Re: [PATCH 3/3] monitor: only run coroutine commands in qemu_aio_context

[Michael Tokarev]

16.01.2024 22:00, Stefan Hajnoczi пишет:
> monitor_qmp_dispatcher_co() runs in the iohandler AioContext that is not
> polled during nested event loops. The coroutine currently reschedules
> itself in the main loop's qemu_aio_context AioContext, which is polled
> during nested event loops. One known problem is that QMP device-add
> calls drain_call_rcu(), which temporarily drops the BQL, leading to all
> sorts of havoc like other vCPU threads re-entering device emulation code
> while another vCPU thread is waiting in device emulation code with
> aio_poll().
>
> Paolo Bonzini suggested running non-coroutine QMP handlers in the
> iohandler AioContext. This avoids trouble with nested event loops. His
> original idea was to move coroutine rescheduling to
> monitor_qmp_dispatch(), but I resorted to moving it to qmp_dispatch()
> because we don't know if the QMP handler needs to run in coroutine
> context in monitor_qmp_dispatch(). monitor_qmp_dispatch() would have
> been nicer since it's associated with the monitor implementation and not
> as general as qmp_dispatch(), which is also used by qemu-ga.
>
> A number of qemu-iotests need updated .out files because the order of
> QMP events vs QMP responses has changed.
>
> Solves Issue #1933.
>
> Fixes: 7bed89958bfbf40df9ca681cefbdca63abdde39d ("device_core: use drain_call_rcu in 
> in qmp_device_add")
> Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2215192
> Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2214985
> Buglink: https://issues.redhat.com/browse/RHEL-17369
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

This change broke something in 7.2. I'm still debugging it, will
come with a follow-up once some more details are found, I'll also
check current master with and without this commit.

The prob happens with multiple suspend-resume cycles, - with this
change applied, guest does not work as expected after *second*
suspend-resume.

JFYI for now.

Thanks,

/mjt