migration: enable multifd and postcopy together

Enable Multifd and Postcopy migration together.
The migration_ioc_process_incoming() routine checks
magic value sent on each channel and helps to properly
setup multifd and postcopy channels.

The Precopy and Multifd threads work during the initial
guest RAM transfer. When migration moves to the Postcopy
phase, the multifd threads cease to send data on multifd
channels and Postcopy threads on the destination
request/pull data from the source side.

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Prasad Pandit <pjp@fedoraproject.org>
Link: https://lore.kernel.org/r/20250512125124.147064-3-ppandit@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>

4 files changed, 11 insertions, 9 deletions

diff --git a/migration/multifd-nocomp.c b/migration/multifd-nocomp.c
index 88fe0f99f2..b48eae3d86 100644
--- a/migration/multifd-nocomp.c
+++ b/migration/multifd-nocomp.c
@@ -17,6 +17,7 @@
 #include "migration-stats.h"
 #include "multifd.h"
 #include "options.h"
+#include "migration.h"
 #include "qapi/error.h"
 #include "qemu/cutils.h"
 #include "qemu/error-report.h"
@@ -398,7 +399,7 @@ int multifd_ram_flush_and_sync(QEMUFile *f)
     MultiFDSyncReq req;
     int ret;
 
-    if (!migrate_multifd()) {
+    if (!migrate_multifd() || migration_in_postcopy()) {
         return 0;
     }
 
diff --git a/migration/multifd.c b/migration/multifd.c
index ec108af624..f18b166bcf 100644
--- a/migration/multifd.c
+++ b/migration/multifd.c
@@ -1379,6 +1379,13 @@ static void *multifd_recv_thread(void *opaque)
         }
 
         if (has_data) {
+            /*
+             * multifd thread should not be active and receive data
+             * when migration is in the Postcopy phase. Two threads
+             * writing the same memory area could easily corrupt
+             * the guest state.
+             */
+            assert(!migration_in_postcopy());
             if (is_device_state) {
                 assert(use_packets);
                 ret = multifd_device_state_recv(p, &local_err);
diff --git a/migration/options.c b/migration/options.c
index b6ae95358d..3fcd577cd7 100644
--- a/migration/options.c
+++ b/migration/options.c
@@ -509,11 +509,6 @@ bool migrate_caps_check(bool *old_caps, bool *new_caps, Error **errp)
             error_setg(errp, "Postcopy is not compatible with ignore-shared");
             return false;
         }
-
-        if (new_caps[MIGRATION_CAPABILITY_MULTIFD]) {
-            error_setg(errp, "Postcopy is not yet compatible with multifd");
-            return false;
-        }
     }
 
     if (new_caps[MIGRATION_CAPABILITY_BACKGROUND_SNAPSHOT]) {
diff --git a/migration/ram.c b/migration/ram.c
index e12913b43e..d26dbd37c4 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -1993,9 +1993,8 @@ static int ram_save_target_page(RAMState *rs, PageSearchStatus *pss)
         }
     }
 
-    if (migrate_multifd()) {
-        RAMBlock *block = pss->block;
-        return ram_save_multifd_page(block, offset);
+    if (migrate_multifd() && !migration_in_postcopy()) {
+        return ram_save_multifd_page(pss->block, offset);
     }
 
     return ram_save_page(rs, pss);