migration/colo: Document qemu_fflush(fb)

COLO caches all device states in a buffer channel `fb'.  Add some comments
explaining the flush, that (1) it's the `fb' not the main channel, (2) on
what it updates.

Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Tested-by: Lukas Straub <lukasstraub2@web.de>
Link: https://lore.kernel.org/qemu-devel/20260127185254.3954634-10-peterx@redhat.com
Signed-off-by: Fabiano Rosas <farosas@suse.de>

diff --git a/migration/colo.c b/migration/colo.c
index f92803dd291daf293fe3203260c9614f723a9805..1b94e0f0ee9fe192f224bacd071d99b75f374344 100644 (file)
--- a/migration/colo.c
+++ b/migration/colo.c
@@ -472,12 +472,14 @@ static int colo_do_checkpoint_transaction(MigrationState *s,
     qemu_savevm_state_complete_precopy_iterable(s->to_dst_file, false);
     qemu_put_byte(s->to_dst_file, QEMU_VM_EOF);
 
-    qemu_fflush(fb);
-
     /*
      * We need the size of the VMstate data in Secondary side,
      * With which we can decide how much data should be read.
+     *
+     * Flush the qemufile cache to make sure both bioc->usage and
+     * bioc->data contains the latest info.
      */
+    qemu_fflush(fb);
     colo_send_message_value(s->to_dst_file, COLO_MESSAGE_VMSTATE_SIZE,
                             bioc->usage, &local_err);
     if (local_err) {