Re: [PATCH RFC v5 07/19] virtio: allow virtio-1 queue layout

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

On Tue, Dec 02, 2014 at 02:00:15PM +0100, Cornelia Huck wrote:
> For virtio-1 devices, we allow a more complex queue layout that doesn't
> require descriptor table and rings on a physically-contigous memory area:
> add virtio_queue_set_rings() to allow transports to set this up.
> 
> Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
> ---
>  hw/virtio/virtio.c         |   16 ++++++++++++++++
>  include/hw/virtio/virtio.h |    2 ++
>  2 files changed, 18 insertions(+)
> 
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index 8f69ffa..508dccf 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -96,6 +96,13 @@ static void virtqueue_init(VirtQueue *vq)
>  {
>      hwaddr pa = vq->pa;
>  
> +    if (pa == -1ULL) {
> +        /*
> +         * This is a virtio-1 style vq that has already been setup
> +         * in virtio_queue_set.
> +         */
> +        return;
> +    }
>      vq->vring.desc = pa;
>      vq->vring.avail = pa + vq->vring.num * sizeof(VRingDesc);
>      vq->vring.used = vring_align(vq->vring.avail +
> @@ -717,6 +724,15 @@ hwaddr virtio_queue_get_addr(VirtIODevice *vdev, int n)
>      return vdev->vq[n].pa;
>  }
>  
> +void virtio_queue_set_rings(VirtIODevice *vdev, int n, hwaddr desc,
> +                            hwaddr avail, hwaddr used)
> +{
> +    vdev->vq[n].pa = -1ULL;
> +    vdev->vq[n].vring.desc = desc;
> +    vdev->vq[n].vring.avail = avail;
> +    vdev->vq[n].vring.used = used;
> +}
> +
>  void virtio_queue_set_num(VirtIODevice *vdev, int n, int num)
>  {
>      /* Don't allow guest to flip queue between existent and

pa == -1ULL tricks look quite ugly.
Can't we set desc/avail/used unconditionally, and drop
the pa value?

> diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
> index 68c40db..80ee313 100644
> --- a/include/hw/virtio/virtio.h
> +++ b/include/hw/virtio/virtio.h
> @@ -224,6 +224,8 @@ void virtio_queue_set_addr(VirtIODevice *vdev, int n, hwaddr addr);
>  hwaddr virtio_queue_get_addr(VirtIODevice *vdev, int n);
>  void virtio_queue_set_num(VirtIODevice *vdev, int n, int num);
>  int virtio_queue_get_num(VirtIODevice *vdev, int n);
> +void virtio_queue_set_rings(VirtIODevice *vdev, int n, hwaddr desc,
> +                            hwaddr avail, hwaddr used);
>  void virtio_queue_set_align(VirtIODevice *vdev, int n, int align);
>  void virtio_queue_notify(VirtIODevice *vdev, int n);
>  uint16_t virtio_queue_vector(VirtIODevice *vdev, int n);
> -- 
> 1.7.9.5