Re: [PATCH] vdpa_sim: fix vringh initialization in vdpasim_queue_ready()

[Eugenio Perez Martin]

On Thu, Nov 10, 2022 at 3:13 PM Stefano Garzarella <sgarzare@xxxxxxxxxx> wrote:
>
> When we initialize vringh, we should pass the features and the
> number of elements in the virtqueue negotiated with the driver,
> otherwise operations with vringh may fail.
>
> This was discovered in a case where the driver sets a number of
> elements in the virtqueue different from the value returned by
> .get_vq_num_max().
>
> In vdpasim_vq_reset() is safe to initialize the vringh with
> default values, since the virtqueue will not be used until
> vdpasim_queue_ready() is called again.
>
> Fixes: 2c53d0f64c06 ("vdpasim: vDPA device simulator")
> Signed-off-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>
> ---
>  drivers/vdpa/vdpa_sim/vdpa_sim.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/drivers/vdpa/vdpa_sim/vdpa_sim.c b/drivers/vdpa/vdpa_sim/vdpa_sim.c
> index b071f0d842fb..b20689f8fe89 100644
> --- a/drivers/vdpa/vdpa_sim/vdpa_sim.c
> +++ b/drivers/vdpa/vdpa_sim/vdpa_sim.c
> @@ -67,8 +67,7 @@ static void vdpasim_queue_ready(struct vdpasim *vdpasim, unsigned int idx)
>  {
>         struct vdpasim_virtqueue *vq = &vdpasim->vqs[idx];
>
> -       vringh_init_iotlb(&vq->vring, vdpasim->dev_attr.supported_features,
> -                         VDPASIM_QUEUE_MAX, false,
> +       vringh_init_iotlb(&vq->vring, vdpasim->features, vq->num, false,
>                           (struct vring_desc *)(uintptr_t)vq->desc_addr,
>                           (struct vring_avail *)
>                           (uintptr_t)vq->driver_addr,
> --
> 2.38.1
>

I think this is definitely an improvement, but I'd say we should go a
step further and rename VDPASIM_QUEUE_MAX to VDPASIM_QUEUE_DEFAULT. As
you point out in the patch message it is not a max anymore.

Another thing to note is that we don't have a way to report that
userspace indicated a bad value for queue length. With the current
code vringh will not initialize at all if I'm not wrong, so we should
prevent userspace to put a bad num.

Ideally, we should repeat the tests of vring_init_kern at
vdpasim_set_vq_num. We could either call it with NULL vring addresses
to check for -EINVAL, or simply repeat the conditional (!num || num >
0xffff || (num & (num - 1))). I'd say the first one is better to not
go out of sync.

All of that can be done on top anyway, so for this patch:

Acked-by: Eugenio Pérez <eperezma@xxxxxxxxxx>