[RFC PATCH 21/21] Documentation/x86: Add documentation for TDX host support

[Kai Huang <kai.huang@intel.com>]

Add <Documentation/x86/tdx_host.rst> for TDX host support.

Signed-off-by: Kai Huang <kai.huang@intel.com>
---
 Documentation/x86/index.rst    |   1 +
 Documentation/x86/tdx_host.rst | 300 +++++++++++++++++++++++++++++++++
 2 files changed, 301 insertions(+)
 create mode 100644 Documentation/x86/tdx_host.rst

diff --git a/Documentation/x86/index.rst b/Documentation/x86/index.rst
index 382e53ca850a..145fc251fbfc 100644
--- a/Documentation/x86/index.rst
+++ b/Documentation/x86/index.rst
@@ -25,6 +25,7 @@ x86-specific Documentation
    intel_txt
    amd-memory-encryption
    tdx
+   tdx_host
    pti
    mds
    microcode
diff --git a/Documentation/x86/tdx_host.rst b/Documentation/x86/tdx_host.rst
new file mode 100644
index 000000000000..a843ede9d45c
--- /dev/null
+++ b/Documentation/x86/tdx_host.rst
@@ -0,0 +1,300 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=========================================================
+Intel Trusted Domain Extensions (TDX) host kernel support
+=========================================================
+
+Intel Trusted Domain Extensions (TDX) protects guest VMs from malicious
+host and certain physical attacks. To support TDX, a new CPU mode called
+Secure Arbitration Mode (SEAM) is added to Intel processors.
+
+SEAM is an extension to the VMX architecture to define a new VMX root
+operation called 'SEAM VMX root' and a new VMX non-root operation called
+'VMX non-root'. Collectively, the SEAM VMX root and SEAM VMX non-root
+execution modes are called operation in SEAM.
+
+SEAM VMX root operation is designed to host a CPU-attested, software
+module called 'Intel TDX module' to manage virtual machine (VM) guests
+called Trust Domains (TD). The TDX module implements the functions to
+build, tear down, and start execution of TD VMs. SEAM VMX root is also
+designed to additionally host a CPU-attested, software module called the
+'Intel Persistent SEAMLDR (Intel P-SEAMLDR)' module to load and update
+the Intel TDX module.
+
+The software in SEAM VMX root runs in the memory region defined by the
+SEAM range register (SEAMRR). Access to this range is restricted to SEAM
+VMX root operation. Code fetches outside of SEAMRR when in SEAM VMX root
+operation are meant to be disallowed and lead to an unbreakable shutdown.
+
+TDX leverages Intel Multi-Key Total Memory Encryption (MKTME) to crypto
+protect TD guests. TDX reserves part of MKTME KeyID space as TDX private
+KeyIDs, which can only be used by software runs in SEAM. The physical
+address bits reserved for encoding TDX private KeyID are treated as
+reserved bits when not in SEAM operation. The partitioning of MKTME
+KeyIDs and TDX private KeyIDs is configured by BIOS.
+
+Host kernel transits to either P-SEAMLDR or TDX module via the new
+SEAMCALL instruction. SEAMCALLs are host-side interface functions
+defined by P-SEAMLDR and TDX module around the new SEAMCALL instruction.
+They are similar to a hypercall, except they are made by host kernel to
+the SEAM software modules.
+
+Before being able to manage TD guests, the TDX module must be loaded
+into SEAMRR and properly initialized using SEAMCALLs defined by TDX
+architecture. The current implementation assumes both P-SEAMLDR and
+TDX module are loaded by BIOS before the kernel boots.
+
+Detection and Initialization
+----------------------------
+
+The presence of SEAMRR is reported via a new SEAMRR bit (15) of the
+IA32_MTRRCAP MSR. The SEAMRR range registers consist of a pair of MSRs:
+IA32_SEAMRR_PHYS_BASE (0x1400) and IA32_SEAMRR_PHYS_MASK (0x1401).
+SEAMRR is enabled when bit 3 of IA32_SEAMRR_PHYS_BASE is set and
+bit 10/11 of IA32_SEAMRR_PHYS_MASK are set.
+
+However, there is no CPUID or MSR for querying the presence of the TDX
+module or the P-SEAMLDR. SEAMCALL fails with VMfailInvalid when SEAM
+software is not loaded, so SEAMCALL can be used to detect P-SEAMLDR and
+TDX module. SEAMLDR.INFO SEAMCALL is used to detect both P-SEAMLDR and
+TDX module.  Success of the SEAMCALL means P-SEAMLDR is loaded, and the
+P-SEAMLDR information returned by the SEAMCALL further tells whether TDX
+module is loaded or not.
+
+User can check whether the TDX module is initialized via dmesg:
+
+  [..] tdx: P-SEAMLDR: version 0x0, vendor_id: 0x8086, build_date: 20211209, build_num 160, major 1, minor 0
+  [..] tdx: TDX module detected.
+  [..] tdx: TDX module: vendor_id 0x8086, major_version 1, minor_version 0, build_date 20211209, build_num 160
+  [..] tdx: TDX module initialized.
+
+Initializing TDX takes time (in seconds) and additional memory space (for
+metadata). Both are affected by the size of total usable memory which the
+TDX module is configured with. In particular, the TDX metadata consumes
+~1/256 of TDX usable memory. This leads to a non-negligible burden as the
+current implementation simply treats all E820 RAM ranges as TDX usable
+memory (all system RAM meets the security requirements on the first
+generation of TDX-capable platforms).
+
+Therefore, kernel uses lazy TDX initialization to avoid such burden for
+all users on a TDX-capable platform. The software component (e.g. KVM)
+which wants to use TDX is expected to call two helpers below to detect
+and initialize the TDX module until TDX is truly needed:
+
+        if (tdx_detect())
+                goto no_tdx;
+        if (tdx_init())
+                goto no_tdx;
+
+TDX detection and initialization are done via SEAMCALLs which require the
+CPU in VMX operation. The caller of the above two helpers should ensure
+that condition.
+
+Currently, only KVM is the only user of TDX and KVM already handles
+entering/leaving VMX operation. Letting KVM initialize TDX on demand
+avoids handling entering/leaving VMX operation, which isn't trivial, in
+core-kernel.
+
+In addition, a new kernel parameter 'tdx_host={on/off}' can be used to
+force disabling the TDX capability by the admin.
+
+TDX Memory Management
+---------------------
+
+TDX architecture manages TDX memory via below data structures:
+
+1) Convertible Memory Regions (CMRs)
+
+TDX provides increased levels of memory confidentiality and integrity.
+This requires special hardware support for features like memory
+encryption and storage of memory integrity checksums. A CMR represents a
+memory range that meets those requirements and can be used as TDX memory.
+The list of CMRs can be queried from TDX module.
+
+2) TD Memory Regions (TDMRs)
+
+The TDX module manages TDX usable memory via TD Memory Regions (TDMR).
+Each TDMR has information of its base and size, its metadata (PAMT)'s
+base and size, and an array of reserved areas to hold the memory region
+address holes and PAMTs. TDMR must be 1G aligned and in 1G granularity.
+
+Host kernel is responsible for choosing which convertible memory regions
+(reside in CMRs) to use as TDX memory, and constructing a list of TDMRs
+to cover all those memory regions, and configure the TDMRs to TDX module.
+
+3) Physical Address Metadata Tables (PAMTs)
+
+This metadata essentially serves as the 'struct page' for the TDX module,
+recording things like which TD guest 'owns' a given page of memory. Each
+TDMR has a dedicated PAMT.
+
+PAMT is not reserved by the hardware upfront and must be allocated by the
+kernel and given to the TDX module. PAMT for a given TDMR doesn't have
+to be within that TDMR, but a PAMT must be within one CMR.  Additionally,
+if a PAMT overlaps with a TDMR, the overlapping part must be marked as
+reserved in that particular TDMR.
+
+Kernel Policy of TDX Memory
+---------------------------
+
+The first generation of TDX essentially guarantees that all system RAM
+memory regions (excluding the memory below 1MB) are covered by CMRs.
+Currently, to avoid having to modify the page allocator to support both
+TDX and non-TDX allocation, the kernel choose to use all system RAM as
+TDX memory. A list of TDMRs are constructed based on all RAM entries in
+e820 table and configured to the TDX module.
+
+Limitations
+-----------
+
+1. Constructing TDMRs
+
+Currently, the kernel tries to create one TDMR for each RAM entry in
+e820. 'e820_table' is used to find all RAM entries to honor 'mem' and
+'memmap' kernel command line. However, 'memmap' command line may also
+result in many discrete RAM entries. TDX architecturally only supports a
+limited number of TDMRs (currently 64). In this case, constructing TDMRs
+may fail due to exceeding the maximum number of TDMRs. The user is
+responsible for not doing so otherwise TDX may not be available. This
+can be further enhanced by supporting merging adjacent TDMRs.
+
+2. PAMT allocation
+
+Currently, the kernel allocates PAMT for each TDMR separately using
+alloc_contig_pages(). alloc_contig_pages() only guarantees the PAMT is
+allocated from a given NUMA node, but doesn't have control over
+allocating PAMT from a given TDMR range. This may result in all PAMTs
+on one NUMA node being within one single TDMR. PAMTs overlapping with
+a given TDMR must be put into the TDMR's reserved areas too. However TDX
+only supports a limited number of reserved areas per TDMR (currently 16),
+thus too many PAMTs in one NUMA node may result in constructing TDMR
+failure due to exceeding TDMR's maximum reserved areas.
+
+The user is responsible for not creating too many discrete RAM entries
+on one NUMA node, which may result in having too many TDMRs on one node,
+which eventually results in constructing TDMR failure due to exceeding
+the maximum reserved areas. This can be further enhanced to support
+per-NUMA-node PAMT allocation, which could reduce the number of PAMT to
+1 for each node.
+
+3. TDMR initialization
+
+Currently, the kernel initialize TDMRs one by one. This may take couple
+of seconds to finish on large memory systems (TBs). This can be further
+enhanced by allowing initializing different TDMRs in parallel on multiple
+cpus.
+
+4. CPU hotplug
+
+The first generation of TDX architecturally doesn't support ACPI CPU
+hotplug. All logical cpus are enabled by BIOS in MADT table. Also, the
+first generation of TDX-capable platforms don't support ACPI CPU hotplug
+either. Since this physically cannot happen, currently kernel doesn't
+have any check in ACPI CPU hotplug code path to disable it.
+
+Also, only TDX module initialization requires all BIOS-enabled cpus are
+online. After the initialization, any logical cpu can be brought down
+and brought up to online again later. Therefore this series doesn't
+change logical CPU hotplug either.
+
+This can be enhanced when any future generation of TDX starts to support
+ACPI cpu hotplug.
+
+5. Memory hotplug
+
+The first generation of TDX architecturally doesn't support memory
+hotplug. The CMRs are generated by BIOS during boot and it is fixed
+during machine's runtime.
+
+However, the first generation of TDX-capable platforms don't support ACPI
+memory hotplug. Since it physically cannot happen, currently kernel
+doesn't have any check in ACPI memory hotplug code path to disable it.
+
+A special case of memory hotplug is adding NVDIMM as system RAM using
+kmem driver. However the first generation of TDX-capable platforms
+cannot turn on TDX and NVDIMM simultaneously, so in practice this cannot
+happen either.
+
+Another case is admin can use 'memmap' kernel command line to create
+legacy PMEMs and use them as TD guest memory, or theoretically, can use
+kmem driver to add them as system RAM. Current implementation always
+includes legacy PMEMs when constructing TDMRs so they are also TDX memory.
+So legacy PMEMs can either be used as TD guest memory directly or can be
+converted to system RAM via kmem driver.
+
+This can be enhanced when future generation of TDX starts to support ACPI
+memory hotplug, or NVDIMM and TDX can be enabled simultaneously on the
+same platform.
+
+6. Online CPUs
+
+TDX initialization includes a step where certain SEAMCALL must be called
+on every BIOS-enabled CPU (with a ACPI MADT entry marked as enabled).
+Otherwise, the initialization process aborts at a later step.
+
+The user should avoid using boot parameters (such as maxcpus, nr_cpus,
+possible_cpus) or offlining CPUs before initializing TDX. Doing so will
+lead to the mismatch between online CPUs and BIOS-enabled CPUs, resulting
+TDX module initialization failure.
+
+It is ok to offline CPUs after TDX initialization is completed.
+
+7. Kexec
+
+The TDX module can be initialized only once during its lifetime. The
+first generation of TDX doesn't have interface to reset TDX module to
+uninitialized state so it can be initialized again.
+
+This implies:
+
+  - If the old kernel fails to initialize TDX, the new kernel cannot
+    use TDX too unless the new kernel fixes the bug which leads to
+    initialization failure in the old kernel and can resume from where
+    the old kernel stops. This requires certain coordination between
+    the two kernels.
+
+  - If the old kernel has initialized TDX successfully, the new kernel
+    may be able to use TDX if the two kernels have exactly the same
+    configurations on the TDX module. It further requires the new kernel
+    to reserve the TDX metadata pages (allocated by the old kernel) in
+    its page allocator. It also requires coordination between the two
+    kernels. Furthermore, if kexec() is done when there are active TD
+    guests running, the new kernel cannot use TDX because it's extremely
+    hard for the old kernel to pass all TDX private pages to the new
+    kernel.
+
+Given that, the current implementation doesn't support TDX after kexec()
+(except the old kernel hasn't initialized TDX at all).
+
+The current implementation doesn't shut down TDX module but leaves it
+open during kexec().  This is because shutting down TDX module requires
+CPU being in VMX operation but there's no guarantee of this during
+kexec(). Leaving the TDX module open is not the best case, but it is OK
+since the new kernel won't be able to use TDX anyway (therefore TDX
+module won't run at all).
+
+This can be further enhanced when core-kernele (non-KVM) can handle
+VMXON.
+
+If TDX is ever enabled and/or used to run any TD guests, the cachelines
+of TDX private memory, including PAMTs, used by TDX module need to be
+flushed before transiting to the new kernel otherwise they may silently
+corrupt the new kernel. Similar to SME, the current implementation
+flushes cache in stop_this_cpu().
+
+8. Initialization error
+
+Currently, any error happened during TDX initialization moves the TDX
+module to the SHUTDOWN state. No SEAMCALL is allowed in this state, and
+the TDX module cannot be re-initialized without a hard reset.
+
+This can be further enhanced to treat some errors as recoverable errors
+and let the caller retry later. A more detailed state machine can be
+added to record the internal state of TDX module, and the initialization
+can resume from that state in the next try.
+
+Specifically, there are three cases that can be treated as recoverable
+error: 1) -ENOMEM (i.e. due to PAMT allocation); 2) TDH.SYS.CONFIG error
+due to TDH.SYS.LP.INIT is not called on all cpus (i.e. due to offline
+cpus); 3) -EPERM when the caller doesn't guarantee all cpus are in VMX
+operation.
-- 
2.33.1