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

[Christian Borntraeger]

On 05.12.2018 17:45, Cornelia Huck wrote:
> On Wed, 5 Dec 2018 17:38:22 +0100
> David Hildenbrand <address@hidden> 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 <address@hidden>
>>> ---
>>>
>>> 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.

> 
> 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?)

agreed. 
>