Re: [PATCH net-next v3 1/2] dma-mapping: introduce DMA_ATTR_CC_DECRYPTED for pre-decrypted memory

Petr Tesarik
2026-03-09T12:56:21Z

On Thu, 5 Mar 2026 13:36:40 +0100
Jiri Pirko <jiri@resnulli.us> wrote:

> From: Jiri Pirko <jiri@nvidia.com>
>
> Current CC designs don't place a vIOMMU in front of untrusted devices.
> Instead, the DMA API forces all untrusted device DMA through swiotlb
> bounce buffers (is_swiotlb_force_bounce()) which copies data into
> decrypted memory on behalf of the device.
>
> When a caller has already arranged for the memory to be decrypted
> via set_memory_decrypted(), the DMA API needs to know so it can map
> directly using the unencrypted physical address rather than bounce
> buffering. Following the pattern of DMA_ATTR_MMIO, add
> DMA_ATTR_CC_DECRYPTED for this purpose. Like the MMIO case, only the
> caller knows what kind of memory it has and must inform the DMA API
> for it to work correctly.
>
> Signed-off-by: Jiri Pirko <jiri@nvidia.com>
> ---
> v1->v2:
> - rebased on top of recent dma-mapping-fixes
> ---
> include/linux/dma-mapping.h | 6 ++++++
> include/trace/events/dma.h | 3 ++-
> kernel/dma/direct.h | 14 +++++++++++---
> 3 files changed, 19 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> index 29973baa0581..ae3d85e494ec 100644
> --- a/include/linux/dma-mapping.h
> +++ b/include/linux/dma-mapping.h
> @@ -85,6 +85,12 @@
> * a cacheline must have this attribute for this to be considered safe.
> */
> #define DMA_ATTR_CPU_CACHE_CLEAN (1UL << 11)
> +/*
> + * DMA_ATTR_CC_DECRYPTED: Indicates memory that has been explicitly decrypted
> + * (shared) for confidential computing guests. The caller must have
> + * called set_memory_decrypted(). A struct page is required.
> + */
> +#define DMA_ATTR_CC_DECRYPTED (1UL << 12)

I don't want to start a bikeshedding discussion, so if everyone else
likes this name, let's keep it. But maybe the "_CC" (meaning
Confidential Comptuing) is not necessary. IIUC it's the same concept as
set_page_encrypted(), set_page_decrypted(), which does not refer to
CoCo either.

Just my two cents
Petr T