vsock.Listen(port, nil) binds the local context ID, which resolves to the loopback context (VMADDR_CID_LOCAL) on kernels with vsock_loopback loaded. a loopback listener only accepts connections from the host itself, so guest agents can never reach any of the four host-side listeners (agent hub, read cache proxy, upload cache proxy, and the spindle-microvm-run dev listener). fedora kernels auto-load vsock_loopback; nixos hosts do not, which is why this only shows up off-nixos. use ListenContextID(vsock.Host, ...) so the listeners always bind the host context (VMADDR_CID_HOST) regardless of which modules are loaded. Signed-off-by: Winter <winter@madoka.systems>