Jason Gunthorpe [off-list ref] writes:
On Wed, Jun 10, 2026 at 01:37:26PM +0530, Aneesh Kumar K.V wrote:
quoted
Jason Gunthorpe [off-list ref] writes:
quoted
On Thu, Jun 04, 2026 at 02:09:43PM +0530, Aneesh Kumar K.V (Arm) wrote:
quoted
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