Recently OVMF got some initial support for IGVM. This article describes what exactly is supported and how to use this. Before taking the deep dive lets start with some basics.

What is IGVM

IGVM is a file format for firmware images. Microsoft has published a git repository with rust crates to generate and parse igvm files. An C library is available as well. The igvm_defs crate defines all the data structures of the file format.

Main motivation for this new format is confidential computing, even though IGVM supports the 'NATIVE' platform (aka non-confidential VMs) too. When setting up a confidential VM the hypervisor needs to know a number of details, for example which pages should be included in the launch measurement, where are special pages like the SEV-SNP cpuid page, and more. OVMF has defined a metadata block at the end of the firmware image where that kind of information is stored. IGVM provides a standard way to store that information in the firmware image.

IGVM also is alot more flexible than traditional firmware images. Traditional firmware images essentially mimic what physical hardware is doing: A single block of memory is mapped as ROM or flash below 4G into the virtual machine address space. IGVM allows to load data into guest ram at any location (in addition to filling the traditional place below 4G of course).

Introducing virt-firmware-rs

The virt-firmware-rs project is a collection of rust crates for virtual machine firmware. Some are for the host, some for the guest. Most interesting for this blog article are the igvm-tools.

igvm-inspect is the first tool in that collection. It will parse igvm files and print the content of the file in a compact format to the console. Here is an example: 

$ igvm-inspect /usr/share/edk2/ovmf/OVMF.igvm
INFO inspecting /usr/share/edk2/ovmf/OVMF.igvm
platform: NATIVE, compat 0x1
platform: SEV_SNP, compat 0x2
SnpVpContext: rm16, cs:ip f000:fff0, compat 0x2 [SEV_SNP]
PageData: gpa 0x800000, type NORMAL, flags 4k, empty, compat 0x2 [SEV_SNP]
PageData: end 0x808fff (8 more pages)
PageData: gpa 0x80a000, type NORMAL, flags 4k, empty, compat 0x2 [SEV_SNP]
PageData: end 0x80cfff (2 more pages)
PageData: gpa 0x80d000, type SECRETS, flags 4k, empty, compat 0x2 [SEV_SNP]
PageData: gpa 0x80e000, type CPUID_DATA, flags 4k, empty, compat 0x2 [SEV_SNP]
PageData: gpa 0x811000, type NORMAL, flags 4k, empty, compat 0x2 [SEV_SNP]
PageData: end 0x82ffff (30 more pages)
PageData: gpa 0xe0000, type NORMAL, flags 4k, compat 0x3 [NATIVE, SEV_SNP]
PageData: end 0xfffff (31 more pages)
PageData: gpa 0xffc84000, type NORMAL, flags 4k, compat 0x3 [NATIVE, SEV_SNP]
PageData: end 0xffffffff (891 more pages)
PageData: gpa 0xffc00000, type NORMAL, flags 4k, compat 0x3 [NATIVE, SEV_SNP]
PageData: end 0xffc83fff (131 more pages)

In this listing you can see some features of the IGVM format. A single file can support multiple platforms. Which file directives are needed for which platform is specified with a compatibility bitmask. There are different page types. "SnpVpContext" is the VMSA (aka initial vCPU state) for the firmware.

Next tool is igvm-wrap, which can convert traditional firmware images into IGVM. The file inspected above has been created with that tool. The image is simply mapped below 4G (with the topmost 128k being mirrored below 1M in real mode adress space). The tool can parse the OVMF metadata section and write that information into IGVM directives. By default the generated file has only support for the native platform, but if supported by the firmware you can turn on snp support. In case you have separate images for firmware code and efi variable store the latter can be added to the command line too: 

$ igvm-wrap \
    --input /usr/share/edk2/ovmf/OVMF_CODE.fd \
    --vars /usr/share/edk2/ovmf/OVMF_VARS.fd \
    --output ovmf.igvm \
    --snp
INFO reading /usr/share/edk2/ovmf/OVMF_CODE.fd
INFO reading /usr/share/edk2/ovmf/OVMF_VARS.fd
INFO writing ovmf.igvm

All of the above works fine without changes to the edk2 code base. But by adding IGVM support in the firmware code we can do more than that.

OVMF and IGVM parameters

IGVM has the concept of parameters to pass information about the virtual machine configuration from the hypervisor to the firmware. For example the memory map and the number of vCPUs. These two are parameters are supported now by OVMF (edk2-stable202511 or newer). There is a reserved page at a fixed location for them, and in case the hypevisor has filled the parameters OVMF will used them for memory detection and MP initialization.

There is a new block in the OVMF metadata area declaring the parameter page and the parameters so tools like igvm-wrap can find them and generate the corresponding IGVM parameter declarations. Looks like this (needs virt-firmware-rs version 25.10 or newer): 

$ igvm-wrap --input OVMF.fd --output ovmf.igvm
INFO reading OVMF.fd
INFO writing ovmf.igvm
$ igvm-inspect ovmf.igvm
INFO inspecting ovmf.igvm
platform: NATIVE, compat 0x1
ParameterArea: idx 0, bytes 0x1000
Parameter/MemoryMap: idx 0, offset 0x0
Parameter/VpCount: idx 0, offset 0xc0
ParameterInsert: gpa 0x812000, idx 0, compat 0x1 [NATIVE]
PageData: gpa 0xe0000, type NORMAL, flags 4k, compat 0x1 [NATIVE]
[ ... ]

OVMF already has a number of different ways to detect memory, depending on the environment it has been booted in (qemu, cloud hypervisor, Xen, ...). So why add another one?

  • One thing less where OVMF depends on a virtual device unter host control (qemu fw_cfg), which is a good thing for confidential guests.
  • When running under coconut svsm this allows svsm to update the memory map (specifically mark the range used by svsm itself as 'reserved') and pass on the modified memory map to OVMF.
  • Maybe this becomes the new standard way to pass the virtual machine memory map to OVMF. Maybe it doesn't.

Load more than just the OVMF firmware

As mentioned above IGVM allows to load data anywhere into guest memory. So we can put additional things into the firmware image. Of course the firmware needs some way to learn about this additional data for this being actually useful. So OVMF got a second special page where it will look for Hand Off Blocks (or HOBs for short), describing where the firmware can find the additional data and what kind of data that is.

So, what "additional things" are supported by OVMF?

  • Secure boot databases ('PK', 'KEK', 'db' and 'dbx'). When running in a confidential VM OVMF will automatically enroll the keys if the system is in setup mode (i.e. no keys enrolled yet). OVMF requires confidential VMs because the secure boot configuratiion can be verified using the launch measurement then.
  • Some EFI binary. Typically this would be a UKI (unified kernel image) containing a linux kernel plus initrd.
  • Some shim.efi binary. Needed for certificate-based secure boot verification of linux distro kernels.

Time to introduce the next virt-firmware-rs igvm utility: igvm-update. This can add all of the above to an igvm image (also needs edk2-stable202511 or newer and virt-firmware-rs 25.10 or newer). 

$ igvm-update \
    --input ovmf.igvm \
    --kernel /boot/vmlinuz-$(uname -r) \
    --output ovmf-kernel.igvm
INFO reading ovmf.igvm
INFO reading /boot/vmlinuz-6.16.9-200.fc42.x86_64
INFO add kernel efi binary
INFO add efidata hoblist
INFO writing ovmf-kernel.igvm

Voila, now you have an igvm image which boots straight into the included linux kernel.

What role does secure boot play?

Why even bother with secure boot in confidential VMs? We have launch measurements, right? Well, problem is, doing the launch measurements is a relatively slow process. So it is desireable to minimize the amount data which must be covered by the launch measurement.

So, the design idea here is to include the firmware itself and the secure boot configuration in the launch measurement. But the UKI containing linux kernel and initrd is not included, this is verified using secure boot instead.

There are two possible ways to handle secure boot verification:

  • First, go the certificate-based route, which is how a typical linux distro install boots with secure boot enabled. The microsoft secure boot certificates are enrolled in 'db', shim.efi (with microsoft signature) is loaded by the firmware, the linux kernel (with distro signature) is loaded by shim.efi. That requires adding both microsoft certificates (--add-cert-ms) and shim binary (--shim /path/to/shimx64.efi) to the igvm file.
  • The other option is to add the authenticode hash of the kernel to 'db' (--add-hash). That way secure boot will allow that one kernel to be loaded. The concept is very simliar to the 'kernel-hashes=on' logic in qemu amd sev support code, except that it uses bog standard secure boot verification instead of some custom hash verification logic, and it also expects UKIs are used to make sure secure boot verification covers initrd and kernel command line too.

Closing remarks

That's it. Have fun checking out the new features.

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