Re: [PATCH v4 18/20] virtio-scsi: implement BlockDevOps->drained_begin()

[Kevin Wolf <kwolf@redhat.com>]

Am 25.04.2023 um 19:27 hat Stefan Hajnoczi geschrieben:
> The virtio-scsi Host Bus Adapter provides access to devices on a SCSI
> bus. Those SCSI devices typically have a BlockBackend. When the
> BlockBackend enters a drained section, the SCSI device must temporarily
> stop submitting new I/O requests.
> 
> Implement this behavior by temporarily stopping virtio-scsi virtqueue
> processing when one of the SCSI devices enters a drained section. The
> new scsi_device_drained_begin() API allows scsi-disk to message the
> virtio-scsi HBA.
> 
> scsi_device_drained_begin() uses a drain counter so that multiple SCSI
> devices can have overlapping drained sections. The HBA only sees one
> pair of .drained_begin/end() calls.
> 
> After this commit, virtio-scsi no longer depends on hw/virtio's
> ioeventfd aio_set_event_notifier(is_external=true). This commit is a
> step towards removing the aio_disable_external() API.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

> @@ -206,9 +208,11 @@ void virtio_scsi_dataplane_stop(VirtIODevice *vdev)
>      }
>      s->dataplane_stopping = true;
>  
> -    aio_context_acquire(s->ctx);
> -    aio_wait_bh_oneshot(s->ctx, virtio_scsi_dataplane_stop_bh, s);
> -    aio_context_release(s->ctx);
> +    if (s->bus.drain_count == 0) {
> +        aio_context_acquire(s->ctx);
> +        aio_wait_bh_oneshot(s->ctx, virtio_scsi_dataplane_stop_bh, s);
> +        aio_context_release(s->ctx);
> +    }

Same question as for virtio-blk: We lose processing the virtqueue one
last time during drain. Is it okay, and if so, why do we need it outside
of drain?

Kevin