migration: Implement switchover ack logic

Implement switchover ack logic. This prevents the source from stopping
the VM and completing the migration until an ACK is received from the
destination that it's OK to do so.

To achieve this, a new SaveVMHandlers handler switchover_ack_needed()
and a new return path message MIG_RP_MSG_SWITCHOVER_ACK are added.

The switchover_ack_needed() handler is called during migration setup in
the destination to check if switchover ack is used by the migrated
device.

When switchover is approved by all migrated devices in the destination
that support this capability, the MIG_RP_MSG_SWITCHOVER_ACK return path
message is sent to the source to notify it that it's OK to do
switchover.

Signed-off-by: Avihai Horon <avihaih@nvidia.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Tested-by: YangHang Liu <yanghliu@redhat.com>
Acked-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/migration/migration.h b/migration/migration.h
index 30c3e97635..c859a0d35e 100644
--- a/migration/migration.h
+++ b/migration/migration.h
@@ -210,6 +210,13 @@ struct MigrationIncomingState {
      * contains valid information.
      */
     QemuMutex page_request_mutex;
+
+    /*
+     * Number of devices that have yet to approve switchover. When this reaches
+     * zero an ACK that it's OK to do switchover is sent to the source. No lock
+     * is needed as this field is updated serially.
+     */
+    unsigned int switchover_ack_pending_num;
 };
 
 MigrationIncomingState *migration_incoming_get_current(void);
@@ -440,6 +447,12 @@ struct MigrationState {
 
     /* QEMU_VM_VMDESCRIPTION content filled for all non-iterable devices. */
     JSONWriter *vmdesc;
+
+    /*
+     * Indicates whether an ACK from the destination that it's OK to do
+     * switchover has been received.
+     */
+    bool switchover_acked;
 };
 
 void migrate_set_state(int *state, int old_state, int new_state);
@@ -480,6 +493,7 @@ int migrate_send_rp_message_req_pages(MigrationIncomingState *mis,
 void migrate_send_rp_recv_bitmap(MigrationIncomingState *mis,
                                  char *block_name);
 void migrate_send_rp_resume_ack(MigrationIncomingState *mis, uint32_t value);
+int migrate_send_rp_switchover_ack(MigrationIncomingState *mis);
 
 void dirty_bitmap_mig_before_vm_start(void);
 void dirty_bitmap_mig_cancel_outgoing(void);