Is guest_cid required to be unique across multiple Firecracker VMs if each has a different uds_path?

[bbaterdene-gh]

In Firecracker, each microVM has its own VMM process and is configured with its own unique vsock.uds_path. Suppose I launch multiple Firecracker VMs on the same host, each with a different uds_path but using the same guest_cid (e.g., 5).

Given that Firecracker’s vsock implementation is userspace-based and uses UNIX domain sockets rather than the host kernel’s AF_VSOCK subsystem (like vhost-vsock does), is it safe to reuse the same guest_cid across VMs?

More specifically:
• Does Firecracker internally require guest_cid to be unique per VM?
• Is there any host-level conflict or routing ambiguity caused by reusing guest_cid if the uds_path is unique per VM?
• Are there any edge cases where this setup could cause issues for host-initiated connections or vsock forwarding logic?

I’m aware that guest_cid must be >2 and unique within a VM, but the Firecracker documentation doesn’t clearly state the scope of uniqueness across multiple VMs.

Thanks.

[bchalios]

Hi @bbaterdene-gh,

As you correctly say:

Given that Firecracker’s vsock implementation is userspace-based and uses UNIX domain sockets rather than the host kernel’s AF_VSOCK subsystem (like vhost-vsock does), is it safe to reuse the same guest_cid across VMs?

We are not using AF_VSOCK on the host side. In our case each microVM has its own namespace for vsock IDs, which means that two different microVMs can have the same CID.