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, 39 insertions, 0 deletions

diff --git a/migration/savevm.c b/migration/savevm.c
index 4046faf009..faebf47ef5 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -90,6 +90,7 @@ enum qemu_vm_cmd {
     MIG_CMD_ENABLE_COLO,       /* Enable COLO */
     MIG_CMD_POSTCOPY_RESUME,   /* resume postcopy on dest */
     MIG_CMD_RECV_BITMAP,       /* Request for recved bitmap on dst */
+    MIG_CMD_SWITCHOVER_START,  /* Switchover start notification */
     MIG_CMD_MAX
 };
 
@@ -109,6 +110,7 @@ static struct mig_cmd_args {
     [MIG_CMD_POSTCOPY_RESUME]  = { .len =  0, .name = "POSTCOPY_RESUME" },
     [MIG_CMD_PACKAGED]         = { .len =  4, .name = "PACKAGED" },
     [MIG_CMD_RECV_BITMAP]      = { .len = -1, .name = "RECV_BITMAP" },
+    [MIG_CMD_SWITCHOVER_START] = { .len =  0, .name = "SWITCHOVER_START" },
     [MIG_CMD_MAX]              = { .len = -1, .name = "MAX" },
 };
 
@@ -1201,6 +1203,19 @@ void qemu_savevm_send_recv_bitmap(QEMUFile *f, char *block_name)
     qemu_savevm_command_send(f, MIG_CMD_RECV_BITMAP, len + 1, (uint8_t *)buf);
 }
 
+static void qemu_savevm_send_switchover_start(QEMUFile *f)
+{
+    trace_savevm_send_switchover_start();
+    qemu_savevm_command_send(f, MIG_CMD_SWITCHOVER_START, 0, NULL);
+}
+
+void qemu_savevm_maybe_send_switchover_start(QEMUFile *f)
+{
+    if (migrate_send_switchover_start()) {
+        qemu_savevm_send_switchover_start(f);
+    }
+}
+
 bool qemu_savevm_state_blocked(Error **errp)
 {
     SaveStateEntry *se;
@@ -1687,6 +1702,7 @@ static int qemu_savevm_state(QEMUFile *f, Error **errp)
 
     ret = qemu_file_get_error(f);
     if (ret == 0) {
+        qemu_savevm_maybe_send_switchover_start(f);
         qemu_savevm_state_complete_precopy(f, false);
         ret = qemu_file_get_error(f);
     }
@@ -2383,6 +2399,26 @@ static int loadvm_process_enable_colo(MigrationIncomingState *mis)
     return ret;
 }
 
+static int loadvm_postcopy_handle_switchover_start(void)
+{
+    SaveStateEntry *se;
+
+    QTAILQ_FOREACH(se, &savevm_state.handlers, entry) {
+        int ret;
+
+        if (!se->ops || !se->ops->switchover_start) {
+            continue;
+        }
+
+        ret = se->ops->switchover_start(se->opaque);
+        if (ret < 0) {
+            return ret;
+        }
+    }
+
+    return 0;
+}
+
 /*
  * Process an incoming 'QEMU_VM_COMMAND'
  * 0           just a normal return
@@ -2481,6 +2517,9 @@ static int loadvm_process_command(QEMUFile *f)
 
     case MIG_CMD_ENABLE_COLO:
         return loadvm_process_enable_colo(mis);
+
+    case MIG_CMD_SWITCHOVER_START:
+        return loadvm_postcopy_handle_switchover_start();
     }
 
     return 0;