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> --- v14 -> v15: - Removed the dmesg shows TDX module version (not printed anymore). v13 -> v14: - Added new sections for "Erratum" and "TDX vs S3/hibernation" --- Documentation/arch/x86/tdx.rst | 222 +++++++++++++++++++++++++++++++-- 1 file changed, 211 insertions(+), 11 deletions(-) diff --git a/Documentation/arch/x86/tdx.rst b/Documentation/arch/x86/tdx.rst index dc8d9fd2c3f7..8969675568d0 100644 --- a/Documentation/arch/x86/tdx.rst +++ b/Documentation/arch/x86/tdx.rst @@ -10,6 +10,206 @@ 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:: + + [..] virt/tdx: BIOS enabled: private KeyID range: [16, 64) + +TDX module initialization +--------------------------------------- + +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. + +If the TDX module isn't loaded, the SEAMCALL instruction fails with a +special error. In this case the kernel fails the module initialization +and reports the module isn't loaded:: + + [..] virt/tdx: module not loaded + +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. + +Besides initializing the TDX module, a per-cpu initialization SEAMCALL +must be done on one cpu before any other SEAMCALLs can be made on that +cpu. + +The kernel provides two functions, tdx_enable() and tdx_cpu_enable() to +allow the user of TDX to enable the TDX module and enable TDX on local +cpu respectively. + +Making SEAMCALL requires VMXON has been done on that CPU. Currently only +KVM implements VMXON. For now both tdx_enable() and tdx_cpu_enable() +don't do VMXON internally (not trivial), but depends on the caller to +guarantee that. + +To enable TDX, the caller of TDX should: 1) temporarily disable CPU +hotplug; 2) do VMXON and tdx_enable_cpu() on all online cpus; 3) call +tdx_enable(). For example:: + + cpus_read_lock(); + on_each_cpu(vmxon_and_tdx_cpu_enable()); + ret = tdx_enable(); + cpus_read_unlock(); + if (ret) + goto no_tdx; + // TDX is ready to use + +And the caller of TDX must guarantee the tdx_cpu_enable() has been +successfully done on any cpu before it wants to run any other SEAMCALL. +A typical usage is do both VMXON and tdx_cpu_enable() in CPU hotplug +online callback, and refuse to online if tdx_cpu_enable() fails. + +User can consult dmesg to see whether the TDX module has been initialized. + +If the TDX module is initialized successfully, dmesg shows something +like below:: + + [..] virt/tdx: 262668 KBs allocated for PAMT + [..] virt/tdx: module initialized + +If the TDX module failed to initialize, dmesg also shows it failed to +initialize:: + + [..] virt/tdx: module initialization failed ... + +TDX Interaction to Other Kernel Components +------------------------------------------ + +TDX Memory Policy +~~~~~~~~~~~~~~~~~ + +TDX reports a list of "Convertible Memory Region" (CMR) to tell the +kernel which memory is TDX compatible. The kernel needs to build a list +of memory regions (out of CMRs) as "TDX-usable" memory and pass those +regions to the TDX module. Once this is done, those "TDX-usable" memory +regions are fixed during module's lifetime. + +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 TDX module initialization" +as TDX memory, and in the meantime, refuses to online any non-TDX-memory +in the memory hotplug. + +Physical Memory Hotplug +~~~~~~~~~~~~~~~~~~~~~~~ + +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 module requires the per-cpu initialization SEAMCALL must be done on +one cpu before any other SEAMCALLs can be made on that cpu. The kernel +provides tdx_cpu_enable() to let the user of TDX to do it when the user +wants to use a new cpu for TDX task. + +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; 2) There might be dirty cachelines associated +with TDX private pages. + +The first problem doesn't matter. KeyID 0 doesn't have integrity check. +Even the new kernel wants use any non-zero KeyID, it needs to convert +the memory to that KeyID and such conversion would work from any KeyID. + +However the old kernel needs to guarantee there's no dirty cacheline +left behind before booting to the new kernel to avoid silent corruption +from later cacheline writeback (Intel hardware doesn't guarantee cache +coherency across different KeyIDs). + +Similar to AMD SME, the kernel does wbinvd() to flush cache before +booting to the new kernel. + +Erratum +~~~~~~~ + +The first few generations of TDX hardware have an erratum. A partial +write to a TDX private memory cacheline will silently "poison" the +line. Subsequent reads will consume the poison and generate a machine +check. + +A partial write is a memory write where a write transaction of less than +cacheline lands at the memory controller. The CPU does these via +non-temporal write instructions (like MOVNTI), or through UC/WC memory +mappings. Devices can also do partial writes via DMA. + +Theoretically, a kernel bug could do partial write to TDX private memory +and trigger unexpected machine check. What's more, the machine check +code will present these as "Hardware error" when they were, in fact, a +software-triggered issue. But in the end, this issue is hard to trigger. + +If the platform has such erratum, the kernel does additional things: +1) resetting TDX private pages using MOVDIR64B in kexec before booting to +the new kernel; 2) Printing additional message in machine check handler +to tell user the machine check may be caused by kernel bug on TDX private +memory. + +Interaction vs S3 and deeper states +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +TDX cannot survive from S3 and deeper states. The hardware resets and +disables TDX completely when platform goes to S3 and deeper. Both TDX +guests and the TDX module get destroyed permanently. + +The kernel uses S3 for suspend-to-ram, and use S4 and deeper states for +hibernation. Currently, for simplicity, the kernel chooses to make TDX +mutually exclusive with S3 and hibernation. + +The kernel disables TDX during early boot when hibernation support is +available:: + + [..] virt/tdx: initialization failed: Hibernation support is enabled + +Add 'nohibernate' kernel command line to disable hibernation in order to +use TDX. + +ACPI S3 is disabled during kernel early boot if TDX is enabled. The user +needs to turn off TDX in the BIOS in order to use S3. + +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 +220,7 @@ 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 +230,7 @@ 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 +241,7 @@ Instruction-based #VE - CPUID* Instruction-based #GP ---------------------- +~~~~~~~~~~~~~~~~~~~~~ - All VMX instructions: INVEPT, INVVPID, VMCLEAR, VMFUNC, VMLAUNCH, VMPTRLD, VMPTRST, VMREAD, VMRESUME, VMWRITE, VMXOFF, VMXON @@ -52,7 +252,7 @@ Instruction-based #GP - RDMSR*,WRMSR* RDMSR/WRMSR Behavior --------------------- +~~~~~~~~~~~~~~~~~~~~ MSR access behavior falls into three categories: @@ -73,7 +273,7 @@ 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 +293,7 @@ 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 +307,7 @@ 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 +327,7 @@ 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 +345,7 @@ 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 +367,7 @@ 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 +389,7 @@ 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 -- 2.41.0
