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@xxxxxxxxx> --- 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 -- 2.35.3
