virtio-scsi: Configure all host notifiers in a single MR transaction

This allows the virtio-scsi-pci device to batch the setup of all its
host notifiers. This significantly improves boot time of VMs with a
high number of vCPUs, e.g. from 6m5.563s down to 1m2.884s for a
pseries machine with 384 vCPUs.

Note that memory_region_transaction_commit() must be called before
virtio_bus_cleanup_host_notifier() because the latter might close
ioeventfds that the transaction still assumes to be around when it
commits.

Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <20210407143501.244343-5-groug@kaod.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/scsi/virtio-scsi-dataplane.c

@@ -152,6 +152,8 @@ int virtio_scsi_dataplane_start(VirtIODevice *vdev)
         goto fail_guest_notifiers;
     }
 
+    memory_region_transaction_begin();
+
     rc = virtio_scsi_set_host_notifier(s, vs->ctrl_vq, 0);
     if (rc != 0) {
         goto fail_host_notifiers;
@@ -173,6 +175,8 @@ int virtio_scsi_dataplane_start(VirtIODevice *vdev)
         vq_init_count++;
     }
 
+    memory_region_transaction_commit();
+
     aio_context_acquire(s->ctx);
     virtio_queue_aio_set_host_notifier_handler(vs->ctrl_vq, s->ctx,
                                             virtio_scsi_data_plane_handle_ctrl);
@@ -192,6 +196,11 @@ int virtio_scsi_dataplane_start(VirtIODevice *vdev)
 fail_host_notifiers:
     for (i = 0; i < vq_init_count; i++) {
         virtio_bus_set_host_notifier(VIRTIO_BUS(qbus), i, false);
+    }
+
+    memory_region_transaction_commit();
+
+    for (i = 0; i < vq_init_count; i++) {
         virtio_bus_cleanup_host_notifier(VIRTIO_BUS(qbus), i);
     }
     k->set_guest_notifiers(qbus->parent, vs->conf.num_queues + 2, false);
@@ -229,8 +238,15 @@ void virtio_scsi_dataplane_stop(VirtIODevice *vdev)
 
     blk_drain_all(); /* ensure there are no in-flight requests */
 
+    memory_region_transaction_begin();
+
     for (i = 0; i < vs->conf.num_queues + 2; i++) {
         virtio_bus_set_host_notifier(VIRTIO_BUS(qbus), i, false);
+    }
+
+    memory_region_transaction_commit();
+
+    for (i = 0; i < vs->conf.num_queues + 2; i++) {
         virtio_bus_cleanup_host_notifier(VIRTIO_BUS(qbus), i);
     }