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

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Date:   Thu, 17 May 2018 09:44:54 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     Tony Krowiak <akrowiak@...ux.vnet.ibm.com>
Cc:     Halil Pasic <pasic@...ux.ibm.com>, 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, jjherne@...ux.vnet.ibm.com,
        thuth@...hat.com, pasic@...ux.vnet.ibm.com, berrange@...hat.com,
        fiuczy@...ux.vnet.ibm.com, buendgen@...ibm.com
Subject: Re: [PATCH v5 05/13] s390: vfio-ap: register matrix device with
 VFIO mdev framework

On Mon, 14 May 2018 15:42:18 -0400
Tony Krowiak <akrowiak@...ux.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".)