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    Subject[PATCH v7 20/20] 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>
    ---

    v6 -> v7:
    - Changed "TDX Memory Policy" and "Kexec()" sections.

    ---
    Documentation/x86/tdx.rst | 181 +++++++++++++++++++++++++++++++++++---
    1 file changed, 170 insertions(+), 11 deletions(-)

    diff --git a/Documentation/x86/tdx.rst b/Documentation/x86/tdx.rst
    index dc8d9fd2c3f7..35092e7c60f7 100644
    --- a/Documentation/x86/tdx.rst
    +++ b/Documentation/x86/tdx.rst
    @@ -10,6 +10,165 @@ 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.
    +
    +Before the TDX module can be used to create and run protected VMs, it
    +must be loaded into the isolated range and properly initialized. The TDX
    +architecture doesn't require the BIOS to load the TDX module, but the
    +kernel assumes it is loaded by the BIOS.
    +
    +TDX boot-time detection
    +-----------------------
    +
    +The kernel detects TDX by detecting TDX private KeyIDs during kernel
    +boot. Below dmesg shows when TDX is enabled by BIOS::
    +
    + [..] tdx: TDX enabled by BIOS. TDX private KeyID range: [16, 64).
    +
    +TDX module detection and initialization
    +---------------------------------------
    +
    +There is no CPUID or MSR to detect the TDX module. The kernel detects it
    +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. The kernel initializes the TDX module at runtime on
    +demand. The caller to call tdx_enable() to initialize the TDX module::
    +
    + ret = tdx_enable();
    + if (ret)
    + goto no_tdx;
    + // TDX is ready to use
    +
    +Initializing the TDX module requires all logical CPUs being online.
    +tdx_enable() internally temporarily disables CPU hotplug to prevent any
    +CPU from going offline, but the caller still needs to guarantee all
    +present CPUs are online before calling tdx_enable().
    +
    +Also, tdx_enable() requires all CPUs are already in VMX operation
    +(requirement of making SEAMCALL). Currently, tdx_enable() doesn't handle
    +VMXON internally, but depends on the caller to guarantee that. So far
    +KVM is the only user of TDX and KVM already handles VMXON.
    +
    +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: attributes 0x0, 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
    +------------------------------------------
    +
    +TDX Memory Policy
    +~~~~~~~~~~~~~~~~~
    +
    +TDX reports a list of "Convertible Memory Region" (CMR) to indicate all
    +memory regions that can possibly be used by the TDX module, but they are
    +not automatically usable to the TDX module. As a step of initializing
    +the TDX module, the kernel needs to choose a list of memory regions (out
    +from convertible memory regions) that the TDX module can use and pass
    +those regions to the TDX module. Once this is done, those "TDX-usable"
    +memory regions are fixed during module's lifetime. No more TDX-usable
    +memory can be added to the TDX module after that.
    +
    +To keep things simple, currently the kernel simply guarantees all pages
    +in the page allocator are TDX memory. Specifically, the kernel uses all
    +system memory in the core-mm at the time of initializing the TDX module
    +as TDX memory, and at the meantime, refuses to add any non-TDX-memory in
    +the memory hotplug.
    +
    +This can be enhanced in the future, i.e. by allowing adding non-TDX
    +memory to a separate NUMA node. In this case, the "TDX-capable" nodes
    +and the "non-TDX-capable" nodes can co-exist, but the kernel/userspace
    +needs to guarantee memory pages for TDX guests are always allocated from
    +the "TDX-capable" nodes.
    +
    +Note TDX assumes convertible memory is always physically present during
    +machine's runtime. A non-buggy BIOS should never support hot-removal of
    +any convertible memory. This implementation doesn't handle ACPI memory
    +removal but depends on the BIOS to behave correctly.
    +
    +CPU Hotplug
    +~~~~~~~~~~~
    +
    +TDX doesn't support physical (ACPI) CPU hotplug. During machine boot,
    +TDX verifies all boot-time present logical CPUs are TDX compatible before
    +enabling TDX. A non-buggy BIOS should never support hot-add/removal of
    +physical CPU. Currently the kernel doesn't handle physical CPU hotplug,
    +but depends on the BIOS to behave correctly.
    +
    +Note TDX works with CPU logical online/offline, thus the kernel still
    +allows to offline logical CPU and online it again.
    +
    +Kexec()
    +~~~~~~~
    +
    +There are two problems in terms of using kexec() to boot to a new kernel
    +when the old kernel has enabled TDX: 1) Part of the memory pages are
    +still TDX private pages (i.e. metadata used by the TDX module, and any
    +TDX guest memory if kexec() is executed when there's live TDX guests).
    +2) There might be dirty cachelines associated with TDX private pages.
    +
    +Because the hardware doesn't guarantee cache coherency among different
    +KeyIDs, the old kernel needs to flush cache (of TDX private pages)
    +before booting to the new kernel. Also, the kernel doesn't convert all
    +TDX private pages back to normal because of below considerations:
    +
    +1) The kernel doesn't have existing infrastructure to track which pages
    + are TDX private page.
    +2) The number of TDX private pages can be large, and converting all of
    + them (cache flush + using MOVDIR64B to clear the page) can be time
    + consuming.
    +3) The new kernel will almost only use KeyID 0 to access memory. KeyID
    + 0 doesn't support integrity-check, so it's OK.
    +4) The kernel doesn't (and may never) support MKTME. If any 3rd party
    + kernel ever supports MKTME, it should do MOVDIR64B to clear the page
    + with the new MKTME KeyID (just like TDX does) before using it.
    +
    +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. Currently, the 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 +179,7 @@ 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 +189,7 @@ 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 +200,7 @@ Instruction-based #VE
    - CPUID*

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

    - All VMX instructions: INVEPT, INVVPID, VMCLEAR, VMFUNC, VMLAUNCH,
    VMPTRLD, VMPTRST, VMREAD, VMRESUME, VMWRITE, VMXOFF, VMXON
    @@ -52,7 +211,7 @@ Instruction-based #GP
    - RDMSR*,WRMSR*

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

    MSR access behavior falls into three categories:

    @@ -73,7 +232,7 @@ 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 +252,7 @@ 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 +266,7 @@ 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 +286,7 @@ 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 +304,7 @@ 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 +326,7 @@ 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 +348,7 @@ 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
    --
    2.38.1
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