Re: [PATCH 1/2] virtio_balloon: add work around for out of spec QEMU

[Jason Wang]

On Fri, Jul 5, 2024 at 6:09 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>
> QEMU implemented the configuration
>         VIRTIO_BALLOON_F_REPORTING && ! VIRTIO_BALLOON_F_FREE_PAGE_HINT
> incorrectly: it then uses vq3 for reporting, spec says it is always 4.
>
> This is masked by a corresponding bug in driver:
> add a work around as I'm going to try and fix the driver bug.
>
> Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> ---
>  drivers/virtio/virtio_balloon.c | 19 +++++++++++++++++--
>  1 file changed, 17 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 9a61febbd2f7..7dc3fcd56238 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -597,8 +597,23 @@ static int init_vqs(struct virtio_balloon *vb)
>
>         err = virtio_find_vqs(vb->vdev, VIRTIO_BALLOON_VQ_MAX, vqs,
>                               callbacks, names, NULL);
> -       if (err)
> -               return err;
> +       if (err) {
> +               /*
> +                * Try to work around QEMU bug which since 2020 confused vq numbers
> +                * when VIRTIO_BALLOON_F_REPORTING but not
> +                * VIRTIO_BALLOON_F_FREE_PAGE_HINT are offered.
> +                */
> +               if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_REPORTING) &&
> +                   !virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
> +                       names[VIRTIO_BALLOON_VQ_FREE_PAGE] = "reporting_vq";
> +                       callbacks[VIRTIO_BALLOON_VQ_FREE_PAGE] = balloon_ack;
> +                       err = virtio_find_vqs(vb->vdev,
> +                                             VIRTIO_BALLOON_VQ_REPORTING, vqs, callbacks, names, NULL);
> +               }
> +
> +               if (err)
> +                       return err;
> +       }
>
>         vb->inflate_vq = vqs[VIRTIO_BALLOON_VQ_INFLATE];
>         vb->deflate_vq = vqs[VIRTIO_BALLOON_VQ_DEFLATE];
> --
> MST
>

Acked-by: Jason Wang <jasowang@xxxxxxxxxx>

Do we need a spec to say this is something that needs to be considered
by the driver?

Thanks