Re: [PATCH v3 4/9] s390: ap: tools to find a queue with a specific APQN

[Tony Krowiak]

On 2/18/19 7:21 AM, Cornelia Huck wrote:
> On Fri, 15 Feb 2019 17:13:21 -0500
> Tony Krowiak <akrowiak@xxxxxxxxxxxxx> wrote:
>
> > On 2/14/19 8:51 AM, Pierre Morel wrote:
> > > We need to find the queue with a specific APQN during the
> > > handling of the interception of the PQAP/AQIC instruction.
> > >
> > > To handle the AP associated device reference count we keep
> > > track of it in the vfio_ap_queue until we put the device.
> > >
> > > Signed-off-by: Pierre Morel <pmorel@xxxxxxxxxxxxx>
> > > ---
> > >    drivers/s390/crypto/vfio_ap_ops.c     | 54 +++++++++++++++++++++++++++++++++++
> > >    drivers/s390/crypto/vfio_ap_private.h |  1 +
> > >    2 files changed, 55 insertions(+)
>
> > > +/**
> > > + * vfio_ap_get_queue: Retrieve a queue with a specific APQN
> > > + * @apqn: The queue APQN
> > > + *
> > > + * Retrieve a queue with a specific APQN from the list of the
> > > + * devices associated to the vfio_ap_driver.
> > > + *
> > > + * The vfio_ap_queue has been already associated with the device
> > > + * during the probe.
> > > + * Store the associated device for reference counting
> > > + *
> > > + * Returns the pointer to the associated vfio_ap_queue
> > > + */
> > > +static  __attribute__((unused))
> > > +	struct vfio_ap_queue *vfio_ap_get_queue(int apqn)
> >
> > I think you should change this signature for the reasons I stated
> > below:
> >
> > struct device *vfio_ap_get_queue_dev(int apqn)
> >
> > > +{
> > > +	struct device *dev;
> > > +	struct vfio_ap_queue *q;
> > > +
> > > +	dev = driver_find_device(&matrix_dev->vfio_ap_drv->driver, NULL, &apqn,
> > > +				 vfio_ap_check_apqn);
> > > +	if (!dev)
> > > +		return NULL;
> > > +	q = dev_get_drvdata(dev);
> > > +	q->dev = dev;
> >
> > Why store the device with the vfio_ap_queue object? Why not just return
> > the device. The caller can get the vfio_ap_queue from the device's
> > driver data. It seems the only reason for the 'dev' field is to
> > temporarily hold a ref to the device so it can be put later. Why not
> > just put the device.

After thinking about this further, I question whether we even need
this function if it is going to return 'struct device *'. In that case,
why not just call driver_find_device() when the device is needed?
If you want to keep the function, then the function needs only one 
statement:

	return driver_find_device(...).

> Having looked at the remainder of the patches, I tend to agree that we
> don't really need the backlink; we walk the driver's list of devices in
> any case IIUC.
>
> We *might* want a mechanism to grab the queue quickly (i.e. without
> walking the list) if there's anything performance sensitive in there;
> but from the patch descriptions, I don't think anything is done in a
> hot path, so it should be fine.