[PATCH v3 2/3] vhost: configure all host notifiers in a single MR transaction

[Longpeng(Mike)]

From: Longpeng <longpeng2@huawei.com>

This allows the vhost device to batch the setup of all its host notifiers.
This significantly reduces the device starting time, e.g. the time spend
on enabling notifiers reduce from 376ms to 9.1ms for a VM with 64 vCPUs
and 3 vhost-vDPA generic devices (vdpa_sim_blk, 64vq per device)

Signed-off-by: Longpeng <longpeng2@huawei.com>
---
 hw/virtio/vhost.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
index 5994559da8..064d4abe5c 100644
--- a/hw/virtio/vhost.c
+++ b/hw/virtio/vhost.c
@@ -1562,16 +1562,25 @@ int vhost_dev_enable_notifiers(struct vhost_dev *hdev, 
VirtIODevice *vdev)
         return r;
     }
 
+    /*
+     * Batch all the host notifiers in a single transaction to avoid
+     * quadratic time complexity in address_space_update_ioeventfds().
+     */
+    memory_region_transaction_begin();
+
     for (i = 0; i < hdev->nvqs; ++i) {
         r = virtio_bus_set_host_notifier(VIRTIO_BUS(qbus), hdev->vq_index + i,
                                          true);
         if (r < 0) {
             error_report("vhost VQ %d notifier binding failed: %d", i, -r);
+            memory_region_transaction_commit();
             vhost_dev_disable_notifiers(hdev, vdev);
             return r;
         }
     }
 
+    memory_region_transaction_commit();
+
     return 0;
 }
 
@@ -1585,6 +1594,12 @@ void vhost_dev_disable_notifiers(struct vhost_dev *hdev, 
VirtIODevice *vdev)
     BusState *qbus = BUS(qdev_get_parent_bus(DEVICE(vdev)));
     int i, r;
 
+    /*
+     * Batch all the host notifiers in a single transaction to avoid
+     * quadratic time complexity in address_space_update_ioeventfds().
+     */
+    memory_region_transaction_begin();
+
     for (i = 0; i < hdev->nvqs; ++i) {
         r = virtio_bus_set_host_notifier(VIRTIO_BUS(qbus), hdev->vq_index + i,
                                          false);
@@ -1592,6 +1607,15 @@ void vhost_dev_disable_notifiers(struct vhost_dev *hdev, 
VirtIODevice *vdev)
             error_report("vhost VQ %d notifier cleanup failed: %d", i, -r);
         }
         assert (r >= 0);
+    }
+
+    /*
+     * The transaction expects the ioeventfds to be open when it
+     * commits. Do it now, before the cleanup loop.
+     */
+    memory_region_transaction_commit();
+
+    for (i = 0; i < hdev->nvqs; ++i) {
         virtio_bus_cleanup_host_notifier(VIRTIO_BUS(qbus), hdev->vq_index + i);
     }
     virtio_device_release_ioeventfd(vdev);
-- 
2.23.0