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

[Rob Herring <robh@xxxxxxxxxx>]

On Wed, Jan 20, 2021 at 11:30 AM Robin Murphy <robin.murphy@xxxxxxx> wrote:
>
> On 2021-01-20 16:53, Rob Herring wrote:
> > 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.
> >
> > If this goes into DT, I think we should be able to use dma-ranges for
> > this purpose instead. Normally, 'dma-ranges' is for physical bus
> > restrictions, but there's no reason it can't be used for policy or to
> > express restrictions the firmware has enabled.
>
> There would still need to be some way to tell SWIOTLB to pick up the
> corresponding chunk of memory and to prevent the kernel from using it
> for anything else, though.

Don't we already have that problem if dma-ranges had a very small
range? We just get lucky because the restriction is generally much
more RAM than needed.

In any case, wouldn't finding all the dma-ranges do this? We're
already walking the tree to find the max DMA address now.

> >> 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.
> >>           - vendor specific string in the form <vendor>,[<device>-]<usage>
> >>   no-map (optional) - empty property
> >>       - Indicates the operating system must not create a virtual mapping
> >> @@ -120,6 +134,11 @@ one for multimedia processing (named 
> >> multimedia-memory@77000000, 64MiB).
> >>                      compatible = "acme,multimedia-memory";
> >>                      reg = <0x77000000 0x4000000>;
> >>              };
> >> +
> >> +            restricted_dma_mem_reserved: restricted_dma_mem_reserved {
> >> +                    compatible = "restricted-dma-pool";
> >> +                    reg = <0x50000000 0x400000>;
> >> +            };
> >>      };
> >>
> >>      /* ... */
> >> @@ -138,4 +157,9 @@ one for multimedia processing (named 
> >> multimedia-memory@77000000, 64MiB).
> >>              memory-region = <&multimedia_reserved>;
> >>              /* ... */
> >>      };
> >> +
> >> +    pcie_device: pcie_device@0,0 {
> >> +            memory-region = <&restricted_dma_mem_reserved>;
> >
> > PCI hosts often have inbound window configurations that limit the
> > address range and translate PCI to bus addresses. Those windows happen
> > to be configured by dma-ranges. In any case, wouldn't you want to put
> > the configuration in the PCI host node? Is there a usecase of
> > restricting one PCIe device and not another?
>
> The general design seems to accommodate devices having their own pools
> such that they can't even snoop on each others' transient DMA data. If
> the interconnect had a way of wiring up, say, PCI RIDs to AMBA NSAIDs,
> then in principle you could certainly apply that to PCI endpoints too
> (presumably you'd also disallow them from peer-to-peer transactions at
> the PCI level too).

At least for PCI, I think we can handle this. We have the BDF in the
3rd address cell in dma-ranges. The Openfirmware spec says those are 0
in the case of ranges. It doesn't talk about dma-ranges though. But I
think we could extend it to allow for BDF. Though typically with PCIe
every device is behind its own bridge and each bridge node can have a
dma-ranges.

Rob