[PATCH v3 1/6] migration: Prioritize RDMA in ram_save_target_page()

[Li Zhijian]

Address an error in RDMA-based migration by ensuring RDMA is prioritized
when saving pages in `ram_save_target_page()`.

Previously, the RDMA protocol's page-saving step was placed after other
protocols due to a refactoring in commit bc38dc2f5f3. This led to migration
failures characterized by unknown control messages and state loading errors
destination:
(qemu) qemu-system-x86_64: Unknown control message QEMU FILE
qemu-system-x86_64: error while loading state section id 1(ram)
qemu-system-x86_64: load of migration failed: Operation not permitted
source:
(qemu) qemu-system-x86_64: RDMA is in an error state waiting migration to abort!
qemu-system-x86_64: failed to save SaveStateEntry with id(name): 1(ram): -1
qemu-system-x86_64: rdma migration: recv polling control error!
qemu-system-x86_64: warning: Early error. Sending error.
qemu-system-x86_64: warning: rdma migration: send polling control error

RDMA migration implemented its own protocol/method to send pages to
destination side, hand over to RDMA first to prevent pages being saved by
other protocol.

Fixes: bc38dc2f5f3 ("migration: refactor ram_save_target_page functions")
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
---
V3:
   collect Reviewed tags
---
 migration/ram.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/migration/ram.c b/migration/ram.c
index 589b6505eb2..424df6d9f13 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -1964,6 +1964,11 @@ static int ram_save_target_page(RAMState *rs, 
PageSearchStatus *pss)
     ram_addr_t offset = ((ram_addr_t)pss->page) << TARGET_PAGE_BITS;
     int res;
 
+    /* Hand over to RDMA first */
+    if (control_save_page(pss, offset, &res)) {
+        return res;
+    }
+
     if (!migrate_multifd()
         || migrate_zero_page_detection() == ZERO_PAGE_DETECTION_LEGACY) {
         if (save_zero_page(rs, pss, offset)) {
@@ -1976,10 +1981,6 @@ static int ram_save_target_page(RAMState *rs, 
PageSearchStatus *pss)
         return ram_save_multifd_page(block, offset);
     }
 
-    if (control_save_page(pss, offset, &res)) {
-        return res;
-    }
-
     return ram_save_page(rs, pss);
 }
 
-- 
2.44.0