[Qemu-devel] [PULL v2 13/13] migration: Do not re-read the clock on pre_save in case of paused guest

[Eduardo Habkost <ehabkost@redhat.com>]

From: "Maxiwell S. Garcia" <maxiwell@linux.ibm.com>

The clock move makes the guest knows about the paused time between the
'stop' and 'migrate' commands. This is an issue in an already-paused
VM because some side effects, like process stalls, could happen
after migration.

So, this patch checks the runstate of guest in the pre_save handler and
do not re-reads the clock in case of paused state (cold migration).

Signed-off-by: Maxiwell S. Garcia <maxiwell@linux.ibm.com>
Message-Id: <20190829210711.6570-1-maxiwell@linux.ibm.com>
Reviewed-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
---
 hw/i386/kvm/clock.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/hw/i386/kvm/clock.c b/hw/i386/kvm/clock.c
index 80c133a724..2c59b6894b 100644
--- a/hw/i386/kvm/clock.c
+++ b/hw/i386/kvm/clock.c
@@ -41,6 +41,9 @@ typedef struct KVMClockState {
     uint64_t clock;
     bool clock_valid;
 
+    /* whether the 'clock' value was obtained in the 'paused' state */
+    bool runstate_paused;
+
     /* whether machine type supports reliable KVM_GET_CLOCK */
     bool mach_use_reliable_get_clock;
 
@@ -202,6 +205,8 @@ static void kvmclock_vm_state_change(void *opaque, int running,
             return;
         }
 
+        s->runstate_paused = runstate_check(RUN_STATE_PAUSED);
+
         kvm_synchronize_all_tsc();
 
         kvm_update_clock(s);
@@ -260,9 +265,9 @@ static int kvmclock_pre_load(void *opaque)
 }
 
 /*
- * When migrating, read the clock just before migration,
- * so that the guest clock counts during the events
- * between:
+ * When migrating a running guest, read the clock just
+ * before migration, so that the guest clock counts
+ * during the events between:
  *
  *  * vm_stop()
  *  *
@@ -277,7 +282,9 @@ static int kvmclock_pre_save(void *opaque)
 {
     KVMClockState *s = opaque;
 
-    kvm_update_clock(s);
+    if (!s->runstate_paused) {
+        kvm_update_clock(s);
+    }
 
     return 0;
 }
-- 
2.21.0