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

[Jason Gunthorpe <jgg@ziepe.ca>]

On Mon, Mar 09, 2026 at 06:51:21PM +0100, Jiri Pirko wrote:
> Mon, Mar 09, 2026 at 04:18:57PM +0100, jgg@ziepe.ca wrote:
> >On Mon, Mar 09, 2026 at 04:02:33PM +0200, Leon Romanovsky wrote:
> >> On Mon, Mar 09, 2026 at 10:15:30AM -0300, Jason Gunthorpe wrote:
> >> > On Sun, Mar 08, 2026 at 12:19:48PM +0200, Leon Romanovsky wrote:
> >> > 
> >> > > > +/*
> >> > > > + * 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)
> >> > > 
> >> > > While adding the new attribute is fine, I would expect additional checks in
> >> > > dma_map_phys() to ensure the attribute cannot be misused. For example,
> >> > > WARN_ON(attrs & (DMA_ATTR_CC_DECRYPTED | DMA_ATTR_MMIO)), along with a check
> >> > > that we are taking the direct path only.
> >> > 
> >> > DECRYPYED and MMIO is something that needs to work, VFIO (inside a
> >> > TVM) should be using that combination.
> >> 
> >> So this sentence "A struct page is required" from the comment above is
> >> not accurate.
> >
> >It would be clearer to say "Unless DMA_ATTR_MMIO is provided a struct
> >page is required"
> >
> >We need to audit if that works properly, IIRC it does, but I don't
> >remember.. Jiri?
> 
> How can you do set_memory_decrypted if you don't have page/folio ?

Alot of device MMIO is decrypted by nature and can't be encrypted, so
you'd have to use both flags. eg in VFIO we'd want to do this.

Jason