Re: [PATCH net-next v3 0/2] dma-buf: heaps: system: add an option to allocate explicitly decrypted memory

Mostafa Saleh
2026-03-17T13:24:19Z

Hi Jiri,

On Thu, Mar 05, 2026 at 01:36:39PM +0100, Jiri Pirko wrote:
> From: Jiri Pirko <jiri@nvidia.com>
>
> Confidential computing (CoCo) VMs/guests, such as AMD SEV and Intel TDX,
> run with encrypted/protected memory which creates a challenge
> for devices that do not support DMA to it (no TDISP support).
>
> For kernel-only DMA operations, swiotlb bounce buffering provides a
> transparent solution by copying data through decrypted memory.
> However, the only way to get this memory into userspace is via the DMA
> API's dma_alloc_pages()/dma_mmap_pages() type interfaces which limits
> the use of the memory to a single DMA device, and is incompatible with
> pin_user_pages().
>
> These limitations are particularly problematic for the RDMA subsystem
> which makes heavy use of pin_user_pages() and expects flexible memory
> usage between many different DMA devices.
>
> This patch series enables userspace to explicitly request decrypted
> (shared) memory allocations from the dma-buf system heap.
> Userspace can mmap this memory and pass the dma-buf fd to other
> existing importers such as RDMA or DRM devices to access the
> memory. The DMA API is improved to allow the dma heap exporter to DMA
> map the shared memory to each importing device.

I have been looking into a similar problem with restricted-dma[1] and
the inability of the DMA API to recognize that a block of memory is
already decrypted.

However, in your case, adding a new attr “DMA_ATTR_CC_DECRYPTED” works
well as dma-buf owns the memory, and is both responsible for the
set_memory_decrypted() and passing the DMA attrs.

On the other hand, for restricted-dma, the memory decryption is deep
in the DMA direct memory allocation and the DMA API callers (for ex
virtio drivers) are clueless about it and can’t pass any attrs.
My proposal was specific to restricted-dma and won’t work for your case.

I am wondering if the kernel should have a more solid, unified method
for identifying already-decrypted memory instead. Perhaps we need a
way for the DMA API to natively recognize the encryption state of a
physical page (working alongside force_dma_unencrypted(dev)), rather
than relying on caller-provided attributes?

[1] https://lore.kernel.org/all/20260305170335.963568-1-smostafa@google.com/

Thanks,
Mostafa

>
> Jiri Pirko (2):
> dma-mapping: introduce DMA_ATTR_CC_DECRYPTED for pre-decrypted memory
> dma-buf: heaps: system: add system_cc_decrypted heap for explicitly
> decrypted memory
>
> drivers/dma-buf/heaps/system_heap.c | 103 ++++++++++++++++++++++++++--
> include/linux/dma-mapping.h | 6 ++
> include/trace/events/dma.h | 3 +-
> kernel/dma/direct.h | 14 +++-
> 4 files changed, 117 insertions(+), 9 deletions(-)
>
> --
> 2.51.1
>