RE: [PATCH v11 00/20] TDX host kernel support

[Dan Williams]

Kai Huang wrote:
> Intel Trusted Domain Extensions (TDX) protects guest VMs from malicious
> host and certain physical attacks.  TDX specs are available in [1].
> 
> This series is the initial support to enable TDX with minimal code to
> allow KVM to create and run TDX guests.  KVM support for TDX is being
> developed separately[2].  A new "userspace inaccessible memfd" approach
> to support TDX private memory is also being developed[3].  The KVM will
> only support the new "userspace inaccessible memfd" as TDX guest memory.

This memfd approach is incompatible with one of the primary ways that
new memory topologies like high-bandwidth-memory and CXL are accessed,
via a device-special-file mapping. There is already precedent for mmap()
to only be used for communicating address value and not CPU accessible
memory. See "Userspace P2PDMA with O_DIRECT NVMe devices" [1].

So before this memfd requirement becomes too baked in to the design I
want to understand if "userspace inaccessible" is the only requirement
so I can look to add that to the device-special-file interface for
"device" / "Soft Reserved" memory like HBM and CXL.

[1]: https://lore.kernel.org/all/20221021174116.7200-1-logang@xxxxxxxxxxxx/