Re: [PATCH v3 1/2] vhost: Change the sequence of device start

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

On Mon, Oct 17, 2022 at 02:44:51PM +0800, Yajun Wu wrote:
> This patch is part of adding vhost-user vhost_dev_start support. The
> motivation is to improve backend configuration speed and reduce live
> migration VM downtime.
> 
> Moving the device start routines after finishing all the necessary device
> and VQ configuration, further aligning to the virtio specification for
> "device initialization sequence".
> 
> Following patch will add vhost-user vhost_dev_start support.
> 
> Signed-off-by: Yajun Wu <yajunw@nvidia.com>
> Acked-by: Parav Pandit <parav@nvidia.com>
> 
> ---
>  hw/block/vhost-user-blk.c | 18 +++++++++++-------
>  hw/net/vhost_net.c        | 12 ++++++------
>  2 files changed, 17 insertions(+), 13 deletions(-)
> 
> diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
> index 84902dde17..f4deb8cd5d 100644
> --- a/hw/block/vhost-user-blk.c
> +++ b/hw/block/vhost-user-blk.c
> @@ -164,13 +164,6 @@ static int vhost_user_blk_start(VirtIODevice *vdev, Error **errp)
>          goto err_guest_notifiers;
>      }
>  
> -    ret = vhost_dev_start(&s->dev, vdev);
> -    if (ret < 0) {
> -        error_setg_errno(errp, -ret, "Error starting vhost");
> -        goto err_guest_notifiers;
> -    }
> -    s->started_vu = true;
> -
>      /* guest_notifier_mask/pending not used yet, so just unmask
>       * everything here. virtio-pci will do the right thing by
>       * enabling/disabling irqfd.
> @@ -179,9 +172,20 @@ static int vhost_user_blk_start(VirtIODevice *vdev, Error **errp)
>          vhost_virtqueue_mask(&s->dev, vdev, i, false);
>      }
>  
> +    s->dev.vq_index_end = s->dev.nvqs;
> +    ret = vhost_dev_start(&s->dev, vdev);
> +    if (ret < 0) {
> +        error_setg_errno(errp, -ret, "Error starting vhost");
> +        goto err_guest_notifiers;
> +    }
> +    s->started_vu = true;
> +
>      return ret;
>  
>  err_guest_notifiers:
> +    for (i = 0; i < s->dev.nvqs; i++) {
> +        vhost_virtqueue_mask(&s->dev, vdev, i, true);
> +    }
>      k->set_guest_notifiers(qbus->parent, s->dev.nvqs, false);
>  err_host_notifiers:
>      vhost_dev_disable_notifiers(&s->dev, vdev);
> diff --git a/hw/net/vhost_net.c b/hw/net/vhost_net.c
> index d28f8b974b..d6924f5e57 100644
> --- a/hw/net/vhost_net.c
> +++ b/hw/net/vhost_net.c
> @@ -387,21 +387,21 @@ int vhost_net_start(VirtIODevice *dev, NetClientState *ncs,
>          } else {
>              peer = qemu_get_peer(ncs, n->max_queue_pairs);
>          }
> -        r = vhost_net_start_one(get_vhost_net(peer), dev);
> -
> -        if (r < 0) {
> -            goto err_start;
> -        }
>  
>          if (peer->vring_enable) {
>              /* restore vring enable state */
>              r = vhost_set_vring_enable(peer, peer->vring_enable);
>  
>              if (r < 0) {
> -                vhost_net_stop_one(get_vhost_net(peer), dev);
>                  goto err_start;
>              }
>          }
> +
> +        r = vhost_net_start_one(get_vhost_net(peer), dev);
> +        if (r < 0) {
> +            vhost_net_stop_one(get_vhost_net(peer), dev);

Error handling broken here. Corrupts memory if triggered.
I fixed it up when applying just because of the freeze
but I won't do this generally.

> +            goto err_start;
> +        }
>      }
>  
>      return 0;
> -- 
> 2.27.0