Re: [PATCH v18 08/18] s390/vfio-ap: allow assignment of unavailable AP queues to mdev device

[Tony Krowiak]

On 3/3/22 10:39, Jason J. Herne wrote:
> On 2/14/22 19:50, Tony Krowiak wrote:
> >   /**
> > - * vfio_ap_mdev_verify_no_sharing - verifies that the AP matrix is 
> > not configured
> > + * vfio_ap_mdev_verify_no_sharing - verify APQNs are not shared by 
> > matrix mdevs
> >    *
> > - * @matrix_mdev: the mediated matrix device
> > + * @mdev_apm: mask indicating the APIDs of the APQNs to be verified
> > + * @mdev_aqm: mask indicating the APQIs of the APQNs to be verified
> >    *
> > - * Verifies that the APQNs derived from the cross product of the AP 
> > adapter IDs
> > - * and AP queue indexes comprising the AP matrix are not configured 
> > for another
> > + * Verifies that each APQN derived from the Cartesian product of a 
> > bitmap of
> > + * AP adapter IDs and AP queue indexes is not configured for any matrix
> >    * mediated device. AP queue sharing is not allowed.
> >    *
> > - * Return: 0 if the APQNs are not shared; otherwise returns 
> > -EADDRINUSE.
> > + * Return: 0 if the APQNs are not shared; otherwise return -EADDRINUSE.
> >    */
> > -static int vfio_ap_mdev_verify_no_sharing(struct ap_matrix_mdev 
> > *matrix_mdev)
> > +static int vfio_ap_mdev_verify_no_sharing(unsigned long *mdev_apm,
> > +                      unsigned long *mdev_aqm)
> >   {
> > -    struct ap_matrix_mdev *lstdev;
> > +    struct ap_matrix_mdev *matrix_mdev;
> >       DECLARE_BITMAP(apm, AP_DEVICES);
> >       DECLARE_BITMAP(aqm, AP_DOMAINS);
> >   -    list_for_each_entry(lstdev, &matrix_dev->mdev_list, node) {
> > -        if (matrix_mdev == lstdev)
> > +    list_for_each_entry(matrix_mdev, &matrix_dev->mdev_list, node) {
> > +        /*
> > +         * If the input apm and aqm belong to the matrix_mdev's matrix,
> > +         * then move on to the next.
> > +         */
> > +        if (mdev_apm == matrix_mdev->matrix.apm &&
> > +            mdev_aqm == matrix_mdev->matrix.aqm)
> >               continue;
>
> We may have a problem here. This check seems like it exists to stop 
> you from
> comparing an mdev's apm/aqm with itself. Obviously comparing an mdev's 
> newly
> updated apm/aqm with itself would cause a false positive sharing 
> check, right?
> If this is the case, I think the comment should be changed to reflect 
> that.

You are correct, this check is performed to prevent comparing an mdev to
itself, I'll improve the comment.

> Aside from the comment, what stops this particular series of if 
> statements from
> allowing us to configure a second mdev with the exact same apm/aqm 
> values as an
> existing mdev? If we do, then this check's continue will short circuit 
> the rest
> of the function thereby allowing that 2nd mdev even though it should be a
> sharing violation.

I don't see how this is possible.

The function above is called from two places: the 
vfio_ap_mdev_validate_masks()
function which is invoked when an adapter or domain is assigned to the 
mdev; and the
vfio_ap_mdev_resource_in_use() function which is a callback registered 
with the
AP bus and is called by the bus when the apmask/aqmask are changed.

In the former case, the addresses passed in are from the apm/aqm fields 
within
the ap_matrix structure. Each ap_matrix structure is a field contained 
within an
ap_matrix_mdev structure, it is not a pointer to storage allocated 
external to
the matrix_mdev, so the apm/aqm addresses passed in from the
vfio_ap_mdev_validate_masks() function will be unique to each 
ap_matrix_mdev
structure.

In the latter case, the addresses are passed in by the AP bus and are 
allocated by the
bus and would definitely not be contained within an ap_matrix_mdev since 
the AP bus
doesn't even have access to that structure.

>