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

[Stefano Garzarella]

On Fri, Nov 11, 2022 at 04:40:33PM +0100, Eugenio Perez Martin wrote:
> 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.

I'm not sure about renaming since it is the value returned by 
vdpasim_get_vq_num_max, so IMHO the _MAX suffix is fine.
But I admit that initially I didn't understand whether it's the maximum 
number of queues or elements, so maybe VDPASIM_VQ_NUM_MAX is better.

> 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.

Right!

> 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.

Or we could do the check in vdpasim_set_vq_ready() and set it not ready 
if the vq_num is wrong.

> All of that can be done on top anyway, so for this patch:
>
> Acked-by: Eugenio Pérez <eperezma@xxxxxxxxxx>

Thanks for the review,
Stefano