[PATCH 3/3] spapr: Migrate SpaprDrc::unplug_requested

[Greg Kurz <groug@kaod.org>]

Hot unplugging a device is an asynchronous operation. If the guest is
migrated after the event was sent but before it could release the
device with RTAS, the destination QEMU doesn't know about the pending
unplug operation and doesn't actually remove the device when the guest
finally releases it. The device

Migrate SpaprDrc::unplug_requested to fix the inconsistency. This is
done with a subsection that is only sent if an unplug request is
pending. This allows to preserve migration with older guests in the
case of a pending hotplug request. This will cause migration to fail
if the destination can't handle the subsection, but this is better
than ending with an inconsistency.

Signed-off-by: Greg Kurz <groug@kaod.org>
---
 hw/ppc/spapr_drc.c |   27 +++++++++++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)

diff --git a/hw/ppc/spapr_drc.c b/hw/ppc/spapr_drc.c
index d512ac6e1e7f..6f5cab70fc6b 100644
--- a/hw/ppc/spapr_drc.c
+++ b/hw/ppc/spapr_drc.c
@@ -455,6 +455,22 @@ void spapr_drc_reset(SpaprDrc *drc)
     }
 }
 
+static bool spapr_drc_unplug_requested_needed(void *opaque)
+{
+    return spapr_drc_unplug_requested(opaque);
+}
+
+static const VMStateDescription vmstate_spapr_drc_unplug_requested = {
+    .name = "spapr_drc/unplug_requested",
+    .version_id = 1,
+    .minimum_version_id = 1,
+    .needed = spapr_drc_unplug_requested_needed,
+    .fields  = (VMStateField []) {
+        VMSTATE_BOOL(unplug_requested, SpaprDrc),
+        VMSTATE_END_OF_LIST()
+    }
+};
+
 static bool spapr_drc_needed(void *opaque)
 {
     SpaprDrc *drc = (SpaprDrc *)opaque;
@@ -467,8 +483,11 @@ static bool spapr_drc_needed(void *opaque)
     /*
      * We need to migrate the state if it's not equal to the expected
      * long-term state, which is the same as the coldplugged initial
-     * state */
-    return !spapr_drc_device_ready(drc);
+     * state, or if an unplug request is pending.
+     */
+    return
+        spapr_drc_unplug_requested_needed(drc) ||
+        !spapr_drc_device_ready(drc);
 }
 
 static const VMStateDescription vmstate_spapr_drc = {
@@ -479,6 +498,10 @@ static const VMStateDescription vmstate_spapr_drc = {
     .fields  = (VMStateField []) {
         VMSTATE_UINT32(state, SpaprDrc),
         VMSTATE_END_OF_LIST()
+    },
+    .subsections = (const VMStateDescription * []) {
+        &vmstate_spapr_drc_unplug_requested,
+        NULL
     }
 };