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Date:   Tue, 22 May 2018 10:19:40 +0200
From:   Cornelia Huck <cohuck@redhat.com>
To:     Tony Krowiak <akrowiak@linux.vnet.ibm.com>
Cc:     Halil Pasic <pasic@linux.ibm.com>, linux-s390@vger.kernel.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        freude@de.ibm.com, schwidefsky@de.ibm.com,
        heiko.carstens@de.ibm.com, borntraeger@de.ibm.com,
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        thuth@redhat.com, pasic@linux.vnet.ibm.com, berrange@redhat.com,
        fiuczy@linux.vnet.ibm.com, buendgen@de.ibm.com
Subject: Re: [PATCH v5 05/13] s390: vfio-ap: register matrix device with
 VFIO mdev framework
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On Mon, 21 May 2018 11:13:58 -0400
Tony Krowiak <akrowiak@linux.vnet.ibm.com> wrote:

> On 05/17/2018 03:44 AM, Cornelia Huck wrote:
> > On Mon, 14 May 2018 15:42:18 -0400
> > Tony Krowiak <akrowiak@linux.vnet.ibm.com> wrote:
> >  
> >> On 05/11/2018 01:18 PM, Halil Pasic wrote:  
> >>>
> >>> On 05/07/2018 05:11 PM, Tony Krowiak wrote:  
> >>>> Registers the matrix device created by the VFIO AP device
> >>>> driver with the VFIO mediated device framework.
> >>>> Registering the matrix device will create the sysfs
> >>>> structures needed to create mediated matrix devices
> >>>> each of which will be used to configure the AP matrix
> >>>> for a guest and connect it to the VFIO AP device driver.
> >>>> +static int vfio_ap_mdev_create(struct kobject *kobj, struct
> >>>> mdev_device *mdev)
> >>>> +{
> >>>> +    struct ap_matrix *ap_matrix = to_ap_matrix(mdev_parent_dev(mdev));
> >>>> +
> >>>> +    ap_matrix->available_instances--;
> >>>> +
> >>>> +    return 0;
> >>>> +}
> >>>> +
> >>>> +static int vfio_ap_mdev_remove(struct mdev_device *mdev)
> >>>> +{
> >>>> +    struct ap_matrix *ap_matrix = to_ap_matrix(mdev_parent_dev(mdev));
> >>>> +
> >>>> +    ap_matrix->available_instances++;
> >>>> +
> >>>> +    return 0;
> >>>> +}
> >>>> +  
> >>> The above functions seem to be called with the lock of this
> >>> auto-generated
> >>> mdev parent device held. That's why we don't have to care about
> >>> synchronization
> >>> ourselves, right?  
> >> I would assume as much. The comments for the 'struct mdev_parent_ops' in
> >> include/linux/mdev.h do not mention anything about synchronization, nor
> >> did I
> >> see any locking or synchronization in the vfio_ccw implementation after
> >> which
> >> I modeled my code, so frankly it is something I did not consider.
> >>  
> >>>
> >>> A small comment in the code could be helpful for mdev non-experts.
> >>> Hell, I would
> >>> even consider documenting it for all mdev -- took me some time to
> >>> figure out.  
> >> You may want to bring this up with the VFIO mdev maintainers, but I'd be
> >> happy to
> >> include a comment in the functions in question if you think it important.  
> > Important note: There's currently a patch on list that removes the mdev
> > parent mutex, and it seems there was never intended to be any
> > serialization in that place by the mdev core. (Look for "vfio/mdev:
> > Check globally for duplicate devices".)  
> 
> The patch on the list holds the mdev_list_lock during create and remove
> of an mdev device, so it looks like no synchronization is necessary on the
> part of the vendor code in the create/remove callbacks; does that sound
> about right?

v1/v2 did that; v3/v4 hold the list lock only while the device is added
to the mdev list. v4 also adds a note regarding locking to the
documentation.