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

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jul 06, 2026 at 09:28:50AM +0200, David Hildenbrand (Arm) wrote:
> On 7/5/26 21:12, Michael S. Tsirkin wrote:
> > The virtio spec requires the driver not to kick the device before
> > DRIVER_OK is set.  init_vqs() primes the stats virtqueue with a buffer
> > and kicks the device before virtio_device_ready() is called in
> > virtballoon_probe(), violating this requirement.
> > 
> > Further, if the device responds to the early kick by processing the
> > buffer before DRIVER_OK, stats_request() fires and queues
> > update_balloon_stats_work.  Should probe then fail and free vb, the work
> > runs against freed memory.
> > 
> > To fix, move buffer setup to after DRIVER_OK. Be careful to
> > disable update_balloon_stats_work while this is going on,
> > to make sure it does not race with the setup.
> > 
> > setup_vqs() warns but does not fail probe or restore if
> > virtqueue_add_outbuf() fails; the call never actually fails in these
> > contexts since the queue is freshly initialized and empty.
> > 
> > Testing: tested that stats still work after the change.
> > 
> > Fixes: 9564e138b1f6 ("virtio: Add memory statistics reporting to the balloon driver (V4)")
> > Reported-by: Sashiko:gemini-3.1-pro-preview
> > Cc: David Hildenbrand <david@kernel.org>
> > Assisted-by: Claude:claude-sonnet-4-6
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> > 
> > changes from v1:
> > 	check that work enable/disable is balanced
> > 	explain how add buf never fails in probe/restore
> > 
> >  drivers/virtio/virtio_balloon.c | 50 +++++++++++++++++++++------------
> >  1 file changed, 32 insertions(+), 18 deletions(-)
> > 
> > diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> > index 088b3a0e6ce6..bc0a2b19ca7d 100644
> > --- a/drivers/virtio/virtio_balloon.c
> > +++ b/drivers/virtio/virtio_balloon.c
> > @@ -611,25 +611,8 @@ static int init_vqs(struct virtio_balloon *vb)
> >  	vb->inflate_vq = vqs[VIRTIO_BALLOON_VQ_INFLATE];
> >  	vb->deflate_vq = vqs[VIRTIO_BALLOON_VQ_DEFLATE];
> >  	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ)) {
> > -		struct scatterlist sg;
> > -		unsigned int num_stats;
> >  		vb->stats_vq = vqs[VIRTIO_BALLOON_VQ_STATS];
> > -
> > -		/*
> > -		 * Prime this virtqueue with one buffer so the hypervisor can
> > -		 * use it to signal us later (it can't be broken yet!).
> > -		 */
> > -		num_stats = update_balloon_stats(vb);
> > -
> > -		sg_init_one(&sg, vb->stats, sizeof(vb->stats[0]) * num_stats);
> > -		err = virtqueue_add_outbuf(vb->stats_vq, &sg, 1, vb,
> > -					   GFP_KERNEL);
> > -		if (err) {
> > -			dev_warn(&vb->vdev->dev, "%s: add stat_vq failed\n",
> > -				 __func__);
> > -			return err;
> > -		}
> > -		virtqueue_kick(vb->stats_vq);
> > +		disable_work(&vb->update_balloon_stats_work);
> 
> That's to stop the stats queue triggering stats_request() I assume?

Yes.

> Is that
> valid before we actually added+kicked ourselves?

Why not?

> Also, can't we simply handle that in stats_request(), just ignoring it there?

Then we'd need to maintain a special flag and I dislike that.

> Then  we wouldn't have to go through the siable + enable.

We'd still need to flip the flag on/off.

> >  	}
> >  
> >  	if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT))
> > @@ -916,6 +899,33 @@ static int virtio_balloon_register_shrinker(struct virtio_balloon *vb)
> >  	return 0;
> >  }
> >  
> > +static void setup_vqs(struct virtio_balloon *vb)
> > +{
> > +	struct scatterlist sg;
> > +	unsigned int num_stats;
> > +	bool ret;
> > +
> > +	if (!virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ))
> > +		return;
> > +
> > +	/*
> > +	 * Prime this virtqueue with one buffer so the hypervisor can
> > +	 * use it to signal us later (it can't be broken yet!).
> > +	 */
> > +	num_stats = update_balloon_stats(vb);
> > +	sg_init_one(&sg, vb->stats, sizeof(vb->stats[0]) * num_stats);
> > +	if (virtqueue_add_outbuf(vb->stats_vq, &sg, 1, vb, GFP_KERNEL)) {
> > +		dev_warn(&vb->vdev->dev, "%s: add stat_vq failed\n", __func__);
> > +		return;
> > +	}
> > +	virtqueue_kick(vb->stats_vq);
> > +
> > +	ret = enable_and_queue_work(system_freezable_wq,
> > +				    &vb->update_balloon_stats_work);
> > +	/* Make sure we balanced enable/disable, or we won't report stats. */
> > +	BUG_ON(!ret);
> 
> A WARN_ON_ONCE() should be good enough here, no need to crash the kernel (no new
> BUG_ON's).

Will do, thanks!

> -- 
> Cheers,
> 
> David