[PATCH 01/12] dma-buf: add dynamic caching of sg_table

[Liam Mark]

On Tue, 16 Apr 2019, Christian König wrote:

> To allow a smooth transition from pinning buffer objects to dynamic
> invalidation we first start to cache the sg_table for an attachment
> unless the driver explicitly says to not do so.
> 
> ---
>  drivers/dma-buf/dma-buf.c | 24 ++++++++++++++++++++++++
>  include/linux/dma-buf.h   | 11 +++++++++++
>  2 files changed, 35 insertions(+)
> 
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index 7c858020d14b..65161a82d4d5 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -573,6 +573,20 @@ struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
>  	list_add(&attach->node, &dmabuf->attachments);
>  
>  	mutex_unlock(&dmabuf->lock);
> +
> +	if (!dmabuf->ops->dynamic_sgt_mapping) {
> +		struct sg_table *sgt;
> +
> +		sgt = dmabuf->ops->map_dma_buf(attach, DMA_BIDIRECTIONAL);
> +		if (!sgt)
> +			sgt = ERR_PTR(-ENOMEM);
> +		if (IS_ERR(sgt)) {
> +			dma_buf_detach(dmabuf, attach);
> +			return ERR_CAST(sgt);
> +		}
> +		attach->sgt = sgt;
> +	}
> +
>  	return attach;
>  
>  err_attach:
> @@ -595,6 +609,10 @@ void dma_buf_detach(struct dma_buf *dmabuf, struct dma_buf_attachment *attach)
>  	if (WARN_ON(!dmabuf || !attach))
>  		return;
>  
> +	if (attach->sgt)
> +		dmabuf->ops->unmap_dma_buf(attach, attach->sgt,
> +					   DMA_BIDIRECTIONAL);
> +
>  	mutex_lock(&dmabuf->lock);
>  	list_del(&attach->node);
>  	if (dmabuf->ops->detach)
> @@ -630,6 +648,9 @@ struct sg_table *dma_buf_map_attachment(struct dma_buf_attachment *attach,
>  	if (WARN_ON(!attach || !attach->dmabuf))
>  		return ERR_PTR(-EINVAL);
>  
> +	if (attach->sgt)
> +		return attach->sgt;
> +

I am concerned by this change to make caching the sg_table the default 
behavior as this will result in the exporter's map_dma_buf/unmap_dma_buf 
calls are no longer being called in 
dma_buf_map_attachment/dma_buf_unmap_attachment.
	
This seems concerning to me as it appears to ignore the cache maintenance 
aspect of the map_dma_buf/unmap_dma_buf calls.
For example won't this potentially cause issues for clients of ION.

If we had the following
- #1 dma_buf_attach coherent_device
- #2 dma_buf attach non_coherent_device
- #3 dma_buf_map_attachment non_coherent_device
- #4 non_coherent_device writes to buffer
- #5 dma_buf_unmap_attachment non_coherent_device
- #6 dma_buf_map_attachment coherent_device
- #7 coherent_device reads buffer
- #8 dma_buf_unmap_attachment coherent_device	

There wouldn't be any CMO at step #5 anymore (specifically no invalidate) 
so now at step #7 the coherent_device could read a stale cache line.

Also, now by default dma_buf_unmap_attachment no longer removes the 
mappings from the iommu, so now by default dma_buf_unmap_attachment is not 
doing what I would expect and clients are losing the potential sandboxing 
benefits of removing the mappings.
Shouldn't this caching behavior be something that clients opt into instead 
of being the default?

Liam

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