migration: Add MIG_CMD_SWITCHOVER_START and its load handler

This QEMU_VM_COMMAND sub-command and its switchover_start SaveVMHandler is
used to mark the switchover point in main migration stream.

It can be used to inform the destination that all pre-switchover main
migration stream data has been sent/received so it can start to process
post-switchover data that it might have received via other migration
channels like the multifd ones.

Add also the relevant MigrationState bit stream compatibility property and
its hw_compat entry.

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Zhang Chen <zhangckid@gmail.com> # for the COLO part
Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Link: https://lore.kernel.org/qemu-devel/311be6da85fc7e49a7598684d80aa631778dcbce.1741124640.git.maciej.szmigiero@oracle.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/migration/colo.c b/migration/colo.c
index 9a8e5fbe9b..c976b3ff34 100644
--- a/migration/colo.c
+++ b/migration/colo.c
@@ -452,6 +452,9 @@ static int colo_do_checkpoint_transaction(MigrationState *s,
         bql_unlock();
         goto out;
     }
+
+    qemu_savevm_maybe_send_switchover_start(s->to_dst_file);
+
     /* Note: device state is saved into buffer */
     ret = qemu_save_device_state(fb);