linux-kernel - Re: [PATCH v7 04/15] s390/vfio-ap: implement in-use callback for vfio

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Date:   Fri, 24 Apr 2020 05:13:15 +0200
From:   Halil Pasic <pasic@...ux.ibm.com>
To:     Tony Krowiak <akrowiak@...ux.ibm.com>
Cc:     Cornelia Huck <cohuck@...hat.com>, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        freude@...ux.ibm.com, borntraeger@...ibm.com,
        mjrosato@...ux.ibm.com, pmorel@...ux.ibm.com,
        alex.williamson@...hat.com, kwankhede@...dia.com,
        jjherne@...ux.ibm.com, fiuczy@...ux.ibm.com
Subject: Re: [PATCH v7 04/15] s390/vfio-ap: implement in-use callback for
 vfio_ap driver

On Thu, 16 Apr 2020 10:45:20 -0400
Tony Krowiak <akrowiak@...ux.ibm.com> wrote:

> 
> 
> On 4/16/20 7:18 AM, Cornelia Huck wrote:
> > On Tue,  7 Apr 2020 15:20:04 -0400
> > Tony Krowiak <akrowiak@...ux.ibm.com> wrote:
> >
> >> Let's implement the callback to indicate when an APQN
> >> is in use by the vfio_ap device driver. The callback is
> >> invoked whenever a change to the apmask or aqmask would
> >> result in one or more queue devices being removed from the driver. The
> >> vfio_ap device driver will indicate a resource is in use
> >> if the APQN of any of the queue devices to be removed are assigned to
> >> any of the matrix mdevs under the driver's control.
> >>
> >> Signed-off-by: Tony Krowiak <akrowiak@...ux.ibm.com>
> >> ---
> >>   drivers/s390/crypto/vfio_ap_drv.c     |  1 +
> >>   drivers/s390/crypto/vfio_ap_ops.c     | 47 +++++++++++++++++----------
> >>   drivers/s390/crypto/vfio_ap_private.h |  2 ++
> >>   3 files changed, 33 insertions(+), 17 deletions(-)
> >> @@ -1369,3 +1371,14 @@ void vfio_ap_mdev_remove_queue(struct ap_queue *queue)
> >>   	kfree(q);
> >>   	mutex_unlock(&matrix_dev->lock);
> >>   }
> >> +
> >> +bool vfio_ap_mdev_resource_in_use(unsigned long *apm, unsigned long *aqm)
> >> +{
> >> +	bool in_use;
> >> +
> >> +	mutex_lock(&matrix_dev->lock);
> >> +	in_use = vfio_ap_mdev_verify_no_sharing(NULL, apm, aqm) ? true : false;
> > Maybe
> >
> > in_use = !!vfio_ap_mdev_verify_no_sharing(NULL, apm, aqm);
> >
> > ?
> 
> To be honest, I find the !! expression very confusing. Every time I see 
> it, I have
> to spend time thinking about what the result of !! is going to be. I think
> the statement should be left as-is because it more clearly expresses
> the intent.
> 

This is discussion is just about cosmetics, I believe. Just a piece of
advice: try to be sensitive about the community. In this community, and
I believe in C general !! is the idiomatic way to convert number to
boolean. Why would one want to do that is a bit longer story. The short
version is in logic condition context the value 0 is false and any
other value is true. !! keeps false value (0) false, and forces a true to
the most true true value. If you keep getting confused every time you
run across a !! that won't help with reading other peoples C.

Regards,
Halil 

> >
> >> +	mutex_unlock(&matrix_dev->lock);
> >> +
> >> +	return in_use;
> >> +}
>