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+.. SPDX-License-Identifier: GPL-2.0
+
+=====================================
+Intel Trust Domain Extensions (TDX)
+=====================================
+
+Intel's Trust Domain Extensions (TDX) protect confidential guest VMs from
+the host and physical attacks by isolating the guest register state and by
+encrypting the guest memory. In TDX, a special module running in a special
+mode sits between the host and the guest and manages the guest/host
+separation.
+
+Since the host cannot directly access guest registers or memory, much
+normal functionality of a hypervisor must be moved into the guest. This is
+implemented using a Virtualization Exception (#VE) that is handled by the
+guest kernel. A #VE is handled entirely inside the guest kernel, but some
+require the hypervisor to be consulted.
+
+TDX includes new hypercall-like mechanisms for communicating from the
+guest to the hypervisor or the TDX module.
+
+New TDX Exceptions
+==================
+
+TDX guests behave differently from bare-metal and traditional VMX guests.
+In TDX guests, otherwise normal instructions or memory accesses can cause
+#VE or #GP exceptions.
+
+Instructions marked with an '*' conditionally cause exceptions.  The
+details for these instructions are discussed below.
+
+Instruction-based #VE
+---------------------
+
+- Port I/O (INS, OUTS, IN, OUT)
+- HLT
+- MONITOR, MWAIT
+- WBINVD, INVD
+- VMCALL
+- RDMSR*,WRMSR*
+- CPUID*
+
+Instruction-based #GP
+---------------------
+
+- All VMX instructions: INVEPT, INVVPID, VMCLEAR, VMFUNC, VMLAUNCH,
+  VMPTRLD, VMPTRST, VMREAD, VMRESUME, VMWRITE, VMXOFF, VMXON
+- ENCLS, ENCLU
+- GETSEC
+- RSM
+- ENQCMD
+- RDMSR*,WRMSR*
+
+RDMSR/WRMSR Behavior
+--------------------
+
+MSR access behavior falls into three categories:
+
+- #GP generated
+- #VE generated
+- "Just works"
+
+In general, the #GP MSRs should not be used in guests.  Their use likely
+indicates a bug in the guest.  The guest may try to handle the #GP with a
+hypercall but it is unlikely to succeed.
+
+The #VE MSRs are typically able to be handled by the hypervisor.  Guests
+can make a hypercall to the hypervisor to handle the #VE.
+
+The "just works" MSRs do not need any special guest handling.  They might
+be implemented by directly passing through the MSR to the hardware or by
+trapping and handling in the TDX module.  Other than possibly being slow,
+these MSRs appear to function just as they would on bare metal.
+
+CPUID Behavior
+--------------
+
+For some CPUID leaves and sub-leaves, the virtualized bit fields of CPUID
+return values (in guest EAX/EBX/ECX/EDX) are configurable by the
+hypervisor. For such cases, the Intel TDX module architecture defines two
+virtualization types:
+
+- Bit fields for which the hypervisor controls the value seen by the guest
+  TD.
+
+- Bit fields for which the hypervisor configures the value such that the
+  guest TD either sees their native value or a value of 0.  For these bit
+  fields, the hypervisor can mask off the native values, but it can not
+  turn *on* values.
+
+A #VE is generated for CPUID leaves and sub-leaves that the TDX module does
+not know how to handle. The guest kernel may ask the hypervisor for the
+value with a hypercall.
+
+#VE on Memory Accesses
+======================
+
+There are essentially two classes of TDX memory: private and shared.
+Private memory receives full TDX protections.  Its content is protected
+against access from the hypervisor.  Shared memory is expected to be
+shared between guest and hypervisor and does not receive full TDX
+protections.
+
+A TD guest is in control of whether its memory accesses are treated as
+private or shared.  It selects the behavior with a bit in its page table
+entries.  This helps ensure that a guest does not place sensitive
+information in shared memory, exposing it to the untrusted hypervisor.
+
+#VE on Shared Memory
+--------------------
+
+Access to shared mappings can cause a #VE.  The hypervisor ultimately
+controls whether a shared memory access causes a #VE, so the guest must be
+careful to only reference shared pages it can safely handle a #VE.  For
+instance, the guest should be careful not to access shared memory in the
+#VE handler before it reads the #VE info structure (TDG.VP.VEINFO.GET).
+
+Shared mapping content is entirely controlled by the hypervisor. The guest
+should only use shared mappings for communicating with the hypervisor.
+Shared mappings must never be used for sensitive memory content like kernel
+stacks.  A good rule of thumb is that hypervisor-shared memory should be
+treated the same as memory mapped to userspace.  Both the hypervisor and
+userspace are completely untrusted.
+
+MMIO for virtual devices is implemented as shared memory.  The guest must
+be careful not to access device MMIO regions unless it is also prepared to
+handle a #VE.
+
+#VE on Private Pages
+--------------------
+
+An access to private mappings can also cause a #VE.  Since all kernel
+memory is also private memory, the kernel might theoretically need to
+handle a #VE on arbitrary kernel memory accesses.  This is not feasible, so
+TDX guests ensure that all guest memory has been "accepted" before memory
+is used by the kernel.
+
+A modest amount of memory (typically 512M) is pre-accepted by the firmware
+before the kernel runs to ensure that the kernel can start up without
+being subjected to a #VE.
+
+The hypervisor is permitted to unilaterally move accepted pages to a
+"blocked" state. However, if it does this, page access will not generate a
+#VE.  It will, instead, cause a "TD Exit" where the hypervisor is required
+to handle the exception.
+
+Linux #VE handler
+=================
+
+Just like page faults or #GP's, #VE exceptions can be either handled or be
+fatal.  Typically, an unhandled userspace #VE results in a SIGSEGV.
+An unhandled kernel #VE results in an oops.
+
+Handling nested exceptions on x86 is typically nasty business.  A #VE
+could be interrupted by an NMI which triggers another #VE and hilarity
+ensues.  The TDX #VE architecture anticipated this scenario and includes a
+feature to make it slightly less nasty.
+
+During #VE handling, the TDX module ensures that all interrupts (including
+NMIs) are blocked.  The block remains in place until the guest makes a
+TDG.VP.VEINFO.GET TDCALL.  This allows the guest to control when interrupts
+or a new #VE can be delivered.
+
+However, the guest kernel must still be careful to avoid potential
+#VE-triggering actions (discussed above) while this block is in place.
+While the block is in place, any #VE is elevated to a double fault (#DF)
+which is not recoverable.
+
+MMIO handling
+=============
+
+In non-TDX VMs, MMIO is usually implemented by giving a guest access to a
+mapping which will cause a VMEXIT on access, and then the hypervisor
+emulates the access.  That is not possible in TDX guests because VMEXIT
+will expose the register state to the host. TDX guests don't trust the host
+and can't have their state exposed to the host.
+
+In TDX, MMIO regions typically trigger a #VE exception in the guest.  The
+guest #VE handler then emulates the MMIO instruction inside the guest and
+converts it into a controlled TDCALL to the host, rather than exposing
+guest state to the host.
+
+MMIO addresses on x86 are just special physical addresses. They can
+theoretically be accessed with any instruction that accesses memory.
+However, the kernel instruction decoding method is limited. It is only
+designed to decode instructions like those generated by io.h macros.
+
+MMIO access via other means (like structure overlays) may result in an
+oops.
+
+Shared Memory Conversions
+=========================
+
+All TDX guest memory starts out as private at boot.  This memory can not
+be accessed by the hypervisor.  However, some kernel users like device
+drivers might have a need to share data with the hypervisor.  To do this,
+memory must be converted between shared and private.  This can be
+accomplished using some existing memory encryption helpers:
+
+ * set_memory_decrypted() converts a range of pages to shared.
+ * set_memory_encrypted() converts memory back to private.
+
+Device drivers are the primary user of shared memory, but there's no need
+to touch every driver. DMA buffers and ioremap() do the conversions
+automatically.
+
+TDX uses SWIOTLB for most DMA allocations. The SWIOTLB buffer is
+converted to shared on boot.
+
+For coherent DMA allocation, the DMA buffer gets converted on the
+allocation. Check force_dma_unencrypted() for details.
+
+Attestation
+===========
+
+Attestation is used to verify the TDX guest trustworthiness to other
+entities before provisioning secrets to the guest. For example, a key
+server may want to use attestation to verify that the guest is the
+desired one before releasing the encryption keys to mount the encrypted
+rootfs or a secondary drive.
+
+The TDX module records the state of the TDX guest in various stages of
+the guest boot process using the build time measurement register (MRTD)
+and runtime measurement registers (RTMR). Measurements related to the
+guest initial configuration and firmware image are recorded in the MRTD
+register. Measurements related to initial state, kernel image, firmware
+image, command line options, initrd, ACPI tables, etc are recorded in
+RTMR registers. For more details, as an example, please refer to TDX
+Virtual Firmware design specification, section titled "TD Measurement".
+At TDX guest runtime, the attestation process is used to attest to these
+measurements.
+
+The attestation process consists of two steps: TDREPORT generation and
+Quote generation.
+
+TDX guest uses TDCALL[TDG.MR.REPORT] to get the TDREPORT (TDREPORT_STRUCT)
+from the TDX module. TDREPORT is a fixed-size data structure generated by
+the TDX module which contains guest-specific information (such as build
+and boot measurements), platform security version, and the MAC to protect
+the integrity of the TDREPORT. A user-provided 64-Byte REPORTDATA is used
+as input and included in the TDREPORT. Typically it can be some nonce
+provided by attestation service so the TDREPORT can be verified uniquely.
+More details about the TDREPORT can be found in Intel TDX Module
+specification, section titled "TDG.MR.REPORT Leaf".
+
+After getting the TDREPORT, the second step of the attestation process
+is to send it to the Quoting Enclave (QE) to generate the Quote. TDREPORT
+by design can only be verified on the local platform as the MAC key is
+bound to the platform. To support remote verification of the TDREPORT,
+TDX leverages Intel SGX Quoting Enclave to verify the TDREPORT locally
+and convert it to a remotely verifiable Quote. Method of sending TDREPORT
+to QE is implementation specific. Attestation software can choose
+whatever communication channel available (i.e. vsock or TCP/IP) to
+send the TDREPORT to QE and receive the Quote.
+
+References
+==========
+
+TDX reference material is collected here:
+
+https://www.intel.com/content/www/us/en/developer/articles/technical/intel-trust-domain-extensions.html