Re: [PATCH 1/3] vfio-ccw: add capabilities chain

[Farhan Ali <alifm@linux.ibm.com>]

On 11/22/2018 11:54 AM, Cornelia Huck wrote:
> diff --git a/drivers/s390/cio/vfio_ccw_private.h b/drivers/s390/cio/vfio_ccw_private.h
> index 078e46f9623d..a6f9f84526e2 100644
> --- a/drivers/s390/cio/vfio_ccw_private.h
> +++ b/drivers/s390/cio/vfio_ccw_private.h
> @@ -3,9 +3,11 @@
>    * Private stuff for vfio_ccw driver
>    *
>    * Copyright IBM Corp. 2017
> + * Copyright Red Hat, Inc. 2018
>    *
>    * Author(s): Dong Jia Shi<bjsdjshi@linux.vnet.ibm.com>
>    *            Xiao Feng Ren<renxiaof@linux.vnet.ibm.com>
> + *            Cornelia Huck<cohuck@redhat.com>
>    */
>   
>   #ifndef_VFIO_CCW_PRIVATE_H_
> @@ -19,6 +21,38 @@
>   #include "css.h"
>   #include "vfio_ccw_cp.h"
>   
> +#define VFIO_CCW_OFFSET_SHIFT   40
> +#define VFIO_CCW_OFFSET_TO_INDEX(off)	(off >> VFIO_CCW_OFFSET_SHIFT)
> +#define VFIO_CCW_INDEX_TO_OFFSET(index)	((u64)(index) << VFIO_CCW_OFFSET_SHIFT)
> +#define VFIO_CCW_OFFSET_MASK	(((u64)(1) << VFIO_CCW_OFFSET_SHIFT) - 1)
> +

Why is the offset shift 40? I know vfio-pci is also using the same 
offset shift, but I am curious about the reasoning behind why we are 
using this? :)