Re: [PATCH net-next v3 2/2] dma-buf: heaps: system: add system_cc_decrypted heap for explicitly decrypted memory

[Jason Gunthorpe <jgg@ziepe.ca>]

On Mon, Mar 09, 2026 at 09:39:44AM -0600, Peter Gonda wrote:
> Great feature to have thanks Jiri! A couple naive questions.
> 
> On Thu, Mar 5, 2026 at 5:38 AM Jiri Pirko <jiri@resnulli.us> wrote:
> >
> > From: Jiri Pirko <jiri@nvidia.com>
> >
> > Add a new "system_cc_decrypted" dma-buf heap to allow userspace to
> > allocate decrypted (shared) memory for confidential computing (CoCo)
> > VMs.
> >
> > On CoCo VMs, guest memory is encrypted by default. The hardware uses an
> > encryption bit in page table entries (C-bit on AMD SEV, "shared" bit on
> > Intel TDX) to control whether a given memory access is encrypted or
> > decrypted. The kernel's direct map is set up with encryption enabled,
> > so pages returned by alloc_pages() are encrypted in the direct map
> > by default. To make this memory usable for devices that do not support
> > DMA to encrypted memory (no TDISP support), it has to be explicitly
> > decrypted. A couple of things are needed to properly handle
> > decrypted memory for the dma-buf use case:
> >
> > - set_memory_decrypted() on the direct map after allocation:
> >   Besides clearing the encryption bit in the direct map PTEs, this
> >   also notifies the hypervisor about the page state change. On free,
> >   the inverse set_memory_encrypted() must be called before returning
> >   pages to the allocator. If re-encryption fails, pages
> >   are intentionally leaked to prevent decrypted memory from being
> >   reused as private.
> >
> > - pgprot_decrypted() for userspace and kernel virtual mappings:
> >   Any new mapping of the decrypted pages, be it to userspace via
> >   mmap or to kernel vmalloc space via vmap, creates PTEs independent
> >   of the direct map. These must also have the encryption bit cleared,
> >   otherwise accesses through them would see encrypted (garbage) data.
> 
> So this only works on new mappings? What if there are existing
> mappings to the memory that will be converted to shared?

The set_memory_decrypted() is called during system_heap_allocate(), it
is not possible to change dynamically between encrypted/decrypted.

Once the heap is created every PTE is always created with the correct
pgprot.

Jason