migration/multifd: Implement ram_save_target_page_multifd to handle multifd version of MigrationOps::ram_save_target_page.

1. Add a dedicated handler for MigrationOps::ram_save_target_page in
multifd live migration.
2. Refactor ram_save_target_page_legacy so that the legacy and multifd
handlers don't have internal functions calling into each other.

Signed-off-by: Hao Xiang <hao.xiang@bytedance.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Message-Id: <20240226195654.934709-4-hao.xiang@bytedance.com>
Link: https://lore.kernel.org/r/20240311180015.3359271-6-hao.xiang@linux.dev
Signed-off-by: Peter Xu <peterx@redhat.com>

migration/ram.c

@@ -2079,7 +2079,6 @@ static bool save_compress_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;
     int res;
 
@@ -2095,17 +2094,33 @@ static int ram_save_target_page_legacy(RAMState *rs, PageSearchStatus *pss)
         return 1;
     }
 
+    return ram_save_page(rs, pss);
+}
+
+/**
+ * ram_save_target_page_multifd: send one target page to multifd workers
+ *
+ * Returns 1 if the page was queued, -1 otherwise.
+ *
+ * @rs: current RAM state
+ * @pss: data about the page we want to send
+ */
+static int ram_save_target_page_multifd(RAMState *rs, PageSearchStatus *pss)
+{
+    RAMBlock *block = pss->block;
+    ram_addr_t offset = ((ram_addr_t)pss->page) << TARGET_PAGE_BITS;
+
     /*
-     * Do not use multifd in postcopy as one whole host page should be
-     * placed.  Meanwhile postcopy requires atomic update of pages, so even
-     * if host page size == guest page size the dest guest during run may
-     * still see partially copied pages which is data corruption.
+     * While using multifd live migration, we still need to handle zero
+     * page checking on the migration main thread.
      */
-    if (migrate_multifd() && !migration_in_postcopy()) {
-        return ram_save_multifd_page(block, offset);
+    if (migrate_zero_page_detection() == ZERO_PAGE_DETECTION_LEGACY) {
+        if (save_zero_page(rs, pss, offset)) {
+            return 1;
+        }
     }
 
-    return ram_save_page(rs, pss);
+    return ram_save_multifd_page(block, offset);
 }
 
 /* Should be called before sending a host page */
@@ -3112,7 +3127,12 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
     }
 
     migration_ops = g_malloc0(sizeof(MigrationOps));
-    migration_ops->ram_save_target_page = ram_save_target_page_legacy;
+
+    if (migrate_multifd()) {
+        migration_ops->ram_save_target_page = ram_save_target_page_multifd;
+    } else {
+        migration_ops->ram_save_target_page = ram_save_target_page_legacy;
+    }
 
     bql_unlock();
     ret = multifd_send_sync_main();