Re: Replace GSource with AioContext for chardev

["Daniel P. Berrangé" <berrange@redhat.com>]

On Thu, Apr 09, 2020 at 03:16:01PM +0200, Marc-André Lureau wrote:
> Hi
> 
> On Thu, Apr 9, 2020 at 2:46 PM Coiby Xu <coiby.xu@gmail.com> wrote:
> >
> >
> > Hi,
> >
> > I'm now implementing vhost-user block device backend
> > https://patchew.org/QEMU/20200309100342.14921-1-coiby.xu@gmail.com/
> > and want to use chardev to help manage vhost-user client connections
> > and read socket message. However there are two issues that need to be
> > addressed.
> >
> > Firstly, chardev isn't suitable for the case when exported drive is
> > run in an IOThread because for mow chardev use GSource to dispatch
> > socket fd events. So I have to specify which IOThread the exported
> > drive is using when launching vhost-user block device backend,
> > for example, the following syntax will be used,
> >
> >    -drive file=file.img,id=disk -device virtio-blk,drive=disk,iothread=iothread0 \
> >     -object vhost-user-blk-server,node-name=disk,chardev=mon1,iothread=iothread0 \
> >     -object iothread,id=iothread0 \
> >     -chardev socket,id=mon1,path=/tmp/vhost-user-blk_vhost.socket,server,nowait
> >
> > then iothread_get_g_main_context(IOThread *iothread) has to be called
> > to run the gcontext in IOThread. If we use AioContext to dispatch socket
> > fd events, we needn't to specify IOThread twice. Besides aio_poll is faster
> > than g_main_loop_run.
> >
> > Secondly, socket chardev's async read handler (set through
> > qemu_chr_fe_set_handlers) doesn't take the case of socket short read
> > into consideration.  I plan to add one which will make use qio_channel_yield.
> >
> > According to
> > [1] Improving the QEMU Event Loop - Linux Foundation Events
> > http://events17.linuxfoundation.org/sites/events/files/slides/Improving%20the%20QEMU%20Event%20Loop%20-%203.pdf
> >
> > "Convert chardev GSource to aio or an equivalent source" (p.30) should have
> > been finished. I'm curious why the plan didn't continue. If it's desirable,
> > I'm going to finish the leftover work to resolve the aforementioned two issues.
> 
> Converting all chardevs to Aio might be challenging, and doesn't bring
> much benefits imho.
> 
> Perhaps a better approach would be to rely on a new chardev API to
> steal the chardev underlying fd or QIO... (mostly keeping -chardev for
> CLI/QMP compatibility reason - although breaking some chardev features
> that imho aren't compatible with all use cases, like replay, muxing,
> swapping etc). The chardev should probably be removed after that...

Yeah, I feel like it was a mistake for us to wire up many of our features
to chardevs. We mostly did it because -chardev provides a pre-existing
syntax for TCP/UNIX sockets and we didn't want to invent new CLI args.
IMHO this was a mistake in retrospect.

Unfortunately the -chardev API is absolutely terrible for any usage that
actually cares about the connection based semantics. Witness the horrible
hacks we do for re-connect and re-try when failing to initially connect
in vhost-user net code.

For features in QEMU where the only desirable chardev backend is one with
connection based, socket semantics, I think we would be better off using
the QIOChannel APIs directly and completely avoiding the chardev code.

Regards,
Daniel
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