Re: [PATCH V5 4/9] virtio: introduce vring_mapping_token

[Jason Wang <jasowang@redhat.com>]

On Thu, Aug 14, 2025 at 6:42 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Thu, Aug 14, 2025 at 11:36:22AM +0800, Jason Wang wrote:
> > > > diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> > > > index addbc209275a..37029df94aaf 100644
> > > > --- a/include/linux/virtio.h
> > > > +++ b/include/linux/virtio.h
> > > > @@ -40,6 +40,13 @@ struct virtqueue {
> > > >       void *priv;
> > > >  };
> > > >
> > > > +union vring_mapping_token {
> > > > +     /* Device that performs DMA */
> > > > +     struct device *dma_dev;
> > > > +     /* Transport specific token used for doing map */
> > > > +     void *opaque;
> > >
> > > Please just declare whatever structure you want it to be.
> >
> > It's an opaque one and so
> >
> > 1) the virtio core knows nothing about that because it could be
> > transport or device specific
> > 2) no assumption of the type and usage, it just receive it from the
> > transport and pass it back when doing the mapping
> >
> > It should work like page->private etc.
> >
> > Does this make sense?
> >
> > Thanks
>
> I fully expect most devices simply to use DMA here and no weird
> tricks. vduse is the weird one, but I don't see us making it
> grow much beyond that.
>
> So I think for now we can just make it vduse_iova_domain *.

It requires exporting VDUSE's internal data structure to the virtio
layer which is a kind of layer violation. And the iova domain will be
soon replaced by virtqueue group in Eungeio's series that support
multiple address spaces

> If we see
> it's getting out of hand with too many types, we can think of solutions.

Considering we've already at it, I think it's better to have a
solution now if it's not complicated:

I can think of two kind of solutions:

1) Stick to the current void * pointer which is simple and straightforward
2) Eugenio's suggestion of having a virtio_map_head and let the
transport/device get the container struct which is the real token.

E.g

struct vduse_iova_domain {
      struct virtio_map_head map;
      ......
};

If you don't like void *, can we go fo 2)?

Thanks


>
> --
> MST
>