Re: [Qemu-devel] [PATCH] virtio: improve virtio devices initialization time

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jan 11, 2018 at 12:16:56PM +0200, Gal Hammer wrote:
> The loading time of a VM is quite significant when its virtio
> devices uses a large amount of virt-queues (e.g. a virtio-serial
> device with max_ports=511). Most of the time is spend in the
> creation of all the required event notifiers (ioeventfd and memory
> regions).
> 
> This patch pack all the changes to the memory regions in a
> single memory transaction.
> 
> Reported-by: Sitong Liu <siliu@redhat.com>
> Reported-by: Xiaoling Gao <xiagao@redhat.com>
> Signed-off-by: Gal Hammer <ghammer@redhat.com>

Nice patch! Any timing numbers to share before/after?

> ---
>  hw/virtio/virtio.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index ad564b0..8d8d93d 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -2574,6 +2574,7 @@ static int virtio_device_start_ioeventfd_impl(VirtIODevice *vdev)
>      VirtioBusState *qbus = VIRTIO_BUS(qdev_get_parent_bus(DEVICE(vdev)));
>      int n, r, err;
>  
> +    memory_region_transaction_begin();
>      for (n = 0; n < VIRTIO_QUEUE_MAX; n++) {
>          VirtQueue *vq = &vdev->vq[n];
>          if (!virtio_queue_get_num(vdev, n)) {
> @@ -2596,6 +2597,7 @@ static int virtio_device_start_ioeventfd_impl(VirtIODevice *vdev)
>          }
>          event_notifier_set(&vq->host_notifier);
>      }
> +    memory_region_transaction_commit();
>      return 0;
>  
>  assign_error:
> @@ -2609,6 +2611,7 @@ assign_error:
>          r = virtio_bus_set_host_notifier(qbus, n, false);
>          assert(r >= 0);
>      }
> +    memory_region_transaction_commit();
>      return err;
>  }
>  
> @@ -2625,6 +2628,7 @@ static void virtio_device_stop_ioeventfd_impl(VirtIODevice *vdev)
>      VirtioBusState *qbus = VIRTIO_BUS(qdev_get_parent_bus(DEVICE(vdev)));
>      int n, r;
>  
> +    memory_region_transaction_begin();
>      for (n = 0; n < VIRTIO_QUEUE_MAX; n++) {
>          VirtQueue *vq = &vdev->vq[n];
>  
> @@ -2632,6 +2636,13 @@ static void virtio_device_stop_ioeventfd_impl(VirtIODevice *vdev)
>              continue;
>          }
>          event_notifier_set_handler(&vq->host_notifier, NULL);
> +    }
> +    memory_region_transaction_commit();
> +
> +    for (n = 0; n < VIRTIO_QUEUE_MAX; n++) {
> +        if (!virtio_queue_get_num(vdev, n)) {
> +            continue;
> +        }
>          r = virtio_bus_set_host_notifier(qbus, n, false);
>          assert(r >= 0);
>      }
> -- 
> 2.7.5