Re: [PATCH v6 60/60] docs: Add TDX documentation

[Francesco Lavra <francescolavra.fl@gmail.com>]

On Tue, 2024-11-05 at 01:24 -0500, Xiaoyao Li wrote:

> diff --git a/docs/system/confidential-guest-support.rst
> b/docs/system/confidential-guest-support.rst
> index 0c490dbda2b7..66129fbab64c 100644
> --- a/docs/system/confidential-guest-support.rst
> +++ b/docs/system/confidential-guest-support.rst
> @@ -38,6 +38,7 @@ Supported mechanisms
>  Currently supported confidential guest mechanisms are:
>  
>  * AMD Secure Encrypted Virtualization (SEV) (see :doc:`i386/amd-
> memory-encryption`)
> +* Intel Trust Domain Extension (TDX) (see :doc:`i386/tdx`)
>  * POWER Protected Execution Facility (PEF) (see :ref:`power-papr-
> protected-execution-facility-pef`)
>  * s390x Protected Virtualization (PV) (see :doc:`s390x/protvirt`)
>  
> diff --git a/docs/system/i386/tdx.rst b/docs/system/i386/tdx.rst
> new file mode 100644
> index 000000000000..60106b29bf72
> --- /dev/null
> +++ b/docs/system/i386/tdx.rst
> @@ -0,0 +1,155 @@
> +Intel Trusted Domain eXtension (TDX)
> +====================================
> +
> +Intel Trusted Domain eXtensions (TDX) refers to an Intel technology
> that extends
> +Virtual Machine Extensions (VMX) and Multi-Key Total Memory
> Encryption (MKTME)
> +with a new kind of virtual machine guest called a Trust Domain (TD).
> A TD runs
> +in a CPU mode that is designed to protect the confidentiality of its
> memory
> +contents and its CPU state from any other software, including the
> hosting
> +Virtual Machine Monitor (VMM), unless explicitly shared by the TD
> itself.
> +
> +Prerequisites
> +-------------
> +
> +To run TD, the physical machine needs to have TDX module loaded and
> initialized
> +while KVM hypervisor has TDX support and has TDX enabled. If those
> requirements
> +are met, the ``KVM_CAP_VM_TYPES`` will report the support of
> ``KVM_X86_TDX_VM``.
> +
> +Trust Domain Virtual Firmware (TDVF)
> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> +
> +Trust Domain Virtual Firmware (TDVF) is required to provide TD
> services to boot
> +TD Guest OS. TDVF needs to be copied to guest private memory and
> measured before
> +the TD boots.
> +
> +KVM vcpu ioctl ``KVM_TDX_INIT_MEM_REGION`` can be used to populates

s/populates/populate

> the TDVF
> +content into its private memory.
> +
> +Since TDX doesn't support readonly memslot, TDVF cannot be mapped as
> pflash
> +device and it actually works as RAM. "-bios" option is chosen to
> load TDVF.
> +
> +OVMF is the opensource firmware that implements the TDVF support.
> Thus the
> +command line to specify and load TDVF is ``-bios OVMF.fd``
> +
> +Feature Configuration
> +---------------------
> +
> +Unlike non-TDX VM, the CPU features (enumerated by CPU or MSR) of a
> TD is not

s/is/are

> +under full control of VMM. VMM can only configure part of features
> of a TD on
> +``KVM_TDX_INIT_VM`` command of VM scope ``MEMORY_ENCRYPT_OP`` ioctl.
> +
> +The configurable features have three types:
> +
> +- Attributes:
> +  - PKS (bit 30) controls whether Supervisor Protection Keys is
> exposed to TD,
> +  which determines related CPUID bit and CR4 bit;
> +  - PERFMON (bit 63) controls whether PMU is exposed to TD.
> +
> +- XSAVE related features (XFAM):
> +  XFAM is a 64b mask, which has the same format as XCR0 or IA32_XSS
> MSR. It
> +  determines the set of extended features available for use by the
> guest TD.
> +
> +- CPUID features:
> +  Only some bits of some CPUID leaves are directly configurable by
> VMM.
> +
> +What features can be configured is reported via TDX capabilities.
> +
> +TDX capabilities
> +~~~~~~~~~~~~~~~~
> +
> +The VM scope ``MEMORY_ENCRYPT_OP`` ioctl provides command
> ``KVM_TDX_CAPABILITIES``
> +to get the TDX capabilities from KVM. It returns a data structure of
> +``struct kvm_tdx_capabilities``, which tells the supported
> configuration of
> +attributes, XFAM and CPUIDs.
> +
> +TD attributes
> +~~~~~~~~~~~~~
> +
> +QEMU supports configuring raw 64-bit TD attributes directly via
> "attributes"
> +property of "tdx-guest" object. Note, it's users' responsibility to
> provide a
> +valid value because some bits may not supported by current QEMU or
> KVM yet.
> +
> +QEMU also supports the configuration of individual attribute bits
> that are
> +supported by it, via propertyies of "tdx-guest" object.

s/propertyies/properties

> +E.g., "sept-ve-disable" (bit 63).
> +
> +MSR based features
> +~~~~~~~~~~~~~~~~
> +
> +Current KVM doesn't support MSR based feature (e.g.,
> MSR_IA32_ARCH_CAPABILITIES)
> +configuration for TDX, and it's a future work to enable it in QEMU
> when KVM adds
> +support of it.
> +
> +Feature check
> +~~~~~~~~~~~~~
> +
> +QEMU checks if the final (CPU) features, determined by given cpu
> model and
> +explicit feature adjustment of "+featureA/-featureB", can be
> supported or not.
> +It can produce feature not supported warnning like
> +
> +  "warning: host doesn't support requested feature:
> CPUID.07H:EBX.intel-pt [bit 25]"
> +
> +It will also procude warning like

s/procude/produce

> +
> +  "warning: TDX forcibly sets the feature:
> CPUID.80000007H:EDX.invtsc [bit 8]"
> +
> +if the fixed-1 feature is requested to be disabled explicitly. This
> is newly
> +added to QEMU for TDX because TDX has fixed-1 features that are
> enfored enabled

s/enfored/enforced

> +by TDX module and VMM cannot disable them.
> +
> +Launching a TD (TDX VM)
> +-----------------------
> +
> +To launch a TDX guest, below are new added and required:

This sentence is missing a subject (such as "command line options").

> +
> +.. parsed-literal::
> +
> +    |qemu_system_x86| \\
> +        -object tdx-guest,id=tdx0 \\
> +        -machine ...,kernel-irqchip=split,confidential-guest-
> support=tdx0 \\
> +        -bios OVMF.fd \\
> +
> +restrictions
> +------------
> +
> + - kernel-irqchip must be split;
> +
> + - No readonly support for private memory;
> +
> + - No SMM support: SMM support requires manipulating the guset

s/guset/guest

> register states
> +   which is not allowed;
> +
> +Debugging
> +---------
> +
> +Bit 0 of TD attributes, is DEBUG bit, which decides if the TD runs
> in off-TD
> +debug mode. When in off-TD debug mode, TD's VCPU state and private
> memory are
> +accessible via given SEAMCALLs. This requires KVM to expose APIs to
> invoke those
> +SEAMCALLs and resonponding QEMU change.

s/resonponding/corresponding