Re: [PATCH v4 17/20] virtio-blk: implement BlockDevOps->drained_begin()

[Kevin Wolf <kwolf@redhat.com>]

Am 25.04.2023 um 19:27 hat Stefan Hajnoczi geschrieben:
> Detach ioeventfds during drained sections to stop I/O submission from
> the guest. virtio-blk is no longer reliant on aio_disable_external()
> after this patch. This will allow us to remove the
> aio_disable_external() API once all other code that relies on it is
> converted.
> 
> Take extra care to avoid attaching/detaching ioeventfds if the data
> plane is started/stopped during a drained section. This should be rare,
> but maybe the mirror block job can trigger it.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  hw/block/dataplane/virtio-blk.c | 17 +++++++++------
>  hw/block/virtio-blk.c           | 38 ++++++++++++++++++++++++++++++++-
>  2 files changed, 48 insertions(+), 7 deletions(-)
> 
> diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
> index bd7cc6e76b..d77fc6028c 100644
> --- a/hw/block/dataplane/virtio-blk.c
> +++ b/hw/block/dataplane/virtio-blk.c
> @@ -245,13 +245,15 @@ int virtio_blk_data_plane_start(VirtIODevice *vdev)
>      }
>  
>      /* Get this show started by hooking up our callbacks */
> -    aio_context_acquire(s->ctx);
> -    for (i = 0; i < nvqs; i++) {
> -        VirtQueue *vq = virtio_get_queue(s->vdev, i);
> +    if (!blk_in_drain(s->conf->conf.blk)) {
> +        aio_context_acquire(s->ctx);
> +        for (i = 0; i < nvqs; i++) {
> +            VirtQueue *vq = virtio_get_queue(s->vdev, i);
>  
> -        virtio_queue_aio_attach_host_notifier(vq, s->ctx);
> +            virtio_queue_aio_attach_host_notifier(vq, s->ctx);
> +        }
> +        aio_context_release(s->ctx);
>      }
> -    aio_context_release(s->ctx);
>      return 0;
>  
>    fail_aio_context:
> @@ -317,7 +319,10 @@ void virtio_blk_data_plane_stop(VirtIODevice *vdev)
>      trace_virtio_blk_data_plane_stop(s);
>  
>      aio_context_acquire(s->ctx);
> -    aio_wait_bh_oneshot(s->ctx, virtio_blk_data_plane_stop_bh, s);
> +
> +    if (!blk_in_drain(s->conf->conf.blk)) {
> +        aio_wait_bh_oneshot(s->ctx, virtio_blk_data_plane_stop_bh, s);
> +    }

So here we actually get a semantic change: What you described as the
second part in the previous patch, processing the virtqueue one last
time, isn't done any more if the device is drained.

If it's okay to just skip this during drain, why do we need to do it
outside of drain?

Kevin