Re: [Qemu-devel] [PATCH RFC] vfio-ap: flag as compatible with balloon

[Cornelia Huck <cohuck@redhat.com>]

On Wed, 5 Dec 2018 17:38:22 +0100
David Hildenbrand <david@redhat.com> wrote:

> On 05.12.18 15:51, Cornelia Huck wrote:
> > vfio-ap devices do not pin any pages in the host. Therefore, they
> > are belived to be compatible with memory ballooning.
> > 
> > Flag them as compatible, so both vfio-ap and a balloon can be
> > used simultaneously.
> > 
> > Signed-off-by: Cornelia Huck <cohuck@redhat.com>
> > ---
> > 
> > As briefly discussed on IRC. RFC as I do not have easy access to
> > hardware I can test this with.
> > 
> > ---
> >  hw/vfio/ap.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> > 
> > diff --git a/hw/vfio/ap.c b/hw/vfio/ap.c
> > index 65de952f44..3bf48eed28 100644
> > --- a/hw/vfio/ap.c
> > +++ b/hw/vfio/ap.c
> > @@ -104,6 +104,14 @@ static void vfio_ap_realize(DeviceState *dev, Error **errp)
> >      vapdev->vdev.name = g_strdup_printf("%s", mdevid);
> >      vapdev->vdev.dev = dev;
> >  
> > +    /*
> > +     * vfio-ap devices are believed to operate in a way compatible with
> > +     * memory ballooning, as no pages are pinned in the host.
> > +     * This needs to be set before vfio_get_device() for vfio common to
> > +     * handle the balloon inhibitor.
> > +     */
> > +    vapdev->vdev.balloon_allowed = true;
> > +
> >      ret = vfio_get_device(vfio_group, mdevid, &vapdev->vdev, &local_err);
> >      if (ret) {
> >          goto out_get_dev_err;
> >   
> 
> What happens if this ever changes? Shouldn't we have an API to at least
> check what the vfio device can guarantee?
> 
> "are believed to operate" doesn't sound like guarantees to me :)

It's the same for ccw :)

While such an API definitely sounds like a good idea, it is probably
overkill to introduce it for this case (do we envision changing the way
vfio-ap operates in the future to make that statement non-true?)