On 09.07.2026 17:59, Thierry Reding wrote: > On Thu, Jul 09, 2026 at 07:56:45AM +0200, Marek Szyprowski wrote: >> On 08.07.2026 10:35, David Hildenbrand (Arm) wrote: >>> On 7/7/26 12:02, Marek Szyprowski wrote: >>>> On 01.07.2026 18:08, Thierry Reding wrote: >>>>> From: Thierry Reding <[email protected]> >>>>> >>>>> There is no technical reason why there should be a limited number of CMA >>>>> regions, so extract some code into helpers and use them to create extra >>>>> functions (cma_create() and cma_free()) that allow creating and freeing, >>>>> respectively, CMA regions dynamically at runtime. >>>> Well, the technical reason for not creating cma regions dynamically at >>>> runtime is that on some architectures (like 32bit ARM) the early fixup >>>> for the region is needed to make it functional for DMA. >>> Can you point me at the code that does that? Thanks! >> Check dma_contiguous_early_fixup() and dma_contiguous_remap() in >> arch/arm/mm/dma-mapping.c. Those functions ensures that the CPU mappings for >> the CMA reserved region in linear map are remapped with 4k pages instead >> of the 1M sections, so later, it will be possible to alter the mappings and >> change them to coherent when needed (altering 1M sections is not possible, >> because each process has it's own level-1 array even for the kernel linear >> mapping). >> >> >> >> However, in the use case in this patchset the reserved region is only shared >> with buddy allocator by using the CMA infrastructure, not registered to the >> regular DMA-mapping API, so it would work fine. I'm not convinced that this >> is the right API to use for this though. > Are you saying you're not convinced that CMA is the right API to use for > this? Or something else? I read this again and indeed CMA seems to be right solution. I only wonder why do You want to create the CMA areas dynamically? Imho it would work if You just create large enough CMA area on boot, what would automatically share the memory with buddy allocator and then allocate dynamic VPR regions with cma_alloc(), potentially unmapping or marking the allocated region as reserved in linear kernel mapping to avoid any potential speculative access to the protected memory.
In both cases You will probably won't need the DMA-mapping API on top of it, although it might be even possible to partially use with by registering custom dma_ops for the devices using the protected region (assuming that it would support only DMA_ATTR_NO_KERNEL_MAPPING allocations). > I certainly don't think we want to get the DMA-mapping API involved for > this because that always implies that we perform cache operations, which > we specifically don't want for this memory. > > Thierry Best regards -- Marek Szyprowski, PhD Samsung R&D Institute Poland
