On Thu, 7 Sept 2023 at 10:26, Paolo Bonzini <pbonz...@redhat.com> wrote: > > On Thu, Sep 7, 2023 at 4:00 PM Stefan Hajnoczi <stefa...@redhat.com> wrote: > > While I agree that the issue would not happen if monitor commands only > > ran in the iohandler AioContext, I don't think we can change that. > > When Kevin implemented coroutine commands in commit 9ce44e2ce267 ("qmp: > > Move dispatcher to a coroutine"), he used qemu_get_aio_context() > > deliberately so that AIO_WAIT_WHILE() can make progress. > > Ah, you are referring to > > + /* > + * Move the coroutine from iohandler_ctx to qemu_aio_context for > + * executing the command handler so that it can make progress if it > + * involves an AIO_WAIT_WHILE(). > + */ > + aio_co_schedule(qemu_get_aio_context(), qmp_dispatcher_co); > + qemu_coroutine_yield(); > > > I'm not clear on the exact scenario though, because coroutines shouldn't > > call AIO_WAIT_WHILE(). > > I think he meant "so that an AIO_WAIT_WHILE() invoked through a bottom > half will make progress on the coroutine as well". > > However I am not sure the comment applies here, because > do_qmp_dispatch_bh() only applies to non-coroutine commands; that > commit allowed monitor commands to run in vCPU threads when they > previously weren't. > > Thinking more about it, I don't like that the > > if (!!(cmd->options & QCO_COROUTINE) == qemu_in_coroutine()) { > } > > check is in qmp_dispatch() rather than monitor_qmp_dispatch(). > > 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.
I'll wait for Kevin's input and will then revisit the patches based on the conclusion we come to. Stefan