[PATCH v2] migration/postcopy: ensure preempt channel is ready before loading states

[Wei Wang <wei.w.wang@intel.com>]

Before loading the guest states, ensure that the preempt channel has been
ready to use, as some of the states (e.g. via virtio_load) might trigger
page faults that will be handled through the preempt channel. So yield to
the main thread in the case that the channel create event hasn't been
dispatched.

Originally-by: Lei Wang <lei4.wang@intel.com>
Link: https://lore.kernel.org/all/9aa5d1be-7801-40dd-83fd-f7e041ced249@intel.com/T/
Suggested-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Lei Wang <lei4.wang@intel.com>
Signed-off-by: Wei Wang <wei.w.wang@intel.com>
---
 migration/savevm.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/migration/savevm.c b/migration/savevm.c
index 388d7af7cd..63f9991a8a 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -2342,6 +2342,23 @@ static int loadvm_handle_cmd_packaged(MigrationIncomingState *mis)
 
     QEMUFile *packf = qemu_file_new_input(QIO_CHANNEL(bioc));
 
+    /*
+     * Before loading the guest states, ensure that the preempt channel has
+     * been ready to use, as some of the states (e.g. via virtio_load) might
+     * trigger page faults that will be handled through the preempt channel.
+     * So yield to the main thread in the case that the channel create event
+     * hasn't been dispatched.
+     */
+    do {
+        if (!migrate_postcopy_preempt() || !qemu_in_coroutine() ||
+            mis->postcopy_qemufile_dst) {
+            break;
+        }
+
+        aio_co_schedule(qemu_get_current_aio_context(), qemu_coroutine_self());
+        qemu_coroutine_yield();
+    } while (1);
+
     ret = qemu_loadvm_state_main(packf, mis);
     trace_loadvm_handle_cmd_packaged_main(ret);
     qemu_fclose(packf);
-- 
2.27.0