[PATCH v4 23/24] x86/virt/tdx: Document TDX Module updates

[Chao Gao <chao.gao@intel.com>]

Document TDX Module updates as a subsection of "TDX Host Kernel Support" to
provide background information and cover key points that developers and
users may need to know, for example:

 - update is done in stop_machine() context
 - update instructions and results
 - update policy and tooling

Signed-off-by: Chao Gao <chao.gao@intel.com>
---
 Documentation/arch/x86/tdx.rst | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/Documentation/arch/x86/tdx.rst b/Documentation/arch/x86/tdx.rst
index 61670e7df2f7..01ae560c7f66 100644
--- a/Documentation/arch/x86/tdx.rst
+++ b/Documentation/arch/x86/tdx.rst
@@ -99,6 +99,40 @@ initialize::
 
   [..] virt/tdx: module initialization failed ...
 
+TDX Module Runtime Updates
+--------------------------
+
+The TDX architecture includes a persistent SEAM loader (P-SEAMLDR) that
+runs in SEAM mode separately from the TDX Module. The kernel can
+communicate with P-SEAMLDR to perform runtime updates of the TDX Module.
+
+During updates, the TDX Module becomes unresponsive to other TDX
+operations. To prevent components using TDX (such as KVM) from experiencing
+unexpected errors during updates, updates are performed in stop_machine()
+context.
+
+TDX Module updates have complex compatibility requirements; the new module
+must be compatible with the current CPU, P-SEAMLDR, and running TDX Module.
+Rather than implementing complex module selection and policy enforcement
+logic in the kernel, userspace is responsible for auditing and selecting
+appropriate updates.
+
+Updates use the standard firmware upload interface. See
+Documentation/driver-api/firmware/fw_upload.rst for detailed instructions
+
+Successful updates are logged in dmesg:
+  [..] virt/tdx: version 1.5.20 -> 1.5.24
+
+If updates failed, running TDs may be killed and further TDX operations may
+be not possible until reboot. For detailed error information, see
+Documentation/ABI/testing/sysfs-devices-faux-tdx-host.
+
+Given the risk of losing existing TDs, userspace should verify that the update
+is compatible with the current system and properly validated before applying it.
+A reference userspace tool that implements necessary checks is available at:
+
+  https://github.com/intel/confidential-computing.tdx.tdx-module.binaries
+
 TDX Interaction to Other Kernel Components
 ------------------------------------------
 
-- 
2.47.3