[Qemu-devel] [PATCH 1/1] spapr: Do not re-read the clock on pre_save handler on migration

["Maxiwell S. Garcia" <maxiwell@linux.ibm.com>]

This handler was added in the commit:
  42043e4f1241: spapr: clock should count only if vm is running

In a scenario without migration, this pre_save handler is not
triggered, so the 'stop/cont' commands save and restore the clock
in the function 'cpu_ppc_clock_vm_state_change.' The SW clock
in the guest doesn't know about this pause.

If the command 'migrate' is called between 'stop' and 'cont',
the pre_save handler re-read the clock, and the SW clock in the
guest will know about the pause between 'stop' and 'migrate.'
If the guest is running a workload like HTC, a side-effect of
this is a lot of process stall messages (with call traces) in
the kernel guest.

Signed-off-by: Maxiwell S. Garcia <maxiwell@linux.ibm.com>
---
 hw/ppc/ppc.c | 24 ------------------------
 1 file changed, 24 deletions(-)

diff --git a/hw/ppc/ppc.c b/hw/ppc/ppc.c
index ad20584f26..3fb50cbeee 100644
--- a/hw/ppc/ppc.c
+++ b/hw/ppc/ppc.c
@@ -1056,35 +1056,11 @@ void cpu_ppc_clock_vm_state_change(void *opaque, int running,
     }
 }
 
-/*
- * When migrating, read the clock just before migration,
- * so that the guest clock counts during the events
- * between:
- *
- *  * vm_stop()
- *  *
- *  * pre_save()
- *
- *  This reduces clock difference on migration from 5s
- *  to 0.1s (when max_downtime == 5s), because sending the
- *  final pages of memory (which happens between vm_stop()
- *  and pre_save()) takes max_downtime.
- */
-static int timebase_pre_save(void *opaque)
-{
-    PPCTimebase *tb = opaque;
-
-    timebase_save(tb);
-
-    return 0;
-}
-
 const VMStateDescription vmstate_ppc_timebase = {
     .name = "timebase",
     .version_id = 1,
     .minimum_version_id = 1,
     .minimum_version_id_old = 1,
-    .pre_save = timebase_pre_save,
     .fields      = (VMStateField []) {
         VMSTATE_UINT64(guest_timebase, PPCTimebase),
         VMSTATE_INT64(time_of_the_day_ns, PPCTimebase),
-- 
2.20.1