Re: [PATCH v5 05/13] s390: vfio-ap: register matrix device with VFIO mdev framework

[Tony Krowiak]

On 05/22/2018 04:19 AM, Cornelia Huck wrote:
> On Mon, 21 May 2018 11:13:58 -0400
> Tony Krowiak <akrowiak@xxxxxxxxxxxxxxxxxx> wrote:
>
> > On 05/17/2018 03:44 AM, Cornelia Huck wrote:
> > > On Mon, 14 May 2018 15:42:18 -0400
> > > Tony Krowiak <akrowiak@xxxxxxxxxxxxxxxxxx> 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.

I'll add some synchronization to the read/update of available instances.

>