Trust Domain Extensions. Intel Trust Domain Extensions (TDX) is a CPU-level technology proposed by Intel in May 2021 for implementing a trusted execution environment in which virtual machines (called "Trust Domains", or TDs) are hardware-isolated from the host's Virtual Machine Monitor (VMM), hypervisor, and other software on the host. This hardware isolation is intended to prevent threat actors with administrative access or physical access to the virtual machine host from compromising aspects of the TD virtual machine's confidentiality and integrity. TDX also supports a remote attestation feature which allows users to determine that a remote system has TDX protections enabled prior to sending it sensitive data.

Intel TDX is of particular use for cloud providers, as it increases isolation of customer virtual machines and provides a higher level of assurance that the cloud provider cannot access the customer's data.

Intel TDX was described in and is pending US patent number 20210141658A1.

Architecture overview 
TDX consists of multiple components including Virtual Machine Extensions (VMX) instruction set architecture (ISA) extensions, a technology for memory encryption, and a new CPU operation mode called SEAM ("Secure Arbitration Mode"), which hosts the TDX module.

Memory protections 
TDX defines two classes of memory: shared memory and private memory. Shared memory is intended to be used for communicating with the TD host and may receive some TDX protections. Private memory received full TDX confidentiality and integrity protections.

TDX implements memory protection by encrypting the TD's memory with a per-TD AES-XTS 128-bit key. To avoid leaking ciphertext, memory access is limited to being from the SEAM mode and direct memory access is unavailable. If memory integrity protections are enabled, a MAC using SHA-3-256 is generated for the private memory and if the MAC validation fails, the TD VM is terminated. TD VM registers are also kept confidential by storing them in a per-TD save state and scrubbing them when the TD returns control to the TD VM.

Guest-hypervisor communication 

TDX provides hardware isolation of TD VMs by brokering all VMM to TD communication through the TDX module and preventing the VMM from accessing the TD's data. The VMM communicates to the TDX module using new SEAMCALL and SEAMRET CPU instructions. SEAMCALL is used by the VMM to invoke the TDX module to create, delete, or execute a TD. SEAMRET is used by the TDX module to return execution back to the VMM.

Remote attestation 
TDX's remote attestation feature builds on the SGX technology to allow someone to determine that a remote TD has TDX protections enabled prior to sending it sensitive data. The remote attestation report can be generated by the TDX module calling the SEAMREPORT instruction. The SEAMREPORT instruction generates a MAC-signed "Report" structure which includes information such as the version numbers of the TDX's components. The VMM would then use SGX enclaves to convert that "Report" structure into a remotely verifiable "Quote", which it would send to the system requesting attestation.

Hardware and operating system support 
TDX is available for 5th generation Intel Xeon processors (codename Emerald Rapids) and Edge Enhanced Compute variants of 4th generation Xeon processors (codename Sapphire Rapids).

First patches to support TDX technology in the Linux kernel were posted in the Linux kernel mailing list around June 2021, were merged on May 24, 2022, and were included in the mainline Linux Kernel version 5.19.

Microsoft Azure has announced that as of April 24, 2023 their new DCesv5-series and ECesv5-series virtual machines would support Intel TDX. They have also published information how to use Intel TDX as part of Microsoft Azure Attestation.

Comparisons to SGX 
TDX is somewhat similar to SGX, as in that both are implementations of trusted execution environments. However, they are significantly different in the scope of the protections and that SGX requires that applications be rewritten to support SGX, while TDX only requires support at the hardware and operating system levels. On the VMM host, TDX involves the use of SGX enclaves to enable support for remote attestation. Additionally, even an operating system which does not support running as a TD VM can be protected by being launched as a nested VM within a TD VM.

References 

Computer-related introductions in 2021
X86 instructions
Intel
Cybersecurity engineering