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

Cornelia Huck
2018-11-28T09:05:34Z

On Tue, 27 Nov 2018 14:04:49 -0500
Farhan Ali <alifm@linux.ibm.com> wrote:

> 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? :)
>

My entire reasoning was "hey, vfio-pci is using this, so it should not
be bad" 8)