kvm/clock.c b/hw/i386/kvm/clock.c
index 1707434db3..7dac319403 100644
--- a/hw/i386/kvm/clock.c
+++ b/hw/i386/kvm/clock.c
@@ -241,6 +241,19 @@ static const VMStateDescription kvmclock_reliable_get_clock = {
     }
 };
 
+/*
+ * When migrating, assume the source has an unreliable
+ * KVM_GET_CLOCK unless told otherwise.
+ */
+static int kvmclock_pre_load(void *opaque)
+{
+    KVMClockState *s = opaque;
+
+    s->clock_is_reliable = false;
+
+    return 0;
+}
+
 /*
  * When migrating, read the clock just before migration,
  * so that the guest clock counts during the events
@@ -268,6 +281,7 @@ static const VMStateDescription kvmclock_vmsd = {
     .name = "kvmclock",
     .version_id = 1,
     .minimum_version_id = 1,
+    .pre_load = kvmclock_pre_load,
     .pre_save = kvmclock_pre_save,
     .fields = (VMStateField[]) {
         VMSTATE_UINT64(clock, KVMClockState),
-- 
2.14.3