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

[Halil Pasic <pasic@linux.ibm.com>]

On Thu, 6 Dec 2018 09:28:34 +0100
David Hildenbrand <david@redhat.com> wrote:

> On 05.12.18 18:25, Christian Borntraeger wrote:
> > 
> > 
> > On 05.12.2018 17:45, Cornelia Huck wrote:
> >> 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 :)
> > 
> > I would actually remove that comment or fix it. We either know or we dont.
> > In the way vfio-works I see no reason to disallow balloon. Even if the guest does
> > something wrong (e.g. crypto I/O on freed pages) the host would handle that the
> > same as it would for normal page accesses. From a host point of view the crypto
> > instructions are just CISC instructions with load/store semantics.
> 
> As long as vfio-ap does not and will never pin pages (and keep them
> pinned), we are fine. I don't know about the details, but if vfio-ap
> really just issues a synchronous instruction for us, we are fine.
> 

I agree with Christian. That comment is best removed.

@Tony, I guess you should have the most elaborate test setup. Can you give
this some testing just in case?

> > 
> >>
> >> It's the same for ccw :)

As a matter of fact, I don't like that comment.

Regards,
Halil

> >>
> >> 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?)
> > 
> > agreed. 
> >>
> > 
> 
>