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

Alexander Graf
2012-10-30T14:05:23Z

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 ;)

Alex