Jason Gunthorpe <[email protected]> writes: > On Wed, Jun 10, 2026 at 01:37:26PM +0530, Aneesh Kumar K.V wrote: >> Jason Gunthorpe <[email protected]> writes: >> >> > On Thu, Jun 04, 2026 at 02:09:43PM +0530, Aneesh Kumar K.V (Arm) wrote: >> >> struct page *dma_alloc_from_pool(struct device *dev, size_t size, >> >> - void **cpu_addr, gfp_t gfp, >> >> + void **cpu_addr, gfp_t gfp, unsigned long attrs, >> >> bool (*phys_addr_ok)(struct device *, phys_addr_t, size_t)) >> >> { >> >> - struct gen_pool *pool = NULL; >> >> + struct dma_gen_pool *dma_pool = NULL; >> >> struct page *page; >> >> bool pool_found = false; >> >> >> >> - while ((pool = dma_guess_pool(pool, gfp))) { >> >> + while ((dma_pool = dma_guess_pool(dma_pool, gfp))) { >> >> + >> >> + if (dma_pool->unencrypted != !!(attrs & DMA_ATTR_CC_SHARED)) >> >> + continue; >> > >> > I don't think you should be overloading DMA_ATTR_CC_SHARED like this. >> > >> > /* >> > * DMA_ATTR_CC_SHARED is not a caller-visible dma_alloc_*() >> > * attribute. The direct allocator uses it internally after it has >> > * decided that the backing pages must be shared/decrypted, so the >> > * rest of the allocation path can consistently select DMA addresses, >> > * choose compatible pools and restore encryption on free. >> > */ >> > if (attrs & DMA_ATTR_CC_SHARED) >> > return NULL; >> > >> > if (force_dma_unencrypted(dev)) { >> > attrs |= DMA_ATTR_CC_SHARED; >> > mark_mem_decrypt = true; >> > } >> > >> > It is fine to have a bit inside the attrs that is only used by the >> > internal logic, but it needs to have a clearer name >> > __DMA_ATTR_REQUIRE_CC_SHARED perhaps. >> > >> >> Are you suggesting adding another attribute in addition to >> DMA_ATTR_CC_SHARED? >> >> Is the idea that __DMA_ATTR_REQUIRE_CC_SHARED would be used in the >> allocation path to request a CC_SHARED allocation, while >> DMA_ATTR_CC_SHARED would be used in the mapping path to describe the >> attribute of the address? > > Yeah, it is a thought at least > > Maybe a comment is good enough. > > I just find it hard to follow when we have this dual usage. Like the > code above for dma_pool->unencrypted is completely wrong if it is an > "attribute of an address". Easy to cut & paste that into the wrong > context. > > Especially if you move things up higher.. having the alloc set both > CC_SHARED and REQUIRE_CC_SHARED or maybe ALLOC_CC_SHARED would make it > clearer that the alloc code lives under that callchain > > Jason >
If we are adding DMA_ATTR_ALLOC_SHARED, should we also allow dma_alloc_attrs() to take that attribute value? Does this look okay? (Note: Parts of the documentation text were updated using Codex.) modified Documentation/core-api/dma-attributes.rst @@ -179,3 +179,32 @@ interface when building their uAPIs, when possible. It must never be used in an in-kernel driver that only works with kernel memory. + +DMA_ATTR_CC_SHARED +------------------ + +This attribute indicates that a DMA mapping is shared, or decrypted, for +confidential computing guests. For normal system memory, the caller must +already have marked the memory decrypted with set_memory_decrypted(). CPU +PTEs for the mapping must use pgprot_decrypted(), and the same shared +semantic may be passed to a vIOMMU when it sets up the IOPTE. + +This attribute describes an existing mapping. It does not allocate shared +backing pages and must not be passed to dma_alloc_attrs(). For MMIO, use +this together with DMA_ATTR_MMIO to indicate shared MMIO. Unless +DMA_ATTR_MMIO is provided, the mapping requires a struct page. + +DMA_ATTR_ALLOC_CC_SHARED +------------------------ + +This attribute indicates that a dma_alloc_attrs() allocation must use +shared, or decrypted, backing pages for confidential computing guests. +Allocation paths use this request when they select shared DMA pools, +decrypt newly allocated pages or restore encryption on free. + +DMA_ATTR_ALLOC_CC_SHARED differs from DMA_ATTR_CC_SHARED in that it +requests shared backing memory from the allocation path. DMA_ATTR_CC_SHARED +describes an already-shared mapping and requires the caller to have +prepared normal system memory before mapping it. Callers that need shared +memory from dma_alloc_attrs() should request DMA_ATTR_ALLOC_CC_SHARED +instead of DMA_ATTR_CC_SHARED. modified include/linux/dma-mapping.h @@ -103,6 +103,13 @@ */ #define DMA_ATTR_CC_SHARED (1UL << 13) +/* + * DMA_ATTR_ALLOC_CC_SHARED: Allocates DMA memory as shared (decrypted) for + * confidential computing guests. Unlike DMA_ATTR_CC_SHARED, this attribute + * is used by dma_alloc_attrs() paths that create shared backing pages; + * DMA_ATTR_CC_SHARED describes an already-shared mapping. + */ +#define DMA_ATTR_ALLOC_CC_SHARED (1UL << 14) /* * A dma_addr_t can hold any valid DMA or bus address for the platform. It can * be given to a device to use as a DMA source or target. It is specific to a
