Re: [PATCH v10 04/16] s390/zcrypt: driver callback to indicate resource in use

[Tony Krowiak]

On 9/14/20 11:29 AM, Cornelia Huck wrote:
> On Fri, 21 Aug 2020 15:56:04 -0400
> Tony Krowiak <akrowiak@xxxxxxxxxxxxx> wrote:
>
> > Introduces a new driver callback to prevent a root user from unbinding
> > an AP queue from its device driver if the queue is in use. The intent of
> > this callback is to provide a driver with the means to prevent a root user
> > from inadvertently taking a queue away from a matrix mdev and giving it to
> > the host while it is assigned to the matrix mdev. The callback will
> > be invoked whenever a change to the AP bus's sysfs apmask or aqmask
> > attributes would result in one or more AP queues being removed from its
> > driver. If the callback responds in the affirmative for any driver
> > queried, the change to the apmask or aqmask will be rejected with a device
> > in use error.
> >
> > For this patch, only non-default drivers will be queried. Currently,
> > there is only one non-default driver, the vfio_ap device driver. The
> > vfio_ap device driver facilitates pass-through of an AP queue to a
> > guest. The idea here is that a guest may be administered by a different
> > sysadmin than the host and we don't want AP resources to unexpectedly
> > disappear from a guest's AP configuration (i.e., adapters, domains and
> > control domains assigned to the matrix mdev). This will enforce the proper
> > procedure for removing AP resources intended for guest usage which is to
> > first unassign them from the matrix mdev, then unbind them from the
> > vfio_ap device driver.
> >
> > Signed-off-by: Tony Krowiak <akrowiak@xxxxxxxxxxxxx>
> > Reported-by: kernel test robot <lkp@xxxxxxxxx>
> This looks a bit odd...

I've removed all of those. These kernel test robot errors were flagged
in the last series. The review comments from the robot suggested
the reported-by, but I assume that was for patches intended to
fix those errors, so I am removing these as per Christian's comments.

> > ---
> >   drivers/s390/crypto/ap_bus.c | 148 ++++++++++++++++++++++++++++++++---
> >   drivers/s390/crypto/ap_bus.h |   4 +
> >   2 files changed, 142 insertions(+), 10 deletions(-)
> (...)
>
> > @@ -1107,12 +1118,70 @@ static ssize_t apmask_show(struct bus_type *bus, char *buf)
> >   	return rc;
> >   }
> >   
> > +static int __verify_card_reservations(struct device_driver *drv, void *data)
> > +{
> > +	int rc = 0;
> > +	struct ap_driver *ap_drv = to_ap_drv(drv);
> > +	unsigned long *newapm = (unsigned long *)data;
> > +
> > +	/*
> > +	 * No need to verify whether the driver is using the queues if it is the
> > +	 * default driver.
> > +	 */
> > +	if (ap_drv->flags & AP_DRIVER_FLAG_DEFAULT)
> > +		return 0;
> > +
> > +	/* The non-default driver's module must be loaded */
> > +	if (!try_module_get(drv->owner))
> > +		return 0;
> > +
> > +	if (ap_drv->in_use)
> > +		if (ap_drv->in_use(newapm, ap_perms.aqm))
> > +			rc = -EADDRINUSE;
> ISTR that Christian suggested -EBUSY in a past revision of this series?
> I think that would be more appropriate.

I went back and looked and sure enough, he did recommend that.
You have a great memory! I didn't respond to that comment, so I
must have missed it at the time.

I personally prefer EADDRINUSE because I think it is more indicative
of the reason an AP resource can not be assigned back to the host
drivers is because it is in use by a guest or, at the very least, reserved
for use by a guest (i.e., assigned to an mdev). To say it is busy implies
that the device is busy performing encryption services which may or
may not be true at a given moment. Even if so, that is not the reason
for refusing to allow reassignment of the device.

> Also, I know we have discussed this before, but it is very hard to
> figure out the offending device(s) if the sysfs manipulation failed. Can
> we at least drop something into the syslog? That would be far from
> perfect, but it gives an admin at least a chance to figure out why they
> got an error. Some more structured way that would be usable from tools
> can still be added later.

I see you found the patch that logged this:)

> > +
> > +	module_put(drv->owner);
> > +
> > +	return rc;
> > +}