migration: Make ram_save_target_page() a pointer

We are going to create a new function for multifd latest in the series.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

migration/ram.c

@@ -452,6 +452,13 @@ void dirty_sync_missed_zero_copy(void)
     ram_counters.dirty_sync_missed_zero_copy++;
 }
 
+struct MigrationOps {
+    int (*ram_save_target_page)(RAMState *rs, PageSearchStatus *pss);
+};
+typedef struct MigrationOps MigrationOps;
+
+MigrationOps *migration_ops;
+
 CompressionStats compression_counters;
 
 struct CompressParam {
@@ -2295,14 +2302,14 @@ static bool save_compress_page(RAMState *rs, PageSearchStatus *pss,
 }
 
 /**
- * ram_save_target_page: save one target page
+ * ram_save_target_page_legacy: save one target page
  *
  * Returns the number of pages written
  *
  * @rs: current RAM state
  * @pss: data about the page we want to send
  */
-static int ram_save_target_page(RAMState *rs, PageSearchStatus *pss)
+static int ram_save_target_page_legacy(RAMState *rs, PageSearchStatus *pss)
 {
     RAMBlock *block = pss->block;
     ram_addr_t offset = ((ram_addr_t)pss->page) << TARGET_PAGE_BITS;
@@ -2428,7 +2435,7 @@ static int ram_save_host_page_urgent(PageSearchStatus *pss)
 
         if (page_dirty) {
             /* Be strict to return code; it must be 1, or what else? */
-            if (ram_save_target_page(rs, pss) != 1) {
+            if (migration_ops->ram_save_target_page(rs, pss) != 1) {
                 error_report_once("%s: ram_save_target_page failed", __func__);
                 ret = -1;
                 goto out;
@@ -2497,7 +2504,7 @@ static int ram_save_host_page(RAMState *rs, PageSearchStatus *pss)
             if (preempt_active) {
                 qemu_mutex_unlock(&rs->bitmap_mutex);
             }
-            tmppages = ram_save_target_page(rs, pss);
+            tmppages = migration_ops->ram_save_target_page(rs, pss);
             if (tmppages >= 0) {
                 pages += tmppages;
                 /*
@@ -2697,6 +2704,8 @@ static void ram_save_cleanup(void *opaque)
     xbzrle_cleanup();
     compress_threads_save_cleanup();
     ram_state_cleanup(rsp);
+    g_free(migration_ops);
+    migration_ops = NULL;
 }
 
 static void ram_state_reset(RAMState *rs)
@@ -3252,6 +3261,8 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
     ram_control_before_iterate(f, RAM_CONTROL_SETUP);
     ram_control_after_iterate(f, RAM_CONTROL_SETUP);
 
+    migration_ops = g_malloc0(sizeof(MigrationOps));
+    migration_ops->ram_save_target_page = ram_save_target_page_legacy;
     ret =  multifd_send_sync_main(f);
     if (ret < 0) {
         return ret;