On Fri, Jan 08, 2021 at 01:39:43AM +0800, Claire Chang wrote: > On Thu, Jan 7, 2021 at 2:58 AM Konrad Rzeszutek Wilk > <konrad.wilk@xxxxxxxxxx> 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. > > > > > > 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? > > It doesn't affect the existing SWIOTLB. It just utilizes the existing SWIOTLB > code to create another DMA pool tied to a specific memory region for a given set > of devices. It bounces the streaming DMA (map/unmap) in and out of that region > and does the memory allocation (dma_direct_alloc) from the same region. Right, so why can't it follow the same mechanism that Xen SWIOTLB does - which had exactly the same problem (needed special handling on the pool) - and do a similar code? > > > > > > > - 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? > > So the system needs to provide a way to lock down the memory access, e.g. MPU. OK! Would it be prudent to have this in the description above perhaps? > > > > > - 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)? > > Since restricted DMA is for the devices that are not behind an IOMMU, I change > the criteria > `if (unlikely(swiotlb_force == SWIOTLB_FORCE))` > to > `if (unlikely(swiotlb_force == SWIOTLB_FORCE) || dev->dma_io_tlb_mem)` > in dma_direct_map_page(). > > Also, even if SWIOTLB=force, the restricted DMA pool is preferred if available > (get_io_tlb_mem in https://lore.kernel.org/patchwork/patch/1360995/). > > Thanks!
