Re: [PATCH v37 0/3] Virtio-balloon: support free page reporting

[Wei Wang]

On 12/27/2018 08:17 PM, Christian Borntraeger wrote:
> On 27.12.2018 12:59, Christian Borntraeger wrote:
> > On 27.12.2018 12:31, Christian Borntraeger wrote:
> > > This patch triggers random crashes in the guest kernel on s390 early during boot.
> > > No migration and no setting of the balloon is involved.
> > Adding Conny and Halil,
> >
> > As the QEMU provides no PAGE_HINT feature yet, this quick hack makes the
> > guest boot fine again:
> >
> >
> > diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> > index 728ecd1eea305..aa2e1864c5736 100644
> > --- a/drivers/virtio/virtio_balloon.c
> > +++ b/drivers/virtio/virtio_balloon.c
> > @@ -492,7 +492,7 @@ static int init_vqs(struct virtio_balloon *vb)
> >                  callbacks[VIRTIO_BALLOON_VQ_FREE_PAGE] = NULL;
> >          }
> >   
> > -       err = vb->vdev->config->find_vqs(vb->vdev, VIRTIO_BALLOON_VQ_MAX,
> > +       err = vb->vdev->config->find_vqs(vb->vdev, 3, //VIRTIO_BALLOON_VQ_MAX,
> >                                           vqs, callbacks, names, NULL, NULL);
> >          if (err)
> >                  return err;
> >
> >
> > To me it looks like that virtio_ccw_find_vqs will abort if any of the virtqueues
> > that it is been asked for does not exist (including the earlier ones).
> This "hack" makes the random crashes go away, but the balloon interface itself
> does not work. (setting the value to anything will hang the guest).
> As patch 1 also modifies the main path, there seem to be additional issues, maybe
> endianess
>
> Looking at things like
>
> +		vb->cmd_id_received = VIRTIO_BALLOON_CMD_ID_STOP;
> +		vb->cmd_id_active = cpu_to_virtio32(vb->vdev,
> +						  VIRTIO_BALLOON_CMD_ID_STOP);
> +		vb->cmd_id_stop = cpu_to_virtio32(vb->vdev,
> +						  VIRTIO_BALLOON_CMD_ID_STOP);
>
>
> Why is cmd_id_received not using cpu_to_virtio32?

That conversion is only needed when we need to send the value to the device.
cmd_id_received doesn't need to be sent to the device.

Best,
Wei