dma-mapping: Add internal shared allocation attribute

DMA_ATTR_CC_SHARED describes an existing DMA mapping whose backing memory
is already shared, or decrypted, for confidential computing. It is a
mapping attribute: callers use it to request a shared DMA address encoding
for memory that has already been prepared for shared DMA.

Allocation paths need a related but different state. Once the DMA core
decides that an allocation must use shared backing pages, the lower-level
allocation helpers need to select shared pools, decrypt newly allocated
pages, derive the DMA address with the shared-memory translation and
restore encryption on free. That state is internal to the DMA-mapping
implementation and should not be passed by drivers to dma_alloc_attrs().

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@kernel.org>
Reviewed-by: Mostafa Saleh <smostafa@google.com>
Link: https://lore.kernel.org/r/20260717180442.110954-10-aneesh.kumar@kernel.org
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>

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 is an internal DMA-mapping attribute for confidential computing guests.
+It is used by allocation paths after the DMA core has determined that the
+backing pages must be shared, or decrypted. For example, the direct DMA and
+SWIOTLB allocation paths use it to select shared DMA pools, decrypt newly
+allocated pages, derive DMA addresses using the shared-memory translation, and
+restore encryption on free.
+
+__DMA_ATTR_ALLOC_CC_SHARED differs from DMA_ATTR_CC_SHARED in that it is not
+a caller-visible DMA API attribute. DMA_ATTR_CC_SHARED describes an
+already-shared mapping and requires the caller to have prepared normal
+system memory before mapping it.

include/linux/dma-mapping.h

@@ -103,6 +103,14 @@
  */
 #define DMA_ATTR_CC_SHARED	(1UL << 13)
 
+/*
+ * __DMA_ATTR_ALLOC_CC_SHARED: Internal DMA-mapping attribute used by
+ * allocation paths that create shared (decrypted) backing pages for
+ * confidential computing guests. Drivers must not pass this attribute to
+ * dma_alloc_attrs().
+ */
+#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

include/trace/events/dma.h

@@ -35,7 +35,8 @@ TRACE_DEFINE_ENUM(DMA_NONE);
 		{ DMA_ATTR_MMIO, "MMIO" }, \
 		{ DMA_ATTR_DEBUGGING_IGNORE_CACHELINES, "CACHELINES_OVERLAP" }, \
 		{ DMA_ATTR_REQUIRE_COHERENT, "REQUIRE_COHERENT" }, \
-		{ DMA_ATTR_CC_SHARED, "CC_SHARED" })
+		{ DMA_ATTR_CC_SHARED, "CC_SHARED" }, \
+		{ __DMA_ATTR_ALLOC_CC_SHARED, "ALLOC_CC_SHARED" })
 
 DECLARE_EVENT_CLASS(dma_map,
 	TP_PROTO(struct device *dev, phys_addr_t phys_addr, dma_addr_t dma_addr,