Re: [PATCH 4/5] KVM: s390: Add a channel I/O based virtio transport driver.

Cornelia Huck
2012-10-30T14:35:50Z

On Tue, 30 Oct 2012 15:05:23 +0100
Alexander Graf <agraf@suse.de> wrote:

>
>
> On 30.10.2012, at 15:00, Cornelia Huck <cornelia.huck@de.ibm.com> wrote:
>
> > On Tue, 30 Oct 2012 14:41:46 +0100
> > Alexander Graf <agraf@suse.de> wrote:
> >
> >> On 10/30/2012 02:03 PM, Cornelia Huck wrote:
> >>> On Mon, 29 Oct 2012 19:37:10 +0100
> >>> Alexander Graf<agraf@suse.de> wrote:
> >>>
> >>>> On 29.10.2012, at 19:34, Cornelia Huck wrote:
> >>>>
> >>>>> On Mon, 29 Oct 2012 19:12:54 +0100
> >>>>> Alexander Graf<agraf@suse.de> wrote:
> >>>>>
> >>>>>> On 29.10.2012, at 14:07, Cornelia Huck wrote:
> >>>>>>> +static void virtio_ccw_kvm_notify(struct virtqueue *vq)
> >>>>>>> +{
> >>>>>>> + struct virtio_ccw_vq_info *info = vq->priv;
> >>>>>>> + struct virtio_ccw_device *vcdev;
> >>>>>>> + struct subchannel_id schid;
> >>>>>>> + __u32 reg2;
> >>>>>>> +
> >>>>>>> + vcdev = to_vc_device(info->vq->vdev);
> >>>>>>> + ccw_device_get_schid(vcdev->cdev,&schid);
> >>>>>>> + reg2 = *(__u32 *)&schid;
> >>>>>> That cast looks quite ugly. Can't you just access the field in there you need? Or if it's multiple fields do a union over them? Or assemble them by hand in C?
> >>>>> I think the cast looks less ugly than using a union to morph it around.
> >>>>> I want the schid with all fields filled out anyway, since this is what
> >>>>> identifies the subchannel.
> >>>> How about a helper function that returns a u32 for a struct subchannel_id in arch/s390/include/asm/schid.h then?
> >>> This would just move the cast around, no? I don't think that would
> >>> improve readability.
> >>
> >> It would take it from the user of that struct close to the definition of
> >> the struct. The fact that it's an ugly cast by then is an implementation
> >> detail. This way it's a public API.
> >
> > But API-wise, returning a struct subchannel_id is cleaner - we don't
> > want "u32 associated with that device", but "subchannel identifier for
> > that device". The fact that we want to stuff it into an integer for the
> > hypercall is the implementaton detail.
>
> The subchannel id is not an implementation detail ;)

And that's why I think it should stay the way it is :)