[PATCH v4 3/7] migration/multifd: Implement ram_save_target_page_multifd to handle multifd version of MigrationOps::ram_save_target_page.

[Hao Xiang <hao.xiang@bytedance.com>]

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>
---
 migration/ram.c | 43 ++++++++++++++++++++++++++++++-------------
 1 file changed, 30 insertions(+), 13 deletions(-)

diff --git a/migration/ram.c b/migration/ram.c
index e1fa229acf..f9d6ea65cc 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -1122,10 +1122,6 @@ static int save_zero_page(RAMState *rs, PageSearchStatus *pss,
     QEMUFile *file = pss->pss_channel;
     int len = 0;
 
-    if (migrate_zero_page_detection() == ZERO_PAGE_DETECTION_NONE) {
-        return 0;
-    }
-
     if (!buffer_is_zero(p, TARGET_PAGE_SIZE)) {
         return 0;
     }
@@ -2045,7 +2041,6 @@ 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;
 
@@ -2061,17 +2056,34 @@ 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.
+     * Backward compatibility support. 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 */
@@ -2983,7 +2995,12 @@ 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();
-- 
2.30.2