linux-kernel - Re: [RFC 05/19] s390/zcrypt: base implementation of AP matrix device driver

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Date:   Wed, 15 Nov 2017 11:31:11 +0100
From:   Cornelia Huck <cohuck@...hat.com>
To:     Tony Krowiak <akrowiak@...ux.vnet.ibm.com>
Cc:     linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, freude@...ibm.com, schwidefsky@...ibm.com,
        heiko.carstens@...ibm.com, borntraeger@...ibm.com,
        kwankhede@...dia.com, bjsdjshi@...ux.vnet.ibm.com,
        pbonzini@...hat.com, alex.williamson@...hat.com,
        pmorel@...ux.vnet.ibm.com, alifm@...ux.vnet.ibm.com,
        mjrosato@...ux.vnet.ibm.com, qemu-s390x@...gnu.org,
        jjherne@...ux.vnet.ibm.com, thuth@...hat.com,
        pasic@...ux.vnet.ibm.com
Subject: Re: [RFC 05/19] s390/zcrypt: base implementation of AP matrix
 device driver

On Tue, 14 Nov 2017 13:15:27 -0500
Tony Krowiak <akrowiak@...ux.vnet.ibm.com> wrote:

> On 11/14/2017 12:00 PM, Cornelia Huck wrote:
> > On Tue, 14 Nov 2017 11:37:05 -0500
> > Tony Krowiak <akrowiak@...ux.vnet.ibm.com> wrote:
> >  
> >> On 11/14/2017 07:40 AM, Cornelia Huck wrote:  
> >>> On Fri, 13 Oct 2017 13:38:50 -0400
> >>> Tony Krowiak <akrowiak@...ux.vnet.ibm.com> wrote:  
> >>>> diff --git a/drivers/s390/crypto/ap_matrix_bus.c b/drivers/s390/crypto/ap_matrix_bus.c
> >>>> index 4eb1e3c..66bfa54 100644
> >>>> --- a/drivers/s390/crypto/ap_matrix_bus.c
> >>>> +++ b/drivers/s390/crypto/ap_matrix_bus.c
> >>>> @@ -75,10 +75,18 @@ static int ap_matrix_dev_create(void)
> >>>>    	return 0;
> >>>>    }
> >>>>    
> >>>> +struct ap_matrix *ap_matrix_get_device(void)
> >>>> +{
> >>>> +	return matrix;  
> >>> See the comments I had for the previous patch. In particular, I think
> >>> it is better to retrieve a pointer to the matrix device via driver core
> >>> methods.  
> >> I got some objections to creating a new bus and since there will only ever
> >> be a single AP matrix device, I decided there really wasn't a need for an
> >> AP matrix bus and got rid of it. I opted instead to create the matrix
> >> device
> >> in the init function of the vfio_ap_matrix driver. Rather than passing
> >> around a
> >> pointer, I put the following in vfio_ap_matrix_private.h:
> >>
> >>       struct ap_matrix {
> >>           struct device device;
> >>           spinlock_t qlock;
> >>           struct list_head queues;
> >>       };
> >>
> >>       extern struct ap_matrix ap_matrix;
> >>
> >> ... and declared the ap_matrix in  the driver (vfio_ap_matrix_drv.c)
> >> file as:
> >>
> >>       struct ap_matrix ap_matrix;
> >>
> >> Does this seem like a reasonable approach?  
> > Getting rid of the bus as overhead is not unreasonable.
> >
> > I'm feeling a bit queasy about the extern, however. I'd prefer a getter
> > function (that also makes sure refcounting rules are followed).  
> I now think I can avoid having to reference the ap_matrix device from
> multiple places. The reason the device is referenced in vfio_ap_matrix_ops.c
> is because there is a need for information about the AP queues that have 
> been
> bound to the vfio_ap_matrix device driver. If interfaces are provided by 
> the
> vfio_ap_matrix device driver to access the needed information, it
> won't be necessary to reference the ap_matrix device directly in
> vfio_ap_matrix_ops. I think this would be a better solution, don't you?

That sounds good, agreed.