Re: [PATCH v8 09/23] dma-mapping: Add internal shared allocation attribute

[Mostafa Saleh <smostafa@google.com>]

On Fri, Jul 17, 2026 at 11:34:27PM +0530, Aneesh Kumar K.V (Arm) wrote:
> 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>

Thanks,
Mostafa

> ---
>  Documentation/core-api/dma-attributes.rst | 29 +++++++++++++++++++++++
>  include/linux/dma-mapping.h               |  8 +++++++
>  include/trace/events/dma.h                |  3 ++-
>  3 files changed, 39 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/core-api/dma-attributes.rst b/Documentation/core-api/dma-attributes.rst
> index 123c8468d58f..eee743184acd 100644
> --- a/Documentation/core-api/dma-attributes.rst
> +++ b/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.
> diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> index cc0823a99cfd..a3e880649fa4 100644
> --- a/include/linux/dma-mapping.h
> +++ b/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
> diff --git a/include/trace/events/dma.h b/include/trace/events/dma.h
> index 31c9ddf72c9d..9df02c1511de 100644
> --- a/include/trace/events/dma.h
> +++ b/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,
> -- 
> 2.43.0
>