Implement save_postcopy_prepare(), preparing for the enablement
of both multifd and postcopy.
Signed-off-by: Peter Xu <pet...@redhat.com>
Signed-off-by: Prasad Pandit <p...@fedoraproject.org>
Reviewed-by: Fabiano Rosas <faro...@suse.de>
Message-ID: <20250411114534.3370816-5-ppan...@redhat.com>
Signed-off-by: Fabiano Rosas <faro...@suse.de>
---
migration/ram.c | 37 +++++++++++++++++++++++++++++++++++++
1 file changed, 37 insertions(+)
diff --git a/migration/ram.c b/migration/ram.c
index 1181a99cf6..a749e4a421 100644
--- a/migration/ram.c
+++ b/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,
--
2.48.1
