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Subject[PATCH v5 22/22] Documentation/x86: Add documentation for TDX host support
Date
Add documentation for TDX host kernel support.  There is already one
file Documentation/x86/tdx.rst containing documentation for TDX guest
internals. Also reuse it for TDX host kernel support.

Introduce a new level menu "TDX Guest Support" and move existing
materials under it, and add a new menu for TDX host kernel support.

Signed-off-by: Kai Huang <kai.huang@intel.com>
---
Documentation/x86/tdx.rst | 190 +++++++++++++++++++++++++++++++++++---
1 file changed, 179 insertions(+), 11 deletions(-)

diff --git a/Documentation/x86/tdx.rst b/Documentation/x86/tdx.rst
index b8fa4329e1a5..6c6b09ca6ba4 100644
--- a/Documentation/x86/tdx.rst
+++ b/Documentation/x86/tdx.rst
@@ -10,6 +10,174 @@ 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.

+TDX Host Kernel Support
+=======================
+
+TDX introduces a new CPU mode called Secure Arbitration Mode (SEAM) and
+a new isolated range pointed by the SEAM Ranger Register (SEAMRR). A
+CPU-attested software module called 'the TDX module' runs inside the new
+isolated range to provide the functionalities to manage and run protected
+VMs.
+
+TDX also leverages Intel Multi-Key Total Memory Encryption (MKTME) to
+provide crypto-protection to the VMs. TDX reserves part of MKTME KeyIDs
+as TDX private KeyIDs, which are only accessible within the SEAM mode.
+BIOS is responsible for partitioning legacy MKTME KeyIDs and TDX KeyIDs.
+
+To enable TDX, BIOS configures SEAMRR and TDX private KeyIDs consistently
+across all CPU packages. TDX doesn't trust BIOS. The MCHECK verifies
+all configurations from BIOS are correct and enables SEAMRR.
+
+After TDX is enabled in BIOS, the TDX module needs to be loaded into the
+SEAMRR range and properly initialized, before it can be used to create
+and run protected VMs.
+
+The TDX architecture doesn't require BIOS to load the TDX module, but
+current kernel assumes it is loaded by BIOS (i.e. either directly or by
+some UEFI shell tool) before booting to the kernel. Current kernel
+detects TDX and initializes the TDX module.
+
+TDX boot-time detection
+-----------------------
+
+Kernel detects TDX and the TDX private KeyIDs during kernel boot. User
+can see below dmesg if TDX is enabled by BIOS:
+
+| [..] tdx: SEAMRR enabled.
+| [..] tdx: TDX private KeyID range: [16, 64).
+| [..] tdx: TDX enabled by BIOS.
+
+TDX module detection and initialization
+---------------------------------------
+
+There is no CPUID or MSR to detect whether the TDX module. The kernel
+detects the TDX module by initializing it.
+
+The kernel talks to the TDX module via the new SEAMCALL instruction. The
+TDX module implements SEAMCALL leaf functions to allow the kernel to
+initialize it.
+
+Initializing the TDX module consumes roughly ~1/256th system RAM size to
+use it as 'metadata' for the TDX memory. It also takes additional CPU
+time to initialize those metadata along with the TDX module itself. Both
+are not trivial. Current kernel doesn't choose to always initialize the
+TDX module during kernel boot, but provides a function tdx_init() to
+allow the caller to initialize TDX when it truly wants to use TDX:
+
+ ret = tdx_init();
+ if (ret)
+ goto no_tdx;
+ // TDX is ready to use
+
+Initializing the TDX module requires all logical CPUs being online and
+are in VMX operation (requirement of making SEAMCALL) during tdx_init().
+Currently, KVM is the only user of TDX. KVM always guarantees all online
+CPUs are in VMX operation when there's any VM. Current kernel doesn't
+handle entering VMX operation in tdx_init() but leaves this to the
+caller.
+
+User can consult dmesg to see the presence of the TDX module, and whether
+it has been initialized.
+
+If the TDX module is not loaded, dmesg shows below:
+
+| [..] tdx: TDX module is not loaded.
+
+If the TDX module is initialized successfully, dmesg shows something
+like below:
+
+| [..] tdx: TDX module: vendor_id 0x8086, major_version 1, minor_version 0, build_date 20211209, build_num 160
+| [..] tdx: 65667 pages allocated for PAMT.
+| [..] tdx: TDX module initialized.
+
+If the TDX module failed to initialize, dmesg shows below:
+
+| [..] tdx: Failed to initialize TDX module. Shut it down.
+
+TDX Interaction to Other Kernel Components
+------------------------------------------
+
+CPU Hotplug
+~~~~~~~~~~~
+
+TDX doesn't work with ACPI CPU hotplug. To guarantee the security MCHECK
+verifies all logical CPUs for all packages during platform boot. Any
+hot-added CPU is not verified thus cannot support TDX. A non-buggy BIOS
+should never deliver ACPI CPU hot-add event to the kernel. Such event is
+reported as BIOS bug and the hot-added CPU is rejected.
+
+TDX requires all boot-time verified logical CPUs being present until
+machine reset. If kernel receives ACPI CPU hot-removal event, assume the
+kernel cannot continue to work normally so just BUG().
+
+Note TDX works with CPU logical online/offline, thus the kernel still
+allows to offline logical CPU and online it again.
+
+Memory Hotplug
+~~~~~~~~~~~~~~
+
+The TDX module reports a list of "Convertible Memory Region" (CMR) to
+indicate which memory regions are TDX-capable. Those regions are
+generated by BIOS and verified by the MCHECK so that they are truly
+present during platform boot and can meet security guarantee.
+
+This means TDX doesn't work with ACPI memory hot-add. A non-buggy BIOS
+should never deliver ACPI memory hot-add event to the kernel. Such event
+is reported as BIOS bug and the hot-added memory is rejected.
+
+TDX also doesn't work with ACPI memory hot-removal. If kernel receives
+ACPI memory hot-removal event, assume the kernel cannot continue to work
+normally so just BUG().
+
+Also, the kernel needs to choose which TDX-capable regions to use as TDX
+memory and pass those regions to the TDX module when it gets initialized.
+Once they are passed to the TDX module, the TDX-usable memory regions are
+fixed during module's lifetime.
+
+To avoid having to modify the page allocator to distinguish TDX and
+non-TDX memory allocation, current kernel guarantees all pages managed by
+the page allocator are TDX memory. This means any hot-added memory to
+the page allocator will break such guarantee thus should be prevented.
+
+There are basically two memory hot-add cases that need to be prevented:
+ACPI memory hot-add and driver managed memory hot-add. The kernel
+rejectes the driver managed memory hot-add too when TDX is enabled by
+BIOS. For instance, dmesg shows below error when using kmem driver to
+add a legacy PMEM as system RAM:
+
+| [..] tdx: Unable to add memory [0x580000000, 0x600000000) on TDX enabled platform.
+| [..] kmem dax0.0: mapping0: 0x580000000-0x5ffffffff memory add failed
+
+However, adding new memory to ZONE_DEVICE should not be prevented as
+those pages are not managed by the page allocator. Therefore,
+memremap_pages() variants are still allowed although they internally
+also uses memory hotplug functions.
+
+Kexec()
+~~~~~~~
+
+TDX (and MKTME) doesn't guarantee cache coherency among different KeyIDs.
+If the TDX module is ever initialized, the kernel needs to flush dirty
+cachelines associated with any TDX private KeyID, otherwise they may
+slightly corrupt the new kernel.
+
+Similar to SME support, the kernel uses wbinvd() to flush cache in
+stop_this_cpu().
+
+The current TDX module architecture doesn't play nicely with kexec().
+The TDX module can only be initialized once during its lifetime, and
+there is no SEAMCALL to reset the module to give a new clean slate to
+the new kernel. Therefore, ideally, if the module is ever initialized,
+it's better to shut down the module. The new kernel won't be able to
+use TDX anyway (as it needs to go through the TDX module initialization
+process which will fail immediately at the first step).
+
+However, there's no guarantee CPU is in VMX operation during kexec(), so
+it's impractical to shut down the module. Current kernel just leaves the
+module in open state.
+
+TDX Guest Support
+=================
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
@@ -20,7 +188,7 @@ 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
@@ -30,7 +198,7 @@ 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
@@ -41,7 +209,7 @@ Instruction-based #VE
- CPUID*

Instruction-based #GP
----------------------
+~~~~~~~~~~~~~~~~~~~~~

- All VMX instructions: INVEPT, INVVPID, VMCLEAR, VMFUNC, VMLAUNCH,
VMPTRLD, VMPTRST, VMREAD, VMRESUME, VMWRITE, VMXOFF, VMXON
@@ -52,7 +220,7 @@ Instruction-based #GP
- RDMSR*,WRMSR*

RDMSR/WRMSR Behavior
---------------------
+~~~~~~~~~~~~~~~~~~~~

MSR access behavior falls into three categories:

@@ -73,7 +241,7 @@ 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
@@ -93,7 +261,7 @@ 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
@@ -107,7 +275,7 @@ 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
@@ -127,7 +295,7 @@ 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
@@ -145,7 +313,7 @@ The hypervisor is permitted to unilaterally move accepted pages to a
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.
@@ -167,7 +335,7 @@ 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
@@ -189,7 +357,7 @@ 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
--
2.36.1
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