[PATCH 1/6] dma-buf: add peer2peer flag

[Daniel Vetter]

On Mon, Mar 30, 2020 at 03:55:31PM +0200, Christian König wrote:
> Add a peer2peer flag noting that the importer can deal with device
> resources which are not backed by pages.
> 
> Signed-off-by: Christian König <christian.koenig at amd.com>
> ---
>  drivers/dma-buf/dma-buf.c |  2 ++
>  include/linux/dma-buf.h   | 10 ++++++++++
>  2 files changed, 12 insertions(+)
> 
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index ccc9eda1bc28..570c923023e6 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -690,6 +690,8 @@ dma_buf_dynamic_attach(struct dma_buf *dmabuf, struct device *dev,
>  
>  	attach->dev = dev;
>  	attach->dmabuf = dmabuf;
> +	if (importer_ops)
> +		attach->peer2peer = importer_ops->allow_peer2peer;

So an idea that crossed my mind to validate this, since we need quite some
bad amounts of bad luck if someone accidentally introduces and access to
struct_page in sg lists in some slowpath.

On map_sg, if ->peer2peer is set, we could mangle the struct_page
pointers, e.g. swap high bits for low bits (so that NULL stays NULL). On
unmap_sg we obviously need to undo that, in case the exporter needs those
pointers for its own book-keeping for some reason. I was also pondering
just setting them all to NULL, but that might break some exporters. With
the pointer mangling trick (especially if we flip high for low bits on 64
where this should result in invalid addresses in almost all cases) we
should be able to catch buggy p2p importers quite quickly.

Thoughts? Maybe add as a follow-up patch for testing?
-Daniel
>  	attach->importer_ops = importer_ops;
>  	attach->importer_priv = importer_priv;
>  
> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> index 1ade486fc2bb..82e0a4a64601 100644
> --- a/include/linux/dma-buf.h
> +++ b/include/linux/dma-buf.h
> @@ -334,6 +334,14 @@ struct dma_buf {
>   * Attachment operations implemented by the importer.
>   */
>  struct dma_buf_attach_ops {
> +	/**
> +	 * @allow_peer2peer:
> +	 *
> +	 * If this is set to true the importer must be able to handle peer
> +	 * resources without struct pages.
> +	 */
> +	bool allow_peer2peer;
> +
>  	/**
>  	 * @move_notify
>  	 *
> @@ -362,6 +370,7 @@ struct dma_buf_attach_ops {
>   * @node: list of dma_buf_attachment, protected by dma_resv lock of the dmabuf.
>   * @sgt: cached mapping.
>   * @dir: direction of cached mapping.
> + * @peer2peer: true if the importer can handle peer resources without pages.
>   * @priv: exporter specific attachment data.
>   * @importer_ops: importer operations for this attachment, if provided
>   * dma_buf_map/unmap_attachment() must be called with the dma_resv lock held.
> @@ -382,6 +391,7 @@ struct dma_buf_attachment {
>  	struct list_head node;
>  	struct sg_table *sgt;
>  	enum dma_data_direction dir;
> +	bool peer2peer;
>  	const struct dma_buf_attach_ops *importer_ops;
>  	void *importer_priv;
>  	void *priv;
> -- 
> 2.17.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch