Re: [RFC 0/3] qmp: make qmp_device_add() a coroutine

[Paolo Bonzini]

On Tue, Sep 12, 2023 at 7:08 PM Kevin Wolf <kwolf@redhat.com> wrote:
> > Any caller of qmp_dispatch() knows if it is in a coroutine or not.
> > qemu-ga uses neither a coroutine dispatcher nor coroutine commands.
> > QEMU uses non-coroutine dispatch for out-of-band commands (and we can
> > forbid coroutine + allow-oob at the same time), and coroutine dispatch
> > for the others.
> >
> > So, moving out of coroutine context (through a bottom half) should be
> > done by monitor_qmp_dispatch(), and likewise moving temporarily out of
> > the iohandler context in the case of coroutine commands. In the case
> > of !req_obj->req you don't need to do either of those. qmp_dispatch()
> > can still assert that the coroutine-ness of the command matches the
> > context in which qmp_dispatch() is called.
> >
> > Once this is done, I think moving out of coroutine context can use a
> > BH that runs in the iohandler context.
>
> Non-coroutine handlers could probably stay in iothread_ctx, but I don't
> think we can avoid switching to a different for coroutine handlers.

Agreed.

> So maybe we can just move the rescheduling down to the coroutine case in
> qmp_dispatch().

Not sure about qmp_dispatch (see above: any caller of the function
knows if it is in a coroutine or not, and qemu-ga need not know about
coroutines at all). But what you said also applies if the rescheduling
is only pushed to monitor_qmp_dispatch(), which would be my first
option.

Thanks!

Paolo