Linux Virtualization - May 2020 - [PATCH v3 1/4] dma-buf: add support for virtio exported objects

On Wed, Mar 11, 2020 at 12:20 PM David Stevens <stevensd at chromium.org>
wrote:
>
> This change adds a new dma-buf operation that allows dma-bufs to be used
> by virtio drivers to share exported objects. The new operation allows
> the importing driver to query the exporting driver for the UUID which
> identifies the underlying exported object.
>
> Signed-off-by: David Stevens <stevensd at chromium.org>
Adding Tomasz Figa, I've discussed this with him at elce last year I
think. Just to make sure.

Bunch of things:
- obviously we need the users of this in a few drivers, can't really
review anything stand-alone
- adding very specific ops to the generic interface is rather awkward,
eventually everyone wants that and we end up in a mess. I think the
best solution here would be if we create a struct virtio_dma_buf which
subclasses dma-buf, add a (hopefully safe) runtime upcasting
functions, and then a virtio_dma_buf_get_uuid() function. Just storing
the uuid should be doable (assuming this doesn't change during the
lifetime of the buffer), so no need for a callback.
- for the runtime upcasting the usual approach is to check the ->ops
pointer. Which means that would need to be the same for all virtio
dma_bufs, which might get a bit awkward. But I'd really prefer we not
add allocator specific stuff like this to dma-buf.
-Daniel
> ---
>  drivers/dma-buf/dma-buf.c | 12 ++++++++++++
>  include/linux/dma-buf.h   | 18 ++++++++++++++++++
>  2 files changed, 30 insertions(+)
>
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index d4097856c86b..fa5210ba6aaa 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -1158,6 +1158,18 @@ void dma_buf_vunmap(struct dma_buf *dmabuf, void
*vaddr)
>  }
>  EXPORT_SYMBOL_GPL(dma_buf_vunmap);
>
> +int dma_buf_get_uuid(struct dma_buf *dmabuf, uuid_t *uuid)
> +{
> +       if (WARN_ON(!dmabuf) || !uuid)
> +               return -EINVAL;
> +
> +       if (!dmabuf->ops->get_uuid)
> +               return -ENODEV;
> +
> +       return dmabuf->ops->get_uuid(dmabuf, uuid);
> +}
> +EXPORT_SYMBOL_GPL(dma_buf_get_uuid);
> +
>  #ifdef CONFIG_DEBUG_FS
>  static int dma_buf_debug_show(struct seq_file *s, void *unused)
>  {
> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> index abf5459a5b9d..00758523597d 100644
> --- a/include/linux/dma-buf.h
> +++ b/include/linux/dma-buf.h
> @@ -251,6 +251,21 @@ struct dma_buf_ops {
>
>         void *(*vmap)(struct dma_buf *);
>         void (*vunmap)(struct dma_buf *, void *vaddr);
> +
> +       /**
> +        * @get_uuid
> +        *
> +        * This is called by dma_buf_get_uuid to get the UUID which
identifies
> +        * the buffer to virtio devices.
> +        *
> +        * This callback is optional.
> +        *
> +        * Returns:
> +        *
> +        * 0 on success or a negative error code on failure. On success
uuid
> +        * will be populated with the buffer's UUID.
> +        */
> +       int (*get_uuid)(struct dma_buf *dmabuf, uuid_t *uuid);
>  };
>
>  /**
> @@ -444,4 +459,7 @@ int dma_buf_mmap(struct dma_buf *, struct
vm_area_struct *,
>                  unsigned long);
>  void *dma_buf_vmap(struct dma_buf *);
>  void dma_buf_vunmap(struct dma_buf *, void *vaddr);
> +
> +int dma_buf_get_uuid(struct dma_buf *dmabuf, uuid_t *uuid);
> +
>  #endif /* __DMA_BUF_H__ */
> --
> 2.25.1.481.gfbce0eb801-goog
>

-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch

Hi,
> - for the runtime upcasting the usual approach is to check the ->ops
> pointer. Which means that would need to be the same for all virtio
> dma_bufs, which might get a bit awkward. But I'd really prefer we not
> add allocator specific stuff like this to dma-buf.
This is exactly the problem, it gets messy quickly, also when it comes
to using the drm_prime.c helpers ...

take care,
  Gerd

On Thu, May 14, 2020 at 09:59:52AM +0200, Gerd Hoffmann
wrote:
>   Hi,
> 
> > - for the runtime upcasting the usual approach is to check the
->ops
> > pointer. Which means that would need to be the same for all virtio
> > dma_bufs, which might get a bit awkward. But I'd really prefer we
not
> > add allocator specific stuff like this to dma-buf.
> 
> This is exactly the problem, it gets messy quickly, also when it comes
> to using the drm_prime.c helpers ...
drm_prime.c helpers (not the core bits) exist becaues nvidia needed
something that wasnt EXPORT_SYMBOL_GPL.

I wouldn't shed a big tear if they don't fit anymore, they're kinda
not
great to begin with. Much midlayer, not much of valued added, but at least
the _GPL is gone.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch