Ping? This can prevent un-predictable symptoms during Libvirt/QEMU live migration, especially when hotpluggable='yes' for vCPU in Libvirt XML file.
For instance, vCPU hotpluggable='yes' is equivalent to: 1. Create source QEMU with "-smp 1,maxvcpus=4". Keep QEMU in prelaunch state. 2. Cold-plug vCPUs=1,2,3 to source QEMU (prelaunch). 3. Continue source QEMU to running status. 4. Create target QEMU with "-smp 1,maxvpus=4". Target QEMU remains prelaunch/stopped. 5. Cold plug vCPUs=1,2,3 to target QEMU. They are expected to remain in stopped status. Unfortunately, due to the bug, vCPUs=1,2,3 are in running status. There are chances for them to start running in KVM guest mode before target QEMU is fully resumed during live migration. As a result, anything abnormal can happen. Thank you very much! Dongli Zhang On 10/10/25 2:36 PM, Dongli Zhang wrote: > When a new vCPU is hotplugged, cpu->stopped is unconditionally set to false > by cpu_common_realizefn(). > > However, there are scenarios where the guest is not running, i.e., when the > guest has been stopped via the HMP 'stop' command, or when the instance is > a live migration target started with "-incoming defer". In these cases, all > existing vCPUs have (cpu->stopped == true), except for the newly hotplugged > vCPU. > > Unpause the hotplugged vCPU only when the guest is running. > > Signed-off-by: Dongli Zhang <[email protected]> > --- > hw/core/cpu-common.c | 6 +++++- > 1 file changed, 5 insertions(+), 1 deletion(-) > > diff --git a/hw/core/cpu-common.c b/hw/core/cpu-common.c > index 8c306c89e4..789382cad5 100644 > --- a/hw/core/cpu-common.c > +++ b/hw/core/cpu-common.c > @@ -30,6 +30,7 @@ > #include "qemu/target-info.h" > #include "exec/log.h" > #include "exec/gdbstub.h" > +#include "system/runstate.h" > #include "system/tcg.h" > #include "hw/boards.h" > #include "hw/qdev-properties.h" > @@ -263,7 +264,10 @@ static void cpu_common_realizefn(DeviceState *dev, Error > **errp) > > if (dev->hotplugged) { > cpu_synchronize_post_init(cpu); > - cpu_resume(cpu); > + > + if (runstate_is_running()) { > + cpu_resume(cpu); > + } > } > > /* NOTE: latest generic point where the cpu is fully realized */
