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

[Halil Pasic]

On 05/15/2018 05:16 PM, Tony Krowiak wrote:
> On 05/15/2018 10:17 AM, Pierre Morel wrote:
> > On 14/05/2018 21:42, Tony Krowiak 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.
> > > > [..]
> > > > > diff --git a/drivers/s390/crypto/vfio_ap_ops.c b/drivers/s390/crypto/vfio_ap_ops.c
> > > > > new file mode 100644
> > > > > index 0000000..d7d36fb
> > > > > --- /dev/null
> > > > > +++ b/drivers/s390/crypto/vfio_ap_ops.c
> > > > > @@ -0,0 +1,106 @@
> > > > > +// SPDX-License-Identifier: GPL-2.0+
> > > > > +/*
> > > > > + * Adjunct processor matrix VFIO device driver callbacks.
> > > > > + *
> > > > > + * Copyright IBM Corp. 2017
> > > > > + * Author(s): Tony Krowiak <akrowiak@xxxxxxxxxxxxxxxxxx>
> > > > > + *
> > > > > + */
> > > > > +#include <linux/string.h>
> > > > > +#include <linux/vfio.h>
> > > > > +#include <linux/device.h>
> > > > > +#include <linux/list.h>
> > > > > +#include <linux/ctype.h>
> > > > > +
> > > > > +#include "vfio_ap_private.h"
> > > > > +
> > > > > +#define VFOP_AP_MDEV_TYPE_HWVIRT "passthrough"
> > > > > +#define VFIO_AP_MDEV_NAME_HWVIRT "VFIO AP Passthrough Device"
> > > > > +
> > > > > +static int vfio_ap_mdev_create(struct kobject *kobj, struct mdev_device *mdev)
> > > > > +{
> > > > > +ÂÂÂ struct ap_matrix *ap_matrix = to_ap_matrix(mdev_parent_dev(mdev));
> > > > > +
> > > > > +ÂÂÂ ap_matrix->available_instances--;
> > > > > +
> > > > > +ÂÂÂ return 0;
> > > > > +}
> > > > > +
> > > > > +static int vfio_ap_mdev_remove(struct mdev_device *mdev)
> > > > > +{
> > > > > +ÂÂÂ struct ap_matrix *ap_matrix = to_ap_matrix(mdev_parent_dev(mdev));
> > > > > +
> > > > > +ÂÂÂ ap_matrix->available_instances++;
> > > > > +
> > > > > +ÂÂÂ 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.
> > >
> > > > [..]
> > > >
> > > >
> > > > > +int vfio_ap_mdev_register(struct ap_matrix *ap_matrix)
> > > > > +{
> > > > > +ÂÂÂ int ret;
> > > > > +
> > > > > +ÂÂÂ ret = mdev_register_device(&ap_matrix->device, &vfio_ap_matrix_ops);
> > > > > +ÂÂÂ if (ret)
> > > > > +ÂÂÂÂÂÂÂ return ret;
> > > > > +
> > > > > +ÂÂÂ ap_matrix->available_instances = AP_MATRIX_MAX_AVAILABLE_INSTANCES;
> > > > > +
> > > > > +ÂÂÂ return 0;
> > > > > +}
> > > > > +
> > > > > +void vfio_ap_mdev_unregister(struct ap_matrix *ap_matrix)
> > > > > +{
> > > > > +ÂÂÂ ap_matrix->available_instances--;
> > > >
> > > > What is this for? I don't understand.
> > >
> > > To control the number of mediated devices one can create for the matrix device.
> > > Once the max is reached, the mdev framework will not allow creation of another
> > > mediated device until one is removed. This counter keeps track of the number
> > > of instances that can be created. This is documented with the mediated
> > > framework. You may want to take a look at:
> > >
> > > Documentation/vfio-mediated-device.txt
> > > Documentation/vfio.txt
> > > Documentation/virtual/kvm/devices/vfio.txt
> >
> > This is what you do in create/remove.
> > But here in unregister I agree with Halil, it does not seem to be usefull.
>
> If that is in fact what Halil was asking, then I misinterpreted his question; I
> thought he was asking what the available_instances was used for. You are
> correct, this does not belong here although it makes little difference given
> this is called only when the driver, which creates the matrix device, is unloaded.
> It is necessary in the register function to initialize its value, but I'll
> remove it from here.

I questioned the dubious usage of ap_matrix->available_instances rather than
asking what is the variable for.

If I've had this deemed damaging I would have asked if it's damaging in a way
I think it is. For example take my comment on 'KVM: s390: interfaces to manage
guest's AP matrix'.

Regards,
Halil

> > > > Regards,
> > > > Halil
> > > >
> > > > > + mdev_unregister_device(&ap_matrix->device);
> > > > > +}