Re: [Qemu-devel] [PATCH 2/2] virtio-blk: use aio handler for data plane

[Paolo Bonzini <pbonzini@redhat.com>]

On 29/03/2016 16:09, Michael S. Tsirkin wrote:
>> > Another small comment, this can be written simply as
>> > 
>> >     if (s->dataplane) {
>> >         virtio_blk_data_plane_start(s->dataplane);
> 
> True, it's best not to poke at dataplane_started.
> 
> >     } else {
> >         virtio_blk_handle_vq(s, vq);
> >     }
> > 
> 
> I prefer the return style I think, to stress the
> fact that this is an unusual, unexpected case.

We're getting dangerously close to personal preference, but I've noticed
virtio-scsi that you get a very common pattern of:

void virtio_scsi_handle_ctrl_vq(VirtIOSCSI *s, VirtQueue *vq)
{
    ... virtqueue_pop ...
}

static void virtio_scsi_data_plane_handle_ctrl(VirtIODevice *vdev,
                                               VirtQueue *vq)
{
    VirtIOSCSI *s = VIRTIO_SCSI(vdev);

    assert(dataplane active and started);
    virtio_scsi_handle_ctrl_vq(s, vq);
}


static void virtio_scsi_handle_ctrl(VirtIODevice *vdev, VirtQueue *vq)
{
    VirtIOSCSI *s = VIRTIO_SCSI(vdev);

    if (dataplane active) {
        virtio_scsi_dataplane_start(s);
    } else {
        virtio_scsi_handle_ctrl_vq(s, vq);
    }
}

so it's not really an unusual, unexpected case but a complete
separation between the dataplane case (handle_output starts
dataplane) and the non-dataplane case (handle_output just does
a cast and calls the actual workhorse).

Paolo