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

[Ruhl, Michael J]

>-----Original Message-----
>From: dri-devel <dri-devel-bounces at lists.freedesktop.org> On Behalf Of
>Daniel Vetter
>Sent: Wednesday, April 1, 2020 7:35 AM
>To: Christian König <ckoenig.leichtzumerken at gmail.com>
>Cc: amd-gfx at lists.freedesktop.org; dri-devel at lists.freedesktop.org
>Subject: Re: [PATCH 1/6] dma-buf: add peer2peer flag
>
>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.

The scatter list usage of the struct page pointer has other information in the
lower bits for keeping track of linking and other stuff.  Swizzling the page
pointers will probably make the scatter list unusable.

Mike

>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
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