Re: [RFC PATCH v3 5/6] dt-bindings: of: Add restricted DMA pool

[Konrad Rzeszutek Wilk]

On Wed, Jan 06, 2021 at 11:41:23AM +0800, Claire Chang wrote:
> Introduce the new compatible string, restricted-dma-pool, for restricted
> DMA. One can specify the address and length of the restricted DMA memory
> region by restricted-dma-pool in the device tree.
> 
> Signed-off-by: Claire Chang <tientzu@xxxxxxxxxxxx>
> ---
>  .../reserved-memory/reserved-memory.txt       | 24 +++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt b/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt
> index e8d3096d922c..44975e2a1fd2 100644
> --- a/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt
> +++ b/Documentation/devicetree/bindings/reserved-memory/reserved-memory.txt
> @@ -51,6 +51,20 @@ compatible (optional) - standard definition
>            used as a shared pool of DMA buffers for a set of devices. It can
>            be used by an operating system to instantiate the necessary pool
>            management subsystem if necessary.
> +        - restricted-dma-pool: This indicates a region of memory meant to be
> +          used as a pool of restricted DMA buffers for a set of devices. The
> +          memory region would be the only region accessible to those devices.
> +          When using this, the no-map and reusable properties must not be set,
> +          so the operating system can create a virtual mapping that will be used
> +          for synchronization. The main purpose for restricted DMA is to
> +          mitigate the lack of DMA access control on systems without an IOMMU,
> +          which could result in the DMA accessing the system memory at
> +          unexpected times and/or unexpected addresses, possibly leading to data
> +          leakage or corruption. The feature on its own provides a basic level
> +          of protection against the DMA overwriting buffer contents at
> +          unexpected times. However, to protect against general data leakage and
> +          system memory corruption, the system needs to provide way to restrict
> +          the DMA to a predefined memory region.

Heya!

I think I am missing something obvious here so please bear with my
questions:

 - This code adds the means of having the SWIOTLB pool tied to a specific
   memory correct?

 - Nothing stops the physical device from bypassing the SWIOTLB buffer.
   That is if an errant device screwed up the length or DMA address, the
   SWIOTLB would gladly do what the device told it do?

 - This has to be combined with SWIOTLB-force-ish to always use the
   bounce buffer, otherwise you could still do DMA without using
   SWIOTLB (by not hitting the criteria for needing to use SWIOTLB)?