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

This allows the virtio-blk-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 3m26.186s down to 0m58.023s 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: <20210407143501.244343-3-groug@kaod.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

hw/block/dataplane/virtio-blk.c

@@ -198,19 +198,30 @@ int virtio_blk_data_plane_start(VirtIODevice *vdev)
         goto fail_guest_notifiers;
     }
 
+    memory_region_transaction_begin();
+
     /* Set up virtqueue notify */
     for (i = 0; i < nvqs; i++) {
         r = virtio_bus_set_host_notifier(VIRTIO_BUS(qbus), i, true);
         if (r != 0) {
+            int j = i;
+
             fprintf(stderr, "virtio-blk failed to set host notifier (%d)\n", r);
             while (i--) {
                 virtio_bus_set_host_notifier(VIRTIO_BUS(qbus), i, false);
+            }
+
+            memory_region_transaction_commit();
+
+            while (j--) {
                 virtio_bus_cleanup_host_notifier(VIRTIO_BUS(qbus), i);
             }
             goto fail_host_notifiers;
         }
     }
 
+    memory_region_transaction_commit();
+
     s->starting = false;
     vblk->dataplane_started = true;
     trace_virtio_blk_data_plane_start(s);
@@ -246,8 +257,15 @@ int virtio_blk_data_plane_start(VirtIODevice *vdev)
     return 0;
 
   fail_aio_context:
+    memory_region_transaction_begin();
+
     for (i = 0; i < nvqs; i++) {
         virtio_bus_set_host_notifier(VIRTIO_BUS(qbus), i, false);
+    }
+
+    memory_region_transaction_commit();
+
+    for (i = 0; i < nvqs; i++) {
         virtio_bus_cleanup_host_notifier(VIRTIO_BUS(qbus), i);
     }
   fail_host_notifiers:
@@ -312,8 +330,15 @@ void virtio_blk_data_plane_stop(VirtIODevice *vdev)
 
     aio_context_release(s->ctx);
 
+    memory_region_transaction_begin();
+
     for (i = 0; i < nvqs; i++) {
         virtio_bus_set_host_notifier(VIRTIO_BUS(qbus), i, false);
+    }
+
+    memory_region_transaction_commit();
+
+    for (i = 0; i < nvqs; i++) {
         virtio_bus_cleanup_host_notifier(VIRTIO_BUS(qbus), i);
     }