[PATCH v2 20/20] Documentation/virt/kvm: Document on Trust Domain Extensions (TDX)

Binbin Wu
2025-02-27T01:19:55Z

Add documentation to Intel Trusted Domain Extensions (TDX) support.

Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
---
TDX "the rest" v2:
- Update for configurable CPUID comment (Tony)
- Add the documentation for KVM_TDX_GET_CPUID
- Add missing white space. (Kai)
- Consolidate description of KVM_MEMORY_ENCRYPT_OP for SEV and TDX. (Kai)
- Update the "Overview" part. (Kai)
- Sync description of struct kvm_tdx_cmd with source code. (Kai)
- Update description of KVM_TDX_CAPABILITIES. (Xiaoyao)
- Use hw_error instead of error and remove the description of "unused"
for TDX specific sub ioctl commands. (Kai)
- Add description for return values for TDX specific sub-ioctl() commands.
(Kai)
- Replace "SEAM call" with "SEAMCALL", and replace "sub ioctl" with
"sub-ioctl()" for consistency. (Kai)
- Remove the detailed SEAMCALLs when describing TDX specific sub-ioctl()
commands. (Kai, Xiaoyao)
- Update description for KVM_TDX_INIT_VM, KVM_TDX_INIT_VCPU, and
KVM_TDX_INIT_MEM_REGION. (Xiaoyao)
- Update "KVM TDX creation flow". (Kai, Xiaoyao)
- Fix typos in enum/struct comments (Xiaoyao)
- Remove design section (Kai)

TDX "the rest" v1:
- Updates to match code changes (Tony)
---
Documentation/virt/kvm/api.rst | 13 +-
Documentation/virt/kvm/x86/index.rst | 1 +
Documentation/virt/kvm/x86/intel-tdx.rst | 255 +++++++++++++++++++++++
3 files changed, 265 insertions(+), 4 deletions(-)
create mode 100644 Documentation/virt/kvm/x86/intel-tdx.rst

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 641d3537a38d..7e155977a8ed 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -1407,6 +1407,9 @@ the memory region are automatically reflected into the guest. For example, an
mmap() that affects the region will be made visible immediately. Another
example is madvise(MADV_DROP).

+For TDX guest, deleting/moving memory region loses guest memory contents.
+Read only region isn't supported. Only as-id 0 is supported.
+
Note: On arm64, a write generated by the page-table walker (to update
the Access and Dirty flags, for example) never results in a
KVM_EXIT_MMIO exit when the slot has the KVM_MEM_READONLY flag. This
@@ -4764,7 +4767,7 @@ H_GET_CPU_CHARACTERISTICS hypercall.

:Capability: basic
:Architectures: x86
-:Type: vm
+:Type: vm ioctl, vcpu ioctl
:Parameters: an opaque platform specific structure (in/out)
:Returns: 0 on success; -1 on error

@@ -4772,9 +4775,11 @@ If the platform supports creating encrypted VMs then this ioctl can be used
for issuing platform-specific memory encryption commands to manage those
encrypted VMs.

-Currently, this ioctl is used for issuing Secure Encrypted Virtualization
-(SEV) commands on AMD Processors. The SEV commands are defined in
-Documentation/virt/kvm/x86/amd-memory-encryption.rst.
+Currently, this ioctl is used for issuing both Secure Encrypted Virtualization
+(SEV) commands on AMD Processors and Trusted Domain Extensions (TDX) commands
+on Intel Processors. The detailed commands are defined in
+Documentation/virt/kvm/x86/amd-memory-encryption.rst and
+Documentation/virt/kvm/x86/intel-tdx.rst.

4.111 KVM_MEMORY_ENCRYPT_REG_REGION
-----------------------------------
diff --git a/Documentation/virt/kvm/x86/index.rst b/Documentation/virt/kvm/x86/index.rst
index 9ece6b8dc817..851e99174762 100644
--- a/Documentation/virt/kvm/x86/index.rst
+++ b/Documentation/virt/kvm/x86/index.rst
@@ -11,6 +11,7 @@ KVM for x86 systems
cpuid
errata
hypercalls
+ intel-tdx
mmu
msr
nested-vmx
diff --git a/Documentation/virt/kvm/x86/intel-tdx.rst b/Documentation/virt/kvm/x86/intel-tdx.rst
new file mode 100644
index 000000000000..de41d4c01e5c
--- /dev/null
+++ b/Documentation/virt/kvm/x86/intel-tdx.rst
@@ -0,0 +1,255 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+===================================
+Intel Trust Domain Extensions (TDX)
+===================================
+
+Overview
+========
+Intel's Trust Domain Extensions (TDX) protect confidential guest VMs from the
+host and physical attacks. A CPU-attested software module called 'the TDX
+module' runs inside a new CPU isolated range to provide the functionalities to
+manage and run protected VMs, a.k.a, TDX guests or TDs.
+
+Please refer to [1] for the whitepaper, specifications and other resources.
+
+This documentation describes TDX-specific KVM ABIs. The TDX module needs to be
+initialized before it can be used by KVM to run any TDX guests. The host
+core-kernel provides the support of initializing the TDX module, which is
+described in the Documentation/arch/x86/tdx.rst.
+
+API description
+===============
+
+KVM_MEMORY_ENCRYPT_OP
+---------------------
+:Type: vm ioctl, vcpu ioctl
+
+For TDX operations, KVM_MEMORY_ENCRYPT_OP is re-purposed to be generic
+ioctl with TDX specific sub-ioctl() commands.
+
+::
+
+ /* Trust Domain Extensions sub-ioctl() commands. */
+ enum kvm_tdx_cmd_id {
+ KVM_TDX_CAPABILITIES = 0,
+ KVM_TDX_INIT_VM,
+ KVM_TDX_INIT_VCPU,
+ KVM_TDX_INIT_MEM_REGION,
+ KVM_TDX_FINALIZE_VM,
+ KVM_TDX_GET_CPUID,
+
+ KVM_TDX_CMD_NR_MAX,
+ };
+
+ struct kvm_tdx_cmd {
+ /* enum kvm_tdx_cmd_id */
+ __u32 id;
+ /* flags for sub-command. If sub-command doesn't use this, set zero. */
+ __u32 flags;
+ /*
+ * data for each sub-command. An immediate or a pointer to the actual
+ * data in process virtual address. If sub-command doesn't use it,
+ * set zero.
+ */
+ __u64 data;
+ /*
+ * Auxiliary error code. The sub-command may return TDX SEAMCALL
+ * status code in addition to -Exxx.
+ */
+ __u64 hw_error;
+ };
+
+KVM_TDX_CAPABILITIES
+--------------------
+:Type: vm ioctl
+:Returns: 0 on success, <0 on error
+
+Return the TDX capabilities that current KVM supports with the specific TDX
+module loaded in the system. It reports what features/capabilities are allowed
+to be configured to the TDX guest.
+
+- id: KVM_TDX_CAPABILITIES
+- flags: must be 0
+- data: pointer to struct kvm_tdx_capabilities
+- hw_error: must be 0
+
+::
+
+ struct kvm_tdx_capabilities {
+ __u64 supported_attrs;
+ __u64 supported_xfam;
+ __u64 reserved[254];
+
+ /* Configurable CPUID bits for userspace */
+ struct kvm_cpuid2 cpuid;
+ };
+
+
+KVM_TDX_INIT_VM
+---------------
+:Type: vm ioctl
+:Returns: 0 on success, <0 on error
+
+Perform TDX specific VM initialization. This needs to be called after
+KVM_CREATE_VM and before creating any VCPUs.
+
+- id: KVM_TDX_INIT_VM
+- flags: must be 0
+- data: pointer to struct kvm_tdx_init_vm
+- hw_error: must be 0
+
+::
+
+ struct kvm_tdx_init_vm {
+ __u64 attributes;
+ __u64 xfam;
+ __u64 mrconfigid[6]; /* sha384 digest */
+ __u64 mrowner[6]; /* sha384 digest */
+ __u64 mrownerconfig[6]; /* sha384 digest */
+
+ /* The total space for TD_PARAMS before the CPUIDs is 256 bytes */
+ __u64 reserved[12];
+
+ /*
+ * Call KVM_TDX_INIT_VM before vcpu creation, thus before
+ * KVM_SET_CPUID2.
+ * This configuration supersedes KVM_SET_CPUID2s for VCPUs because the
+ * TDX module directly virtualizes those CPUIDs without VMM. The user
+ * space VMM, e.g. qemu, should make KVM_SET_CPUID2 consistent with
+ * those values. If it doesn't, KVM may have wrong idea of vCPUIDs of
+ * the guest, and KVM may wrongly emulate CPUIDs or MSRs that the TDX
+ * module doesn't virtualize.
+ */
+ struct kvm_cpuid2 cpuid;
+ };
+
+
+KVM_TDX_INIT_VCPU
+-----------------
+:Type: vcpu ioctl
+:Returns: 0 on success, <0 on error
+
+Perform TDX specific VCPU initialization.
+
+- id: KVM_TDX_INIT_VCPU
+- flags: must be 0
+- data: initial value of the guest TD VCPU RCX
+- hw_error: must be 0
+
+KVM_TDX_INIT_MEM_REGION
+-----------------------
+:Type: vcpu ioctl
+:Returns: 0 on success, <0 on error
+
+Initialize @nr_pages TDX guest private memory starting from @gpa with userspace
+provided data from @source_addr.
+
+Note, before calling this sub command, memory attribute of the range
+[gpa, gpa + nr_pages] needs to be private. Userspace can use
+KVM_SET_MEMORY_ATTRIBUTES to set the attribute.
+
+If KVM_TDX_MEASURE_MEMORY_REGION flag is specified, it also extends measurement.
+
+- id: KVM_TDX_INIT_MEM_REGION
+- flags: currently only KVM_TDX_MEASURE_MEMORY_REGION is defined
+- data: pointer to struct kvm_tdx_init_mem_region
+- hw_error: must be 0
+
+::
+
+ #define KVM_TDX_MEASURE_MEMORY_REGION (1UL << 0)
+
+ struct kvm_tdx_init_mem_region {
+ __u64 source_addr;
+ __u64 gpa;
+ __u64 nr_pages;
+ };
+
+
+KVM_TDX_FINALIZE_VM
+-------------------
+:Type: vm ioctl
+:Returns: 0 on success, <0 on error
+
+Complete measurement of the initial TD contents and mark it ready to run.
+
+- id: KVM_TDX_FINALIZE_VM
+- flags: must be 0
+- data: must be 0
+- hw_error: must be 0
+
+
+KVM_TDX_GET_CPUID
+-----------------
+:Type: vcpu ioctl
+:Returns: 0 on success, <0 on error
+
+Get the CPUID values that the TDX module virtualizes for the TD guest.
+When it returns -E2BIG, the user space should allocate a larger buffer and
+retry. The minimum buffer size is updated in the nent field of the
+struct kvm_cpuid2.
+
+- id: KVM_TDX_GET_CPUID
+- flags: must be 0
+- data: pointer to struct kvm_cpuid2 (in/out)
+- hw_error: must be 0 (out)
+
+::
+
+ struct kvm_cpuid2 {
+ __u32 nent;
+ __u32 padding;
+ struct kvm_cpuid_entry2 entries[0];
+ };
+
+ struct kvm_cpuid_entry2 {
+ __u32 function;
+ __u32 index;
+ __u32 flags;
+ __u32 eax;
+ __u32 ebx;
+ __u32 ecx;
+ __u32 edx;
+ __u32 padding[3];
+ };
+
+KVM TDX creation flow
+=====================
+In addition to the standard KVM flow, new TDX ioctls need to be called. The
+control flow is as follows:
+
+#. Check system wide capability
+
+ * KVM_CAP_VM_TYPES: Check if VM type is supported and if KVM_X86_TDX_VM
+ is supported.
+
+#. Create VM
+
+ * KVM_CREATE_VM
+ * KVM_TDX_CAPABILITIES: Query TDX capabilities for creating TDX guests.
+ * KVM_CHECK_EXTENSION(KVM_CAP_MAX_VCPUS): Query maximum VCPUs the TD can
+ support at VM level (TDX has its own limitation on this).
+ * KVM_SET_TSC_KHZ: Configure TD's TSC frequency if a different TSC frequency
+ than host is desired. This is Optional.
+ * KVM_TDX_INIT_VM: Pass TDX specific VM parameters.
+
+#. Create VCPU
+
+ * KVM_CREATE_VCPU
+ * KVM_TDX_INIT_VCPU: Pass TDX specific VCPU parameters.
+ * KVM_SET_CPUID2: Configure TD's CPUIDs.
+ * KVM_SET_MSRS: Configure TD's MSRs.
+
+#. Initialize initial guest memory
+
+ * Prepare content of initial guest memory.
+ * KVM_TDX_INIT_MEM_REGION: Add initial guest memory.
+ * KVM_TDX_FINALIZE_VM: Finalize the measurement of the TDX guest.
+
+#. Run VCPU
+
+References
+==========
+
+.. [1] https://www.intel.com/content/www/us/en/developer/tools/trust-domain-extensions/documentation.html
--
2.46.0