migration: fix parsing snapshots with x-ignore-shared flag

Snapshots made with mapped-ram and x-ignore-shared flags are
not parsed properly.

The ignore-shared feature adds and extra field in the stream, which
needs to be consumed on the destination side. Even though mapped-ram has
a fixed header format, the ignore-shared is part of the "generic" stream
infomation so the mapped-ram code is currently skipping that be64 read
which incorrectly offsets every subsequent read from the stream.

The current ignore-shared handling can simply be moved earlier in the code
to encompass mapped-ram as well since the ignore-shared doubleword is the
first one read when parsing the ramblock section of the stream.

Co-authored-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Pawel Zmarzly <pzmarzly0@gmail.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20251126121233.542473-1-pzmarzly0@gmail.com
[peterx: enhance commit log per fabiano]
Signed-off-by: Peter Xu <peterx@redhat.com>

migration/ram.c

@@ -4205,6 +4205,17 @@ static int parse_ramblock(QEMUFile *f, RAMBlock *block, ram_addr_t length)
 
     assert(block);
 
+    if (migrate_ignore_shared()) {
+        hwaddr addr = qemu_get_be64(f);
+        if (migrate_ram_is_ignored(block) &&
+            block->mr->addr != addr) {
+            error_report("Mismatched GPAs for block %s "
+                         "%" PRId64 "!= %" PRId64, block->idstr,
+                         (uint64_t)addr, (uint64_t)block->mr->addr);
+            return -EINVAL;
+        }
+    }
+
     if (migrate_mapped_ram()) {
         parse_ramblock_mapped_ram(f, block, length, &local_err);
         if (local_err) {
@@ -4244,16 +4255,6 @@ static int parse_ramblock(QEMUFile *f, RAMBlock *block, ram_addr_t length)
             return -EINVAL;
         }
     }
-    if (migrate_ignore_shared()) {
-        hwaddr addr = qemu_get_be64(f);
-        if (migrate_ram_is_ignored(block) &&
-            block->mr->addr != addr) {
-            error_report("Mismatched GPAs for block %s "
-                         "%" PRId64 "!= %" PRId64, block->idstr,
-                         (uint64_t)addr, (uint64_t)block->mr->addr);
-            return -EINVAL;
-        }
-    }
     ret = rdma_block_notification_handle(f, block->idstr);
     if (ret < 0) {
         qemu_file_set_error(f, ret);

tests/qtest/migration/file-tests.c

@@ -303,6 +303,22 @@ static void migration_test_add_file_smoke(MigrationTestEnv *env)
                        test_multifd_file_mapped_ram_dio);
 }
 
+static void test_precopy_file_mapped_ram_ignore_shared(void)
+{
+    g_autofree char *uri = g_strdup_printf("file:%s/%s", tmpfs,
+                                           FILE_TEST_FILENAME);
+    MigrateCommon args = {
+        .connect_uri = uri,
+        .listen_uri = "defer",
+        .start = {
+            .caps[MIGRATION_CAPABILITY_MAPPED_RAM] = true,
+            .caps[MIGRATION_CAPABILITY_X_IGNORE_SHARED] = true,
+        },
+    };
+
+    test_file_common(&args, true);
+}
+
 void migration_test_add_file(MigrationTestEnv *env)
 {
     tmpfs = env->tmpfs;
@@ -329,6 +345,8 @@ void migration_test_add_file(MigrationTestEnv *env)
 
     migration_test_add("/migration/multifd/file/mapped-ram",
                        test_multifd_file_mapped_ram);
+    migration_test_add("/migration/multifd/file/mapped-ram/ignore-shared",
+                       test_precopy_file_mapped_ram_ignore_shared);
     migration_test_add("/migration/multifd/file/mapped-ram/live",
                        test_multifd_file_mapped_ram_live);