[PATCH v8 55/55] docs: Add TDX documentation

[Xiaoyao Li <xiaoyao.li@intel.com>]

Add docs/system/i386/tdx.rst for TDX support, and add tdx in
confidential-guest-support.rst

Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
---
Changes in v6:
 - Add more information of "Feature configuration"
 - Mark TD Attestation as future work because KVM now drops the support
   of it.

Changes in v5:
 - Add TD attestation section and update the QEMU parameter;

Changes since v1:
 - Add prerequisite of private gmem;
 - update example command to launch TD;

Changes since RFC v4:
 - add the restriction that kernel-irqchip must be split
---
 docs/system/confidential-guest-support.rst |   1 +
 docs/system/i386/tdx.rst                   | 156 +++++++++++++++++++++
 docs/system/target-i386.rst                |   1 +
 3 files changed, 158 insertions(+)
 create mode 100644 docs/system/i386/tdx.rst

diff --git a/docs/system/confidential-guest-support.rst b/docs/system/confidential-guest-support.rst
index 0c490dbda2b7..66129fbab64c 100644
--- a/docs/system/confidential-guest-support.rst
+++ b/docs/system/confidential-guest-support.rst
@@ -38,6 +38,7 @@ Supported mechanisms
 Currently supported confidential guest mechanisms are:
 
 * AMD Secure Encrypted Virtualization (SEV) (see :doc:`i386/amd-memory-encryption`)
+* Intel Trust Domain Extension (TDX) (see :doc:`i386/tdx`)
 * POWER Protected Execution Facility (PEF) (see :ref:`power-papr-protected-execution-facility-pef`)
 * s390x Protected Virtualization (PV) (see :doc:`s390x/protvirt`)
 
diff --git a/docs/system/i386/tdx.rst b/docs/system/i386/tdx.rst
new file mode 100644
index 000000000000..ea2e601dde9a
--- /dev/null
+++ b/docs/system/i386/tdx.rst
@@ -0,0 +1,156 @@
+Intel Trusted Domain eXtension (TDX)
+====================================
+
+Intel Trusted Domain eXtensions (TDX) refers to an Intel technology that extends
+Virtual Machine Extensions (VMX) and Multi-Key Total Memory Encryption (MKTME)
+with a new kind of virtual machine guest called a Trust Domain (TD). A TD runs
+in a CPU mode that is designed to protect the confidentiality of its memory
+contents and its CPU state from any other software, including the hosting
+Virtual Machine Monitor (VMM), unless explicitly shared by the TD itself.
+
+Prerequisites
+-------------
+
+To run TD, the physical machine needs to have TDX module loaded and initialized
+while KVM hypervisor has TDX support and has TDX enabled. If those requirements
+are met, the ``KVM_CAP_VM_TYPES`` will report the support of ``KVM_X86_TDX_VM``.
+
+Trust Domain Virtual Firmware (TDVF)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Trust Domain Virtual Firmware (TDVF) is required to provide TD services to boot
+TD Guest OS. TDVF needs to be copied to guest private memory and measured before
+the TD boots.
+
+KVM vcpu ioctl ``KVM_TDX_INIT_MEM_REGION`` can be used to populate the TDVF
+content into its private memory.
+
+Since TDX doesn't support readonly memslot, TDVF cannot be mapped as pflash
+device and it actually works as RAM. "-bios" option is chosen to load TDVF.
+
+OVMF is the opensource firmware that implements the TDVF support. Thus the
+command line to specify and load TDVF is ``-bios OVMF.fd``
+
+Feature Configuration
+---------------------
+
+Unlike non-TDX VM, the CPU features (enumerated by CPU or MSR) of a TD are not
+under full control of VMM. VMM can only configure part of features of a TD on
+``KVM_TDX_INIT_VM`` command of VM scope ``MEMORY_ENCRYPT_OP`` ioctl.
+
+The configurable features have three types:
+
+- Attributes:
+  - PKS (bit 30) controls whether Supervisor Protection Keys is exposed to TD,
+  which determines related CPUID bit and CR4 bit;
+  - PERFMON (bit 63) controls whether PMU is exposed to TD.
+
+- XSAVE related features (XFAM):
+  XFAM is a 64b mask, which has the same format as XCR0 or IA32_XSS MSR. It
+  determines the set of extended features available for use by the guest TD.
+
+- CPUID features:
+  Only some bits of some CPUID leaves are directly configurable by VMM.
+
+What features can be configured is reported via TDX capabilities.
+
+TDX capabilities
+~~~~~~~~~~~~~~~~
+
+The VM scope ``MEMORY_ENCRYPT_OP`` ioctl provides command ``KVM_TDX_CAPABILITIES``
+to get the TDX capabilities from KVM. It returns a data structure of
+``struct kvm_tdx_capabilities``, which tells the supported configuration of
+attributes, XFAM and CPUIDs.
+
+TD attributes
+~~~~~~~~~~~~~
+
+QEMU supports configuring raw 64-bit TD attributes directly via "attributes"
+property of "tdx-guest" object. Note, it's users' responsibility to provide a
+valid value because some bits may not supported by current QEMU or KVM yet.
+
+QEMU also supports the configuration of individual attribute bits that are
+supported by it, via properties of "tdx-guest" object.
+E.g., "sept-ve-disable" (bit 28).
+
+MSR based features
+~~~~~~~~~~~~~~~~~~
+
+Current KVM doesn't support MSR based feature (e.g., MSR_IA32_ARCH_CAPABILITIES)
+configuration for TDX, and it's a future work to enable it in QEMU when KVM adds
+support of it.
+
+Feature check
+~~~~~~~~~~~~~
+
+QEMU checks if the final (CPU) features, determined by given cpu model and
+explicit feature adjustment of "+featureA/-featureB", can be supported or not.
+It can produce feature not supported warning like
+
+  "warning: host doesn't support requested feature: CPUID.07H:EBX.intel-pt [bit 25]"
+
+It can also produce warning like
+
+  "warning: TDX forcibly sets the feature: CPUID.80000007H:EDX.invtsc [bit 8]"
+
+if the fixed-1 feature is requested to be disabled explicitly. This is newly
+added to QEMU for TDX because TDX has fixed-1 features that are forcibly enabled
+by TDX module and VMM cannot disable them.
+
+Launching a TD (TDX VM)
+-----------------------
+
+To launch a TD, the necessary command line options are tdx-guest object and
+split kernel-irqchip, as below:
+
+.. parsed-literal::
+
+    |qemu_system_x86| \\
+        -object tdx-guest,id=tdx0 \\
+        -machine ...,kernel-irqchip=split,confidential-guest-support=tdx0 \\
+        -bios OVMF.fd \\
+
+Restrictions
+------------
+
+ - kernel-irqchip must be split;
+
+ - No readonly support for private memory;
+
+ - No SMM support: SMM support requires manipulating the guest register states
+   which is not allowed;
+
+Debugging
+---------
+
+Bit 0 of TD attributes, is DEBUG bit, which decides if the TD runs in off-TD
+debug mode. When in off-TD debug mode, TD's VCPU state and private memory are
+accessible via given SEAMCALLs. This requires KVM to expose APIs to invoke those
+SEAMCALLs and corresonponding QEMU change.
+
+It's targeted as future work.
+
+TD attestation
+--------------
+
+In TD guest, the attestation process is used to verify the TDX guest
+trustworthiness to other entities before provisioning secrets to the guest.
+
+TD attestation is initiated first by calling TDG.MR.REPORT inside TD to get the
+REPORT. Then the REPORT data needs to be converted into a remotely verifiable
+Quote by SGX Quoting Enclave (QE).
+
+It's a future work in QEMU to add support of TD attestation since it lacks
+support in current KVM.
+
+Live Migration
+--------------
+
+Future work.
+
+References
+----------
+
+- `TDX Homepage <https://www.intel.com/content/www/us/en/developer/articles/technical/intel-trust-domain-extensions.html>`__
+
+- `SGX QE <https://github.com/intel/SGXDataCenterAttestationPrimitives/tree/master/QuoteGeneration>`__
diff --git a/docs/system/target-i386.rst b/docs/system/target-i386.rst
index ab7af1a75d6e..43b09c79d6be 100644
--- a/docs/system/target-i386.rst
+++ b/docs/system/target-i386.rst
@@ -31,6 +31,7 @@ Architectural features
    i386/kvm-pv
    i386/sgx
    i386/amd-memory-encryption
+   i386/tdx
 
 OS requirements
 ~~~~~~~~~~~~~~~
-- 
2.34.1