[PATCH v9 5/7] migration: enable multifd and postcopy together

[Prasad Pandit <ppandit@redhat.com>]

From: Prasad Pandit <pjp@fedoraproject.org>

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.

Signed-off-by: Prasad Pandit <pjp@fedoraproject.org>
---
 migration/multifd-nocomp.c | 3 ++-
 migration/multifd.c        | 7 +++++++
 migration/options.c        | 5 -----
 migration/ram.c            | 5 ++---
 4 files changed, 11 insertions(+), 9 deletions(-)

v8: remove a !migration_in_postcopy() check
- https://lore.kernel.org/qemu-devel/20250318123846.1370312-1-ppandit@redhat.com/T/#t

diff --git a/migration/multifd-nocomp.c b/migration/multifd-nocomp.c
index ffe75256c9..02f8bf8ce8 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"
@@ -399,7 +400,7 @@ 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 6139cabe44..074d16d07d 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 b0ac2ea408..48aa6076de 100644
--- a/migration/options.c
+++ b/migration/options.c
@@ -491,11 +491,6 @@ 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 753042456e..533c64b941 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -1976,9 +1976,8 @@ 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);
-- 
2.49.0