Re: [PATCH v5 01/16] x86/mm: Move force_dma_unencrypted() to common code

[Sathyanarayanan Kuppuswamy]

On 10/20/21 9:11 AM, Tom Lendacky wrote:
> > Intel TDX doesn't allow VMM to access guest private memory. Any memory
> > that is required for communication with VMM must be shared explicitly
> > by setting the bit in page table entry. After setting the shared bit,
> > the conversion must be completed with MapGPA hypercall. Details about
> > MapGPA hypercall can be found in [1], sec 3.2.
> >
> > The call informs VMM about the conversion between private/shared
> > mappings. The shared memory is similar to unencrypted memory in AMD
> > SME/SEV terminology but the underlying process of sharing/un-sharing
> > the memory is different for Intel TDX guest platform.
> >
> > SEV assumes that I/O devices can only do DMA to "decrypted" physical
> > addresses without the C-bit set. In order for the CPU to interact with
> > this memory, the CPU needs a decrypted mapping. To add this support,
> > AMD SME code forces force_dma_unencrypted() to return true for
> > platforms that support AMD SEV feature. It will be used for DMA memory
> > allocation API to trigger set_memory_decrypted() for platforms that
> > support AMD SEV feature.
> >
> > TDX is similar. So, to communicate with I/O devices, related pages need
> > to be marked as shared. As mentioned above, shared memory in TDX
> > architecture is similar to decrypted memory in AMD SME/SEV. So similar
> > to AMD SEV, force_dma_unencrypted() has to forced to return true. This
> > support is added in other patches in this series.
> >
> > So move force_dma_unencrypted() out of AMD specific code and call AMD
> > specific (amd_force_dma_unencrypted()) initialization function from it.
> > force_dma_unencrypted() will be modified by later patches to include
> > Intel TDX guest platform specific initialization.
> >
> > Also, introduce new config option X86_MEM_ENCRYPT_COMMON that has to be
> > selected by all x86 memory encryption features. This will be selected
> > by both AMD SEV and Intel TDX guest config options.
> >
> > This is preparation for TDX changes in DMA code and it has no
> > functional change.
>
> Can force_dma_unencrypted() be moved to arch/x86/kernel/cc_platform.c, 
> instead of creating a new file? It might fit better with patch #6.

Please check the final version of mem_encrypt_common.c

https://github.com/intel/tdx/blob/guest/arch/x86/mm/mem_encrypt_common.c

I am not sure whether it is alright to move mem_encrypt_init() and
arch_has_restricted_virtio_memory_access() to cc_platform.c

If this is fine, I can get rid of mem_encrypt_common.c

--
Sathyanarayanan Kuppuswamy
Linux Kernel Developer