COLO: Establish a new communicating path for COLO

This new communication path will be used for returning messages
from Secondary side to Primary side.
Signed-off-by: Nzhanghailiang <zhang.zhanghailiang@huawei.com>
Signed-off-by: NLi Zhijian <lizhijian@cn.fujitsu.com>
Reviewed-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: NAmit Shah <amit.shah@redhat.com>
Signed-off-by: NAmit Shah <amit@amitshah.net>

migration/colo.c

@@ -14,6 +14,7 @@
 #include "sysemu/sysemu.h"
 #include "migration/colo.h"
 #include "trace.h"
+#include "qemu/error-report.h"
 
 bool colo_supported(void)
 {
@@ -36,6 +37,12 @@ bool migration_incoming_in_colo_state(void)
 
 static void colo_process_checkpoint(MigrationState *s)
 {
+    s->rp_state.from_dst_file = qemu_file_get_return_path(s->to_dst_file);
+    if (!s->rp_state.from_dst_file) {
+        error_report("Open QEMUFile from_dst_file failed");
+        goto out;
+    }
+
     qemu_mutex_lock_iothread();
     vm_start();
     qemu_mutex_unlock_iothread();
@@ -43,6 +50,10 @@ static void colo_process_checkpoint(MigrationState *s)
 
     /* TODO: COLO checkpoint savevm loop */
 
+out:
+    if (s->rp_state.from_dst_file) {
+        qemu_fclose(s->rp_state.from_dst_file);
+    }
 }
 
 void migrate_start_colo_process(MigrationState *s)
@@ -61,8 +72,25 @@ void *colo_process_incoming_thread(void *opaque)
     migrate_set_state(&mis->state, MIGRATION_STATUS_ACTIVE,
                       MIGRATION_STATUS_COLO);
 
+    mis->to_src_file = qemu_file_get_return_path(mis->from_src_file);
+    if (!mis->to_src_file) {
+        error_report("COLO incoming thread: Open QEMUFile to_src_file failed");
+        goto out;
+    }
+    /*
+     * Note: the communication between Primary side and Secondary side
+     * should be sequential, we set the fd to unblocked in migration incoming
+     * coroutine, and here we are in the COLO incoming thread, so it is ok to
+     * set the fd back to blocked.
+     */
+    qemu_file_set_blocking(mis->from_src_file, true);
+
     /* TODO: COLO checkpoint restore loop */
 
+out:
+    if (mis->to_src_file) {
+        qemu_fclose(mis->to_src_file);
+    }
     migration_incoming_exit_colo();
 
     return NULL;