The duration of loading non-iterable vmstate accounts for a significant
portion of downtime (starting with the timestamp of source qemu stop and
ending with the timestamp of target qemu start). Most of the time is spent
committing memory region changes repeatedly.

This patch packs all the changes to memory region during the period of
loading non-iterable vmstate in a single memory transaction. With the
increase of devices, this patch will greatly improve the performance.

Here are the test1 results:
test info:
- Host
  - Intel(R) Xeon(R) Platinum 8362 CPU
  - Mellanox Technologies MT28841
- VM
  - 32 CPUs 128GB RAM VM
  - 8 16-queue vhost-net device
  - 16 4-queue vhost-user-blk device.

	time of loading non-iterable vmstate     downtime
before		about 112 ms			  285 ms
after		about 44 ms			  208 ms

In test2, we keep the number of the device the same as test1, reduce the
number of queues per device:

Here are the test2 results:
test info:
- Host
  - Intel(R) Xeon(R) Platinum 8362 CPU
  - Mellanox Technologies MT28841
- VM
  - 32 CPUs 128GB RAM VM
  - 8 1-queue vhost-net device
  - 16 1-queue vhost-user-blk device.

	time of loading non-iterable vmstate     downtime
before		about 65 ms			 about 151 ms

after		about 30 ms			 about 110 ms

In test3, we keep the number of queues per device the same as test1, reduce
the number of devices:

Here are the test3 results:
test info:
- Host
  - Intel(R) Xeon(R) Platinum 8362 CPU
  - Mellanox Technologies MT28841
- VM
  - 32 CPUs 128GB RAM VM
  - 1 16-queue vhost-net device
  - 1 4-queue vhost-user-blk device.

	time of loading non-iterable vmstate     downtime
before		about 24 ms			 about 51 ms
after		about 12 ms			 about 38 ms

As we can see from the test results above, both the number of queues and
the number of devices have a great impact on the time of loading non-iterable
vmstate. The growth of the number of devices and queues will lead to more
mr commits, and the time consumption caused by the flatview reconstruction
will also increase.

Signed-off-by: Chuang Xu <xuchuangxclwt@bytedance.com>
---
 migration/savevm.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/migration/savevm.c b/migration/savevm.c
index b5e6962bb6..3dd9daabd8 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -2770,6 +2770,7 @@ out:
             goto retry;
         }
     }
+
     return ret;
 }
 
@@ -2795,7 +2796,25 @@ int qemu_loadvm_state(QEMUFile *f)
 
     cpu_synchronize_all_pre_loadvm();
 
+    /*
+     * Call memory_region_transaction_begin() before loading vmstate.
+     * This call is paired with memory_region_transaction_commit() at
+     * the end of qemu_loadvm_state_main(), in order to pack all the
+     * changes to memory region during the period of loading
+     * non-iterable vmstate in a single memory transaction.
+     * This operation will reduce time of loading non-iterable vmstate
+     */
+    memory_region_transaction_begin();
+
     ret = qemu_loadvm_state_main(f, mis);
+
+    /*
+     * Call memory_region_transaction_commit() after loading vmstate.
+     * At this point, qemu actually completes all the previous memory
+     * region transactions.
+     */
+    memory_region_transaction_commit();
+
     qemu_event_set(&mis->main_thread_load_event);
 
     trace_qemu_loadvm_state_post_main(ret);
-- 
2.20.1