linux-kernel - Re: [PATCH v11 04/14] s390/zcrypt: driver callback to indicate resource in use

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Date:   Tue, 27 Oct 2020 14:01:57 +0100
From:   Halil Pasic <pasic@...ux.ibm.com>
To:     Tony Krowiak <akrowiak@...ux.ibm.com>
Cc:     linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, freude@...ux.ibm.com, borntraeger@...ibm.com,
        cohuck@...hat.com, mjrosato@...ux.ibm.com,
        alex.williamson@...hat.com, kwankhede@...dia.com,
        fiuczy@...ux.ibm.com, frankja@...ux.ibm.com, david@...hat.com,
        hca@...ux.ibm.com, gor@...ux.ibm.com
Subject: Re: [PATCH v11 04/14] s390/zcrypt: driver callback to indicate
 resource in use

On Thu, 22 Oct 2020 13:11:59 -0400
Tony Krowiak <akrowiak@...ux.ibm.com> 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 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.

Like discussed last time, there seems to be nothing, that would prevent
a resource becoming in use between the in_use() callback returned false
and the resource being removed as a result of ap_bus_revise_bindings().

Another thing that may be of interest, is that now we hold the
ap_perms_mutex for the in_use() checks. The ap_perms_mutex is used
in ap_device_probe() and I don't quite understand some
usages of in zcrypt_api.c My feeling is that the extra pressure on that
lock should not be a problem, except if in_use() were to not return
because of some deadlock.

With all that said if Harald is fine with it, so am I.

Acked-by: Halil Pasic <pasic@...ux.ibm.com>

> 
> 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 and 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@...ux.ibm.com>
>