Re: [PATCH V2] python/machine: QEMUMachine reopen_qmp_connection

[Steven Sistare]

On 2/7/2023 3:28 PM, John Snow wrote:
> On Tue, Feb 7, 2023 at 2:03 PM Steve Sistare <steven.sistare@oracle.com> 
> wrote:
>>
>> Provide reopen_qmp_connection() to reopen a closed monitor connection.
>> This will be needed by cpr, because qemu exec closes the monitor socket.
>>
>> Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
>> Reviewed-by: John Snow <jsnow@redhat.com>
>> ---
>>  python/qemu/machine/machine.py | 10 ++++++++++
>>  1 file changed, 10 insertions(+)
>>
>> diff --git a/python/qemu/machine/machine.py b/python/qemu/machine/machine.py
>> index ef94dcf..557209a 100644
>> --- a/python/qemu/machine/machine.py
>> +++ b/python/qemu/machine/machine.py
>> @@ -501,6 +501,16 @@ def _close_qmp_connection(self) -> None:
>>          finally:
>>              self._qmp_connection = None
>>
>> +    def reopen_qmp_connection(self) -> None:
>> +        """Close and re-open the QMP connection."""
>> +        self._close_qmp_connection()
>> +        self._qmp_connection = QEMUMonitorProtocol(
>> +            self._monitor_address,
>> +            server=True,
>> +            nickname=self._name
>> +        )
>> +        self._qmp.accept(self._qmp_timer)
>> +
>>      def _early_cleanup(self) -> None:
>>          """
>>          Perform any cleanup that needs to happen before the VM exits.
>> --
>> 1.8.3.1
>>
> 
> This code is still mechanically fine as far as I can tell, but I lost
> the plot on why it's needed - Can you please elaborate for me on what
> you mean by "qemu exec will close the socket"?
> 
> (R-B still stands, but since I am rewriting machine.py to be natively
> async, I should be aware of your new use case.)

Sure, thanks for asking.

During cpr (aka live update), the old qemu process directly execv's the new
qemu binary.  The monitor socket fd is marked cloexec, so the kernel closes it
during execv.

- Steve