migration: optimize the downtime

Qemu_savevm_state_cleanup takes about 300ms in my ram migration tests
with a 8U24G vm(20G is really occupied), the main cost comes from
KVM_SET_USER_MEMORY_REGION ioctl when mem.memory_size = 0 in
kvm_set_user_memory_region. In kmod, the main cost is
kvm_zap_obsolete_pages, which traverses the active_mmu_pages list to
zap the unsync sptes.

It can be optimized by delaying memory_global_dirty_log_stop to the next
vm_start.

Changes v2->v3:
 - NULL VMChangeStateHandler if it is deleted and protect the scenario
   of nested invocations of memory_global_dirty_log_start/stop [Paolo]

Changes v1->v2:
 - create a VMChangeStateHandler in memory.c to reduce the coupling [Paolo]

Signed-off-by: Jay Zhou <jianjay.zhou@huawei.com>
Message-Id: <1501237733-2736-1-git-send-email-jianjay.zhou@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 35 insertions, 1 deletions

diff --git a/memory.c b/memory.c
index a7bc70a..c0adc35 100644
--- a/memory.c
+++ b/memory.c
@@ -2357,8 +2357,15 @@ void memory_global_dirty_log_sync(void)
     }
 }
 
+static VMChangeStateEntry *vmstate_change;
+
 void memory_global_dirty_log_start(void)
 {
+    if (vmstate_change) {
+        qemu_del_vm_change_state_handler(vmstate_change);
+        vmstate_change = NULL;
+    }
+
     global_dirty_log = true;
 
     MEMORY_LISTENER_CALL_GLOBAL(log_global_start, Forward);
@@ -2369,7 +2376,7 @@ void memory_global_dirty_log_start(void)
     memory_region_transaction_commit();
 }
 
-void memory_global_dirty_log_stop(void)
+static void memory_global_dirty_log_do_stop(void)
 {
     global_dirty_log = false;
 
@@ -2381,6 +2388,33 @@ void memory_global_dirty_log_stop(void)
     MEMORY_LISTENER_CALL_GLOBAL(log_global_stop, Reverse);
 }
 
+static void memory_vm_change_state_handler(void *opaque, int running,
+                                           RunState state)
+{
+    if (running) {
+        memory_global_dirty_log_do_stop();
+
+        if (vmstate_change) {
+            qemu_del_vm_change_state_handler(vmstate_change);
+            vmstate_change = NULL;
+        }
+    }
+}
+
+void memory_global_dirty_log_stop(void)
+{
+    if (!runstate_is_running()) {
+        if (vmstate_change) {
+            return;
+        }
+        vmstate_change = qemu_add_vm_change_state_handler(
+                                memory_vm_change_state_handler, NULL);
+        return;
+    }
+
+    memory_global_dirty_log_do_stop();
+}
+
 static void listener_add_address_space(MemoryListener *listener,
                                        AddressSpace *as)
 {