Re: [PATCH v2] virtio_balloon: prime stats vq after virtio_device_ready()

["David Hildenbrand (Arm)" <david@kernel.org>]

On 7/6/26 14:59, Michael S. Tsirkin wrote:
> On Mon, Jul 06, 2026 at 10:57:33AM +0200, David Hildenbrand (Arm) wrote:
>> On 7/6/26 10:43, Michael S. Tsirkin wrote:
>>>
>>> Sorry I do not get the question. Virtio API expects
>>> callers to serialize calls for each VQ. So this just prevents the work
>>> from running and accessing the VQ while probe calls add_buf/kick.
>>> I'll add a comment explaining that.
>>> But it has nothing to do with the spec.
>>
>> The device is clearly not properly initialized yet.
>>
>> Yet, we expect that we get a stats_request() callback that would try to
>> queue_work(). And IIUC, the stats_request() will be directly issued by the device.
> 
> With an in-spec device, it can happen right after virtio_device_ready.
> We could defer it a bit, but I don't see what this gets us, and
> this also protects against any out of spec ones.

Agreed.

> 
> 
>> In QEMU, that would mean that balloon_stats_poll_cb() runs, which would do a
>>
>> if (s->stats_vq_elem == NULL) {
>> 	...
>> 	return
>> }
>>
>> virtqueue_push(s->svq, s->stats_vq_elem, 0);
>> virtio_notify(vdev, s->svq);
>>
>>
>> That's why I'm confused :)
> 
> First the spec says there could be interrupts for no reasons right after
> DRIVER_OK. Second there are situations in which qemu might
> consume a buffer without a kick and then an interrupt
> races with the kick.

Okay, makes sense, thanks.

-- 
Cheers,

David