migration: Fix a potential issue

At the end of live migration and before vm_start() on the destination
side, we should make sure all the decompression tasks are finished, if
this can not be guaranteed, the VM may get the incorrect memory data,
or the updated memory may be overwritten by the decompression thread.
Add the code to fix this potential issue.
Suggested-by: NDavid Alan Gilbert <dgilbert@redhat.com>
Suggested-by: NJuan Quintela <quintela@redhat.com>
Signed-off-by: NLiang Li <liang.z.li@intel.com>
Message-Id: <1462433579-13691-3-git-send-email-liang.z.li@intel.com>
Signed-off-by: NAmit Shah <amit.shah@redhat.com>

migration/ram.c

@@ -2223,6 +2223,24 @@ static void *do_data_decompress(void *opaque)
     return NULL;
 }
 
+static void wait_for_decompress_done(void)
+{
+    int idx, thread_count;
+
+    if (!migrate_use_compression()) {
+        return;
+    }
+
+    thread_count = migrate_decompress_threads();
+    qemu_mutex_lock(&decomp_done_lock);
+    for (idx = 0; idx < thread_count; idx++) {
+        while (!decomp_param[idx].done) {
+            qemu_cond_wait(&decomp_done_cond, &decomp_done_lock);
+        }
+    }
+    qemu_mutex_unlock(&decomp_done_lock);
+}
+
 void migrate_decompress_threads_create(void)
 {
     int i, thread_count;
@@ -2557,6 +2575,7 @@ static int ram_load(QEMUFile *f, void *opaque, int version_id)
         }
     }
 
+    wait_for_decompress_done();
     rcu_read_unlock();
     DPRINTF("Completed load of VM with exit code %d seq iteration "
             "%" PRIu64 "\n", ret, seq_iter);