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> --- 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
