Re: [PATCH v11 07/14] s390/vfio-ap: sysfs attribute to display the guest's matrix

[Tony Krowiak]

On 11/13/20 6:12 PM, Halil Pasic wrote:
> On Fri, 13 Nov 2020 12:27:32 -0500
> Tony Krowiak <akrowiak@xxxxxxxxxxxxx> wrote:
>
> > On 10/28/20 4:17 AM, Halil Pasic wrote:
> > > On Thu, 22 Oct 2020 13:12:02 -0400
> > > Tony Krowiak <akrowiak@xxxxxxxxxxxxx> wrote:
> > >
> > > > +static ssize_t guest_matrix_show(struct device *dev,
> > > > +				 struct device_attribute *attr, char *buf)
> > > > +{
> > > > +	ssize_t nchars;
> > > > +	struct mdev_device *mdev = mdev_from_dev(dev);
> > > > +	struct ap_matrix_mdev *matrix_mdev = mdev_get_drvdata(mdev);
> > > > +
> > > > +	if (!vfio_ap_mdev_has_crycb(matrix_mdev))
> > > > +		return -ENODEV;
> > > I'm wondering, would it make sense to have guest_matrix display the would
> > > be guest matrix when we don't have a KVM? With the filtering in
> > > place, the question in what guest_matrix would my (assign) matrix result
> > > right now if I were to hook up my vfio_ap_mdev to a guest seems a
> > > legitimate one.
> > A couple of thoughts here:
> > * The ENODEV informs the user that there is no guest running
> >      which makes sense to me given this interface displays the
> >      guest matrix. The alternative, which I considered, was to
> >      display an empty matrix (i.e., nothing).
> > * This would be a pretty drastic change to the design because
> >      the shadow_apcb - which is what is displayed via this interface - is
> >      only updated when the guest is started and while it is running (i.e.,
> >      hot plug of new adapters/domains). Making this change would
> >      require changing that entire design concept which I am reluctant
> >      to do at this point in the game.
> No problem. My thinking was, that, because we can do the
> assign/unassing ops also for the running guest, that we also have
> the code to do the maintenance on the shadow_apcb. In this
> series this code is conditional with respect to vfio_ap_mdev_has_crycb().
> E.g.
>
> static ssize_t assign_adapter_store(struct device *dev,
>                                      struct device_attribute *attr,
>                                      const char *buf, size_t count)
> {
> [..]
>          if (vfio_ap_mdev_has_crycb(matrix_mdev))
>                  if (vfio_ap_mdev_filter_guest_matrix(matrix_mdev, true))
>                          vfio_ap_mdev_commit_shadow_apcb(matrix_mdev);
>
> If one were to move the
> vfio_ap_mdev_has_crycb() check into vfio_ap_mdev_commit_shadow_apcb()
> then we would have an always up to date shatdow_apcb, we could display.
>
> I don't feel strongly about this. Was just an idea, because if the result
> of the filtering is surprising, currently the only to see, without
> knowing the algorithm, and possibly the state, and the history of the
> system, is to actually start a guest.

Okay, I can buy this and will make the change.

> Regards,
> Halil