Re: [PATCH v6 04/20] dma-pool: track decrypted atomic pools and select them via attrs

[Aneesh Kumar K.V <aneesh.kumar@kernel.org>]

Jason Gunthorpe <jgg@ziepe.ca> writes:

> On Wed, Jun 10, 2026 at 01:37:26PM +0530, Aneesh Kumar K.V wrote:
>> Jason Gunthorpe <jgg@ziepe.ca> 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