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

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Date:   Mon, 18 Feb 2019 13:21:11 +0100
From:   Cornelia Huck <cohuck@...hat.com>
To:     Tony Krowiak <akrowiak@...ux.ibm.com>
Cc:     Pierre Morel <pmorel@...ux.ibm.com>, borntraeger@...ibm.com,
        alex.williamson@...hat.com, linux-kernel@...r.kernel.org,
        linux-s390@...r.kernel.org, kvm@...r.kernel.org,
        frankja@...ux.ibm.com, pasic@...ux.ibm.com, david@...hat.com,
        schwidefsky@...ibm.com, heiko.carstens@...ibm.com,
        freude@...ux.ibm.com, mimu@...ux.ibm.com
Subject: Re: [PATCH v3 4/9] s390: ap: tools to find a queue with a specific
 APQN

On Fri, 15 Feb 2019 17:13:21 -0500
Tony Krowiak <akrowiak@...ux.ibm.com> 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@...ux.ibm.com>
> > ---
> >   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.

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.