migration/ram: Implement save_postcopy_prepare()

Implement save_postcopy_prepare(), preparing for the enablement
of both multifd and postcopy.

Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Prasad Pandit <pjp@fedoraproject.org>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Message-ID: <20250411114534.3370816-5-ppandit@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>

migration/ram.c

@@ -4398,6 +4398,42 @@ static int ram_resume_prepare(MigrationState *s, void *opaque)
     return 0;
 }
 
+static bool ram_save_postcopy_prepare(QEMUFile *f, void *opaque, Error **errp)
+{
+    int ret;
+
+    if (migrate_multifd()) {
+        /*
+         * When multifd is enabled, source QEMU needs to make sure all the
+         * pages queued before postcopy starts have been flushed.
+         *
+         * The load of these pages must happen before switching to postcopy.
+         * It's because loading of guest pages (so far) in multifd recv
+         * threads is still non-atomic, so the load cannot happen with vCPUs
+         * running on the destination side.
+         *
+         * This flush and sync will guarantee that those pages are loaded
+         * _before_ postcopy starts on the destination. The rationale is,
+         * this happens before VM stops (and before source QEMU sends all
+         * the rest of the postcopy messages).  So when the destination QEMU
+         * receives the postcopy messages, it must have received the sync
+         * message on the main channel (either RAM_SAVE_FLAG_MULTIFD_FLUSH,
+         * or RAM_SAVE_FLAG_EOS), and such message would guarantee that
+         * all previous guest pages queued in the multifd channels are
+         * completely loaded.
+         */
+        ret = multifd_ram_flush_and_sync(f);
+        if (ret < 0) {
+            error_setg(errp, "%s: multifd flush and sync failed", __func__);
+            return false;
+        }
+    }
+
+    qemu_put_be64(f, RAM_SAVE_FLAG_EOS);
+
+    return true;
+}
+
 void postcopy_preempt_shutdown_file(MigrationState *s)
 {
     qemu_put_be64(s->postcopy_qemufile_src, RAM_SAVE_FLAG_EOS);
@@ -4417,6 +4453,7 @@ static SaveVMHandlers savevm_ram_handlers = {
     .load_setup = ram_load_setup,
     .load_cleanup = ram_load_cleanup,
     .resume_prepare = ram_resume_prepare,
+    .save_postcopy_prepare = ram_save_postcopy_prepare,
 };
 
 static void ram_mig_ram_block_resized(RAMBlockNotifier *n, void *host,