firecracker-microvm/firecracker · error

vCPU already started

Error message

vCPU already started

What it means

Error "vCPU already started" thrown in firecracker-microvm/firecracker.

Source

Thrown at src/vmm/src/vstate/vcpu.rs:188

    pub fn copy_kvm_vcpu_fd(&self, vm: &KvmVm) -> Result<VcpuFd, CopyKvmFdError> {
        // SAFETY: We own this fd so it is considered safe to clone
        let r = unsafe { libc::dup(self.kvm_vcpu.fd.as_raw_fd()) };
        if r < 0 {
            return Err(std::io::Error::last_os_error().into());
        }
        // SAFETY: We assert this is a valid fd by checking the result from the dup
        unsafe { Ok(vm.fd().create_vcpu_from_rawfd(r)?) }
    }

    /// Moves the vcpu to its own thread and constructs a VcpuHandle.
    /// The handle can be used to control the remote vcpu.
    pub fn start_threaded(
        mut self,
        vm: &KvmVm,
        seccomp_filter: Arc<BpfProgram>,
        barrier: Arc<Barrier>,
    ) -> Result<VcpuHandle, StartThreadedError> {
        let event_sender = self.event_sender.take().expect("vCPU already started");
        let response_receiver = self.response_receiver.take().unwrap();
        let vcpu_fd = self
            .copy_kvm_vcpu_fd(vm)
            .map_err(StartThreadedError::CopyFd)?;
        let vcpu_thread = thread::Builder::new()
            .name(format!("fc_vcpu {}", self.kvm_vcpu.index))
            .spawn(move || {
                let filter = &*seccomp_filter;
                self.register_kick_signal_handler();
                // Synchronization to make sure thread local data is initialized.
                barrier.wait();
                self.run(filter);
            })
            .map_err(StartThreadedError::Spawn)?;

        Ok(VcpuHandle::new(
            event_sender,
            response_receiver,

View on GitHub (pinned to 81b38b9dad)

Solutions

1. Do not start a vCPU that has already been started; check vCPU state before issuing a start.
2. Ensure resume/start API calls are idempotent or rejected when the vCPU is running.

When it happens

Trigger: Thrown at src/vmm/src/vstate/vcpu.rs:188 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of firecracker-microvm/firecracker@81b38b9dad (2026-08-19). Data as JSON: /api/errors/f995386755d9b37a. Report an issue: GitHub.