[PULL 26/77] docs/system: Add documentation on support for IGVM

[Paolo Bonzini <pbonzini@redhat.com>]

From: Roy Hopkins <roy.hopkins@randomman.co.uk>

IGVM support has been implemented for Confidential Guests that support
AMD SEV and AMD SEV-ES. Add some documentation that gives some
background on the IGVM format and how to use it to configure a
confidential guest.

Signed-off-by: Roy Hopkins <roy.hopkins@randomman.co.uk>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Gerd Hoffman <kraxel@redhat.com>
Reviewed-by: Ani Sinha <anisinha@redhat.com>
Link: https://lore.kernel.org/r/b4dc920a30717e19cd79bbbe2cc769f3b9ff3d37.1751554099.git.roy.hopkins@randomman.co.uk
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 docs/system/i386/amd-memory-encryption.rst |   2 +
 docs/system/igvm.rst                       | 173 +++++++++++++++++++++
 docs/system/index.rst                      |   1 +
 3 files changed, 176 insertions(+)
 create mode 100644 docs/system/igvm.rst

diff --git a/docs/system/i386/amd-memory-encryption.rst b/docs/system/i386/amd-memory-encryption.rst
index 748f5094baf..6c23f3535f4 100644
--- a/docs/system/i386/amd-memory-encryption.rst
+++ b/docs/system/i386/amd-memory-encryption.rst
@@ -1,3 +1,5 @@
+.. _amd-sev:
+
 AMD Secure Encrypted Virtualization (SEV)
 =========================================
 
diff --git a/docs/system/igvm.rst b/docs/system/igvm.rst
new file mode 100644
index 00000000000..79508d95880
--- /dev/null
+++ b/docs/system/igvm.rst
@@ -0,0 +1,173 @@
+Independent Guest Virtual Machine (IGVM) support
+================================================
+
+IGVM files are designed to encapsulate all the information required to launch a
+virtual machine on any given virtualization stack in a deterministic way. This
+allows the cryptographic measurement of initial guest state for Confidential
+Guests to be calculated when the IGVM file is built, allowing a relying party to
+verify the initial state of a guest via a remote attestation.
+
+Although IGVM files are designed with Confidential Computing in mind, they can
+also be used to configure non-confidential guests. Multiple platforms can be
+defined by a single IGVM file, allowing a single IGVM file to configure a
+virtual machine that can run on, for example, TDX, SEV and non-confidential
+hosts.
+
+QEMU supports IGVM files through the user-creatable ``igvm-cfg`` object. This
+object is used to define the filename of the IGVM file to process. A reference
+to the object is added to the ``-machine`` to configure the virtual machine
+to use the IGVM file for configuration.
+
+Confidential platform support is provided through the use of
+the ``ConfidentialGuestSupport`` object. If the virtual machine provides an
+instance of this object then this is used by the IGVM loader to configure the
+isolation properties of the directives within the file.
+
+Further Information on IGVM
+---------------------------
+
+Information about the IGVM format, including links to the format specification
+and documentation for the Rust and C libraries can be found at the project
+repository:
+
+https://github.com/microsoft/igvm
+
+
+Supported Platforms
+-------------------
+
+Currently, IGVM files can be provided for Confidential Guests on host systems
+that support AMD SEV, SEV-ES and SEV-SNP with KVM. IGVM files can also be
+provided for non-confidential guests.
+
+
+Limitations when using IGVM with AMD SEV, SEV-ES and SEV-SNP
+------------------------------------------------------------
+
+IGVM files configure the initial state of the guest using a set of directives.
+Not every directive is supported by every Confidential Guest type. For example,
+AMD SEV does not support encrypted save state regions, therefore setting the
+initial CPU state using IGVM for SEV is not possible. When an IGVM file contains
+directives that are not supported for the active platform, an error is generated
+and the guest launch is aborted.
+
+The table below describes the list of directives that are supported for SEV,
+SEV-ES, SEV-SNP and non-confidential platforms.
+
+.. list-table:: SEV, SEV-ES, SEV-SNP & non-confidential Supported Directives
+   :widths: 35 65
+   :header-rows: 1
+
+   * - IGVM directive
+     - Notes
+   * - IGVM_VHT_PAGE_DATA
+     - ``NORMAL`` zero, measured and unmeasured page types are supported. Other
+       page types result in an error.
+   * - IGVM_VHT_PARAMETER_AREA
+     -
+   * - IGVM_VHT_PARAMETER_INSERT
+     -
+   * - IGVM_VHT_VP_COUNT_PARAMETER
+     - The guest parameter page is populated with the CPU count.
+   * - IGVM_VHT_ENVIRONMENT_INFO_PARAMETER
+     - The ``memory_is_shared`` parameter is set to 1 in the guest parameter
+       page.
+
+.. list-table:: Additional SEV, SEV-ES & SEV_SNP Supported Directives
+   :widths: 25 75
+   :header-rows: 1
+
+   * - IGVM directive
+     - Notes
+   * - IGVM_VHT_MEMORY_MAP
+     - The memory map page is populated using entries from the E820 table.
+   * - IGVM_VHT_REQUIRED_MEMORY
+     - Ensures memory is available in the guest at the specified range.
+
+.. list-table:: Additional SEV-ES & SEV-SNP Supported Directives
+   :widths: 25 75
+   :header-rows: 1
+
+   * - IGVM directive
+     - Notes
+   * - IGVM_VHT_VP_CONTEXT
+     - Setting of the initial CPU state for the boot CPU and additional CPUs is
+       supported with limitations on the fields that can be provided in the
+       VMSA. See below for details on which fields are supported.
+
+Initial CPU state with VMSA
+---------------------------
+
+The initial state of guest CPUs can be defined in the IGVM file for AMD SEV-ES
+and SEV-SNP. The state data is provided as a VMSA structure as defined in Table
+B-4 in the AMD64 Architecture Programmer's Manual, Volume 2 [1].
+
+The IGVM VMSA is translated to CPU state in QEMU which is then synchronized
+by KVM to the guest VMSA during the launch process where it contributes to the
+launch measurement. See :ref:`amd-sev` for details on the launch process and
+guest launch measurement.
+
+It is important that no information is lost or changed when translating the
+VMSA provided by the IGVM file into the VSMA that is used to launch the guest.
+Therefore, QEMU restricts the VMSA fields that can be provided in the IGVM
+VMSA structure to the following registers:
+
+RAX, RCX, RDX, RBX, RBP, RSI, RDI, R8-R15, RSP, RIP, CS, DS, ES, FS, GS, SS,
+CR0, CR3, CR4, XCR0, EFER, PAT, GDT, IDT, LDTR, TR, DR6, DR7, RFLAGS, X87_FCW,
+MXCSR.
+
+When processing the IGVM file, QEMU will check if any fields other than the
+above are non-zero and generate an error if this is the case.
+
+KVM uses a hardcoded GPA of 0xFFFFFFFFF000 for the VMSA. When an IGVM file
+defines initial CPU state, the GPA for each VMSA must match this hardcoded
+value.
+
+Firmware Images with IGVM
+-------------------------
+
+When an IGVM filename is specified for a Confidential Guest Support object it
+overrides the default handling of system firmware: the firmware image, such as
+an OVMF binary should be contained as a payload of the IGVM file and not
+provided as a flash drive or via the ``-bios`` parameter. The default QEMU
+firmware is not automatically populated into the guest memory space.
+
+If an IGVM file is provided along with either the ``-bios`` parameter or pflash
+devices then an error is displayed and the guest startup is aborted.
+
+Running a guest configured using IGVM
+-------------------------------------
+
+To run a guest configured with IGVM you firstly need to generate an IGVM file
+that contains a guest configuration compatible with the platform you are
+targeting.
+
+The ``buildigvm`` tool [2] is an example of a tool that can be used to generate
+IGVM files for non-confidential X86 platforms as well as for SEV, SEV-ES and
+SEV-SNP confidential platforms.
+
+Example using this tool to generate an IGVM file for AMD SEV-SNP::
+
+    buildigvm --firmware /path/to/OVMF.fd --output sev-snp.igvm \
+              --cpucount 4 sev-snp
+
+To run a guest configured with the generated IGVM you need to add an
+``igvm-cfg`` object and refer to it from the ``-machine`` parameter:
+
+Example (for AMD SEV)::
+
+    qemu-system-x86_64 \
+        <other parameters> \
+        -machine ...,confidential-guest-support=sev0,igvm-cfg=igvm0 \
+        -object sev-guest,id=sev0,cbitpos=47,reduced-phys-bits=1 \
+        -object igvm-cfg,id=igvm0,file=/path/to/sev-snp.igvm
+
+References
+----------
+
+[1] AMD64 Architecture Programmer's Manual, Volume 2: System Programming
+  Rev 3.41
+  https://www.amd.com/content/dam/amd/en/documents/processor-tech-docs/programmer-references/24593.pdf
+
+[2] ``buildigvm`` - A tool to build example IGVM files containing OVMF firmware
+  https://github.com/roy-hopkins/buildigvm
\ No newline at end of file
diff --git a/docs/system/index.rst b/docs/system/index.rst
index 718e9d3c56b..427b0204831 100644
--- a/docs/system/index.rst
+++ b/docs/system/index.rst
@@ -38,5 +38,6 @@ or Hypervisor.Framework.
    security
    multi-process
    confidential-guest-support
+   igvm
    vm-templating
    sriov
-- 
2.50.0