What works on which cloud? Currently, Confidential VMs (CVMs) are available in varying quality on the different clouds and software stacks.
For Constellation, the ideal environment provides the following:
1. Ability to run arbitrary software and images inside CVMs
2. CVMs based on AMD SEV-SNP (available in EPYC CPUs since the Milan generation) or, in the future, Intel TDX (available in Xeon CPUs from the Sapphire Rapids generation onward)
(1) is a functional must-have. (2)--(5) are required for remote attestation that fully keeps the infrastructure/cloud out. Constellation can work without them or with approximations, but won't protect against certain privileged attackers anymore.
With its [CVM offering](https://docs.microsoft.com/en-us/azure/confidential-computing/confidential-vm-overview), Azure provides the best foundations for Constellation.
Regarding (3), Azure provides direct access to remote-attestation statements.
The CVM firmware running in VM Privilege Level (VMPL) 0 provides a vTPM (5), but it's closed source (4).
This firmware is signed by Azure.
The signature is reflected in the remote-attestation statements of CVMs.
Thus, the Azure closed-source firmware becomes part of Constellation's trusted computing base (TCB).
\* Recently, Azure [announced](https://techcommunity.microsoft.com/t5/azure-confidential-computing/azure-confidential-vms-using-sev-snp-dcasv5-ecasv5-are-now/ba-p/3573747) the *limited preview* of CVMs with customizable firmware. With this CVM type, (4) switches from *No* to *Yes*. Constellation will support customizable firmware on Azure in the future.
The [CVMs Generally Available in GCP](https://cloud.google.com/confidential-computing/confidential-vm/docs/confidential-vm-overview#amd_sev) are based on AMD SEV but don't have SNP features enabled.
CVMs with SEV-SNP enabled are currently in [private preview](https://cloud.google.com/blog/products/identity-security/rsa-snp-vm-more-confidential). Regarding (3), with their SEV-SNP offering Google provides direct access to remote-attestation statements.
However, regarding (4), the CVMs still include closed-source firmware.
Intel and Google have [collaborated](https://cloud.google.com/blog/products/identity-security/rsa-google-intel-confidential-computing-more-secure) to enhance the security of TDX, and have recently [revealed](https://venturebeat.com/security/intel-launches-confidential-computing-solution-for-virtual-machines/) their plans to make TDX compatible with Google Cloud.
Regarding (3), AWS provides direct access to remote-attestation statements.
However, regarding (5), attestation is partially based on the [NitroTPM](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/nitrotpm.html) for [measured boot](../architecture/attestation.md#measured-boot), which is a vTPM managed by the Nitro hypervisor.
Hence, the hypervisor is currently part of Constellation's TCB.
Regarding (4), the [firmware is open source](https://github.com/aws/uefi) and can be reproducibly built.
OpenStack is an open-source cloud and infrastructure management software. It's used by many smaller CSPs and datacenters. In the latest *Yoga* version, OpenStack has basic support for CVMs. However, much depends on the employed kernel and hypervisor. Features (2)--(4) are likely to be a *Yes* with Linux kernel version 6.2. Thus, going forward, OpenStack on corresponding AMD or Intel hardware will be a viable underpinning for Constellation.
## Conclusion
The different clouds and software like the Linux kernel and OpenStack are in the process of building out their support for state-of-the-art CVMs. Azure has already most features in place. For Constellation, the status quo means that the TCB has different shapes on different infrastructures. With broad SEV-SNP support coming to the Linux kernel, we soon expect a normalization of features across infrastructures.
