Re: [Qemu-devel] [PATCH 3/3] replay: introduce block devices record/replay

["Pavel Dovgalyuk" <dovgaluk@ispras.ru>]

> From: Kevin Wolf [mailto:kwolf@redhat.com]
> > >
> > > First of all, I'm not sure if running replay events from
> > > qemu_clock_get_ns() is such a great idea. This is not a function that
> > > callers expect to change any state. If you absolutely have to do it
> > > there instead of in the clock device emulations, maybe restricting it to
> > > replaying clock events could make it a bit more harmless.
> >
> > Only virtual clock is emulated, and host clock is read from the host
> > real time sources and therefore has to be saved into the log.
> 
> Isn't the host clock invisible to the guest anyway?

It isn't. Host clock is used by guest RTC.

> > There could be asynchronous events that occur in non-cpu threads.
> > For now these events are shutdown request and block task execution.
> > They may "hide" following clock (or another one) events. That is why
> > we process them until synchronous event (like clock, instructions
> > execution, or checkpoint) is met.
> >
> >
> > > Anyway, what does "can't proceed" mean? The coroutine yields because
> > > it's waiting for I/O, but it is never reentered? Or is it hanging while
> > > trying to acquire a lock?
> >
> > I've solved this problem by slightly modifying the queue.
> > I haven't yet made BlockDriverState assignment to the request ids.
> > Therefore aio_poll was temporarily replaced with usleep.
> > Now execution starts and hangs at some random moment of OS loading.
> >
> > Here is the current version of blkreplay functions:
> >
> > static int coroutine_fn blkreplay_co_readv(BlockDriverState *bs,
> >     int64_t sector_num, int nb_sectors, QEMUIOVector *qiov)
> > {
> >     uint32_t reqid = request_id++;
> >     Request *req;
> >     req = block_request_insert(reqid, bs, qemu_coroutine_self());
> >     bdrv_co_readv(bs->file->bs, sector_num, nb_sectors, qiov);
> >
> >     if (replay_mode == REPLAY_MODE_RECORD) {
> >         replay_save_block_event(reqid);
> >     } else {
> >         assert(replay_mode == REPLAY_MODE_PLAY);
> >         qemu_coroutine_yield();
> >     }
> >     block_request_remove(req);
> >
> >     return 0;
> > }
> >
> > void replay_run_block_event(uint32_t id)
> > {
> >     Request *req;
> >     if (replay_mode == REPLAY_MODE_PLAY) {
> >         while (!(req = block_request_find(id))) {
> >             //aio_poll(bdrv_get_aio_context(req->bs), true);
> >             usleep(1);
> >         }
> 
> How is this loop supposed to make any progress?

This loop does not supposed to make any progress. It waits until block_request_insert
call is added to the queue.

> And I still don't understand why aio_poll() doesn't work and where it
> hangs.

aio_poll hangs if "req = block_request_insert(reqid, bs, qemu_coroutine_self());" line
is executed after bdrv_co_readv. When bdrv_co_readv yields, replay_run_block_event has no
information about pending request and cannot jump to its coroutine.
Maybe I should implement aio_poll execution there to make progress in that case?

> >         qemu_coroutine_enter(req->co, NULL);
> >     }
> > }


Pavel Dovgalyuk