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Live migration can deadlock if the guest triggers a virtio device
activation while the migration worker owns the VM.
The failure shows up when starting live migrations during boot and
firmware startup, where the guest can reset and reinitialize virtio
devices while precopy is running. In the failing case, the source log
shows a pending virtio activation that never completes:
8.115833s _virtio-pci-net_0: Needs activation; returning barrier
8.115854s vmm/src/vm.rs:464 -- Waiting for barrier
24.875452s Entering downtime phase
24.875481s stopping vcpu throttling thread
...
vCPU thread did not respond in 10ms to signal - retrying
vCPU thread did not respond in 20ms to signal - retrying
...
thread 'throttle-vcpu' (1029) panicked
...
Pause(Error signalling vCPUs: Timeout when waiting for signal
to be acknowledged)
The vCPU blocks on the activation barrier and never reaches the normal
pause checkpoint. Later, migration enters downtime and stops the vCPU
throttle thread. In the failing case, that thread is still inside a
CpuManager::pause() call, which waits for every vCPU to acknowledge
the signal. The blocked vCPU never does, so the pause times out.
Fix this by storing the DeviceManager inside VmOwnership::Migration.
This keeps just enough state on the VMM thread to drain pending virtio
activations while the migration worker owns the Vm. The barrier logic
stays unchanged. The VMM now releases the same activation barrier during
migration that it already released before migration started.
This keeps the guest from getting stuck in the activation wait and
lets the later pause succeed.
Co-authored-by: Leander Kohler <leander.kohler@cyberus-technology.de>
On-behalf-of: SAP leander.kohler@sap.com
Signed-off-by: Leander Kohler <leander.kohler@cyberus-technology.de>
On-behalf-of: SAP philipp.schuster@sap.com
Signed-off-by: Philipp Schuster <philipp.schuster@cyberus-technology.de>